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AM452 - Voltage-to-current transducer IC with a differential input
PRINCIPLE FUNCTION
Amplification and conversion of differential input voltages (400mV with a CMIR of 1.5 - VCC-3V) into an adjustable current output of 0/4...20mA, for example. The offset and maximum output currents are independently adjustable in a wide range. The IC is suitable for both 2- and 3-wire applications and as a HART(R) carrier IC.
V CC = 6...35V
Differential input 400mV
AM452
IOUT = z.B.0/4...20mA
VREF = 5/10V
IS = max 10mA
TYPICAL APPLICATIONS
Transducers for differential input signals in current output values for: * * * * * Transducers for sensor applications with an internal sensing element supply Drivers for the analog industrial power grid (e.g. remote display in current loop operation) Differential impedance converters Carrier for standard HART(R) protocol communications Modular signal conditioning with digital correction (Frame concept [1])
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 1/21
AM452 - Voltage-to-current transducer IC with a differential input
TABLE OF CONTENTS
PRINCIPLE FUNCTION FEATURES SCHEMATIC GENERAL DESCRIPTION ELECTRICAL SPECIFICATIONS BOUNDARY CONDITIONS DETAILED DESCRIPTION OF FUNCTIONS Instrumentation amplifier (IA) Operational amplifier stage (OP1) Zero adjust stage SET stage Voltage-to-current converter (V/I converter) Reference voltage source Additional operational amplifier OP2 OPERATING AM452 2- and 3-wire applications in general [2] Differences in the AM452 circuitry with 2- and 3-wire applications Selecting the supply voltage Setting the offset and output current range for VIN = 0 OPERATING AM452: IMPORTANT POINTS TO NOTE DIMENSIONING APPLICATIONS Typical 3-wire application with a differential input signal Typical 2-wire application with a differential input signal Offset compensation using a voltage divider at SET stage Using OP2 as a current source Using OP2 as a voltage source BLOCK DIAGRAM AND PINOUT DELIVERY PACKAGE DIMENSIONS FURTHER READING NOTES 1 3 3 3 4 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8
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AM452 - Voltage-to-current transducer IC with a differential input
FEATURES
* Instrumentation amplifier input with a wide voltage range of 400mV * Adjustable gain and offset * Adjustable current output (e.g. of 0/4...20mA) * 2- and 3-wire operation * Suitable for HART(R) applications * Protection against reverse polarity and short-circuiting * Output signal limiting * Integrated current source * Adjustable integrated reference voltage source of 5 to 10V * Modular configuration * Supply voltage of 6...35V * Temperature range of -40C...+85C * RoHS compliant
GENERAL DESCRIPTION
AM452 is an integrated transducer with an adjustable current output which has been specifically designed for the conditioning of differential input signals. It permits the independent adjustment of the offset and fullscale current using just a few components. The IC consists of various functional modules. In addition to the instrumentation amplifier in the signal path there is an operational amplifier which is used to set the gain. The offset can be adjusted using the Zero adjust stage and/or the SET stage module. An additional operational amplifier can supply external components. The adjustable current output stage permits 2- and 3-wire operation by way of a simple amendment to the circuitry. The IC is distinguished by its many protective functions which include protection against reverse polarity and short-circuiting and also an internal current limit.
SCHEMATIC
Prog. Reference Reference Offset OP2 Output
1 12 15 16
OP2 Input Differential input voltages
2 VBG 3 4
SET Stage
11 10
OP2
Voltage Reference V/I Converter
VCC Output
9 8
IA
ZERO Stage 13 5
OP1
AM452
7 14
Offset
Amplifier
GND
Figure 1: Block diagram of AM452 Figure 1: Schematic of AM452
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AM452 - Voltage-to-current transducer IC with a differential input
ELECTRICAL SPECIFICATIONS
Tamb = 25C, VCC = 24V, VREF = 5V, IREF = 1mA (unless otherwise stated)
Parameter Supply Voltage Range Quiescent Current Temperature Specifications Operating Storage Junction Voltage Reference Voltage Current VREF vs. Temperature Line Regulation Load Regulation Load Capacitance Current/Voltage Source OP2 Internal Reference VBG vs. Temperature Current Source: ICV = VBG/REXT Adjustable Current Range* Output Voltage ICV * VCV VCV Voltage Source: VCV = VBG (REXT1 + REXT2) / REXT2 Adjustable Voltage Range Output Current Load Capacitance VCV VCV ICV * ICV CL VCC < 19V VCC 19V Source Sink Source mode 0 1 0.4 0.4 VCC - 4 15 10 -100 10 V V mA A nF VCC < 19V VCC 19V 0 VBG VBG 10 VCC - 4 15 mA V V VBG dVBG/dT Tamb = - 40...+85C 1.20 1.27 60 1.35 140 V ppm/C VREF VREF IREF * dVREF/dT dVREF/dV dVREF/dV dVREF/dI dVREF/dI CL IREF 5mA 1.9 Tamb = - 40...+85C VCC = 6V...35V VCC = 6V...35V, IREF 5mA VSET not connected VSET = GND, VCC 11V 4.75 9.5 0 90 30 60 0.05 0.06 2.2 5.00 10.0 5.25 10.5 10.0 140 80 150 0.10 0.15 5.0 V V mA ppm/C ppm/V ppm/V %/mA %/mA F Tamb Tst TJ -40 -55 85 125 150 C C C Symbol VCC ICC Conditions VSET not connected Tamb = - 40...+85C, IREF = 0mA Min. 6 Typ. Max. 35 1.5 Unit V mA
* In 2-wire operation IS has to fulfill the condition ICC +IS < IOUTmin with IOUTmin = 4mA
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AM452 - Voltage-to-current transducer IC with a differential input
Parameter Instrumentation Amplifier (cont.) Internal Gain Differential Input Voltage Range Common Mode Input Range Common Mode Rejection Ratio Power Supply Rejection Ratio Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature Output Voltage Minimum Output Voltage Load Capacitance Zero Adjust Stage Internal Gain Zero Adjust Voltage GZA VZA VZA VOUTIAmax - GIA VIN ; Vcc<9V, VIN=400mV, GIA=5 VZA VOUTIAmax - GIA VIN; Vcc9V, VIN =400mV, GIA=5 Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature Adjustable Gain Input Range Power Supply Rejection Ratio Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature Output Voltage Limitation Output Voltage Range Load Capacitance VOS dVOS/dT IB dIB/dT GGAIN IR IR PSRR VOS dVOS/dT IB dIB/dT VLIM VOP VOP CL VCC < 10V VCC 10V 0 0 VCC < 10V VCC 10V 1 0 0 80 -3.0 90 -1.0 3 10 7 VREF VCC - 5 VREF 250 1.0 7 25 20 VCC - 5 5 V V dB mV V/C nA pA/C V V V pF 0.94 0 0 0.5 1.6 47 18 1 1.06 Vcc-6 3 2.0 5 120 30 V V mV V/C nA pA/C GIA VIN CMIR CMIR CMRR PSRR VOS dVOS/dT IB dIB/dT VOUTIA VOUTIA VOUTIAmin CL VCC < 9V VCC 9V 4.5 VCC < 9V, ICV < 2mA VCC 9V, ICV < 2mA 4.9 0 1.5 1.5 80 80 -9.0 90 90 -1.5 5 -100 -0.4 -250 -0.9 VCC - 4 5 16 250 +6.0 5 5.1 400 VCC - 3 6.0 mV V V dB dB mV V/C nA nA/C V V mV pF Symbol Conditions Min. Typ. Max. Unit
Operational Amplifier - Gain Stage (OP1)
NB: The current in the IC is given as a negative quantity.
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AM452 - Voltage-to-current transducer IC with a differential input
Parameter V/I Converter Internal Gain Trim Range Voltage Range at R0 FS Offset Voltage VOS vs. Temperature Input Resistance RIN vs. Temperature Output Offset Current IOUTOS vs. Temperature Output Offset Current IOUTOS vs. Temperature Output Control Current IOUTC vs. Temperature Output Voltage Range Output Current Range FS Output Resistance Load Capacitance SET Stage Internal Gain Input Voltage Offset Voltage VOS vs. Temperature Input Bias Current IB vs. Temperature Protective Functions Voltage Limitation at R0 Protection against reverse polarity Current in event of reverse polarity System Parameters Nonlinearity 3-dB-frequency Statistical output impedance Dynamical output impedance f3db Rstat. Rdyn. Ideal input RL = 600, C2 = 1nF RL =600, C2 = 1nF, For f= 2,2kHz, RL =600, C2 = 1nF, 0.05 0.15 5 4*103 3*103 %FS kHz M VLIMR0 VLIMR0 VR0 = VIN GI, SET = GND VIN = 0, VR0 = VSET/2 Ground vs. VS vs. VOUT Ground vs. VS vs. IOUT Ground = 35V, VS = IOUT = 0 4.5 580 VREF/8 635 690 35 35 mV mV V V mA GSET VSET VOS dVOS/dT IB dIB/dT 0 -4.0 -1.0 1.6 8 7 0.5 1.15 +2.0 5 20 18 V mV V/C nA pA/C VR0FS VOS dVOS/dT RIN dRIN/dT IOUTOS dIOUTOS/dT IOUTOS dIOUTOS/dT IOUTC dIOUTC/dT VOUT VOUT IOUTFS ROUT CL 3-wire operation 3-wire operation 2-wire operation 2-wire operation 2-wire operation, VR0/100mV 2-wire operation VOUT = RL IOUT, VCC < 18V VOUT = RL IOUT, VCC 18V IOUT = VR0/R0, 3-wire operation 0.5 0 0 0 20 1.0 500 GVI Adjustable by R0 0.122 0.60 320 0.125 1.00 540 2 7 120 0.2 160 0.3 -25 16 9.5 6 6 -10 -35 26 14 8 8 -15 VCC - 6 12 0.128 1.40 760 4 14 mV mV V/C k k/C A nA/C A nA/C A nA/C V V mA M nF Symbol Conditions Min. Typ. Max. Unit
F 100 F 100
Table 1: Specifications
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AM452 - Voltage-to-current transducer IC with a differential input
BOUNDARY CONDITIONS
Parameter Sense Resistor Stabilization Resistor Load Resistance Sum Gain Resistors Sum Offset Resistors Sum IA Offset Resistor VREF Capacitance Output Capacitance D1 Breakdown Voltage T1 Forward Current Gain Symbol R0 R0 R5 R5 RL R1 + R2 R3 + R4 R6 + R7 C1 C2 VBR Min. value for Tamb 85C Only for 2-wire operation e.g. BCX54/55/56 Conditions IOUTFS = 20mA c = 20mA/IOUTFS IOUTFS = 20mA c = 20mA/IOUTFS Only for 3-wire operation Min. 16 c 16 35 c 35 0 20 20 20 1.9 90 35 50 2.2 100 50 150 Typ. 27 c 27 40 c 40 Max. 38 c 38 45 c 45 600 200 200 200 5.0 250 Unit k k k F nF V
F
Table 2: Boundary conditions NB: In 2-wire operation and with the connected resistors capacitance C2 acts as a low pass filter with a time constant of = RL C2.
VC VREF
1
VSET
12
VREF C1
15
R3
16
R4 VS
GSET SET Stage
VC VSET
2
AM452
OP2
Voltage Reference
GS ET GV I
11 10
R0
VIN+ VIN-
VBG
3
IA
4
ZERO Stage
GVI V/I Converter
OP1
9 8
T1 D1
VR EF R6
VZA
13
5
7
14
Figure 2 shows AM452 as a 3-wire application where output current IOUT min > 0mA is set using the instrumentation amplifier (with a negative offset at the IA input) and the SET stage. The gain on the maximum output current is adjusted using OP1.
GND
R7
R2
R1
R5 IOUT Ground
V OP
Figure 2: Block diagram of AM452 (3-wire version).
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AM452 - Voltage-to-current transducer IC with a differential input
DETAILED DESCRIPTION OF FUNCTIONS
AM452 is a modular, monolithically integrated transducer which has been specially developed for the conditioning of differential voltage signals. It consists of several function blocks, the values of which are described in detail in the electrical specifications. Its various function blocks are depicted in the block diagram (Figure 2) and described in the following. Instrumentation amplifier (IA) The instrumentation amplifier (IA) with an internal fixed gain of GIA = 5 acts as an input stage for differential voltage signals of 400mV maximum. Thanks to the device's special construction a high input impedance and high common mode rejection ratio (CMRR) are achieved. The reference potential of the amplifier can be set externally using pin 13 or ZA, with which the offset current at the output (e.g. 4mA) can be increased. It is thus possible to compensate for the negative offset of the signal source (up to -400mV) or to correct that of the instrumentation amplifier. The following applies to the transfer function of the instrumentation amplifier:
VOUTIA = GIA VIN + VZA with VOUTIA > 0
(1)
where VIN describes the differential voltage between the two inputs VIN+ and VIN- and VZA the voltage at pin 13 (ZA) of instrumentation amplifier IA. Operational amplifier stage (OP1) The operational amplifier stage (OP1) permits variable amplification of the IA output signal. OP1 gain GGAIN can be set via external resistors R1 and R2 (see Figure 2). Protective circuitry against overvoltage is integrated into the chip, limiting the voltage to the set value of the reference voltage. The output voltage at OP1 can be tapped for control purposes at pin 7 (VOP). This is calculated as: VOP = VOUTIA GGAIN with GGAIN = 1 + 1 R 2 where VOUTIA is not externally accessible but is connected internally to the OP1 input. Zero adjust stage The zero adjust stage enables a negative signal to be raised to a maximum of -400mV at the instrumentation amplifier input by adding an additional voltage of VZA. A zero setting which is practically offset free with regard to the following circuit modules can thus be achieved, for example. The following applies: VZA VOUTIA max - GIA VIN
R
(2)
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AM452 - Voltage-to-current transducer IC with a differential input
SET stage The SET stage permits the adjustment of the offset output current IOUTmin > 0mA. Together with the V/I converter it effects the output current IOUT. Via pin 16 (SET) an offset current ISET can be set at pin 8 (IOUT) e.g. with the help of the internal voltage reference and an external voltage divider as shown in Figure 2, for example. Voltage-to-current converter (V/I converter) The voltage-to-current converter (V/I converter) compares the voltage drop across the external sensing resistor R0 with a value of VSET GSET + VOP GVI and uses the result to regulate transistor T1. It generates a suitable signal at the IC output pin 8 (IOUT) which activates external transistor T1. This in turn supplies an output current of IOUT and accepts the power dissipation of the output stage. External resistor R0 permits the output current to be finely adjusted. For the output current IOUT amplified by T1 the following ratio applies:
I OUT =
V G V VOP GVI V + I SET = OP + I SET with I SET = SET SET = SET 8 R0 R0 2R 0 R0
(3)/(4)
where VOP is the input voltage of the V/I converter and VSET the voltage at pin 16 (SET). Reference voltage source The reference voltage source (bandgap voltage source) enables voltage to be supplied to external components (such as sensors, microprocessors, etc.). The reference voltage value VREF can be set using pin 12 (VSET). If pin 12 is not connected, VREF = 5V; if pin 12 is switched to ground, VREF = 10V. Values between these can be set if two external resistors are used (inserted between pin 15 (VREF) and pin 12 (VSET) and between pin 12 (VSET) and GND).
External capacitor C1 stabilizes the reference voltage. It must be connected even if the voltage reference is not in use. It may not undershoot the given minimum value.
Additional operational amplifier OP2 The additional operational amplifier OP2 can be used as a current or voltage source to supply external components. OP2's positive input must be connected internally to bandgap voltage VBG so that the OP2 output voltage at pin 1 or CVREF can be set across a wide range using external resistors. The individual modules are described separately in the specifications. The reference voltage source and the operational amplifier OP2 can be operated as independent circuit elements or modules. Instrumentation amplifier IA, operational amplifier OP1 and the V/I converter form a unit within the circuit and have the task of converting the voltage input signal into the required output current.
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AM452 - Voltage-to-current transducer IC with a differential input
OPERATING AM452
2- and 3-wire applications in general [2]
As AM452 can function in both 2- and 3-wire operation through external contacting, it is important to first differentiate between the two versions of the circuit. In 2-wire operation the IC ground is "virtual" (floating), as with a constant load resistance the IC supply voltage VCC changes according to the current. The following equation can generally be applied to 2-wire operation:
VCC = VS - I OUT (VIN ) RL
(5)
The reason for this is that in 2-wire operation the IC is connected in series to the actual load resistor RL. This is illustrated in Figure 3. In a 2-wire system the power consumption of the overall system (AM452 plus all external components including the signal source and adjusting resistors) may not be more than IOUTmin (e.g. 4mA). In 3-wire operation Equation (5) no longer applies as the IC ground is connected to the ground of the system. In 3-wire operation the supply voltage can be expressed as: VCC = VS (6)
2-wire system signal source and conditioning IC
GND Ground VCC VS GND
VCC IOUT RL VS
3-wire system signal source and conditioning IC
GND = Ground VCC = VS
IOUT RL VCC = VS
Ground
Ground = GND
Figure 3: The basic difference between a 2- and 3-wire circuit
NB: The difference between GND and Ground must be clearly acknowledged!
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AM452 - Voltage-to-current transducer IC with a differential input
Differences in the AM452 circuitry with 2- and 3-wire applications
3-wire connection
VS R0
VS
SET Stage
11 10
C2 R0
SET Stage
11 10 9
V/I Converter
9 8
V/I Converter T1 D1
8
T1 D1
R5
14
14
2-wire connection
IOUT RL
R5 GND
GND
IOUT
RL
Ground
Ground
Figure 4: Differences in 2-and 3-wire circuitry in conjunction with AM452
AM452 is constructed in such a way that by changing the external circuitry it is suitable for both 2-wire and 3-wire operation. In 3-wire operation (see Figure 4, right) the IC's connection to ground (pin 14 or GND) is connected to the system ground (Ground) which is applied externally. System supply voltage VS is connected to pin 10 (VCC) and pin VCC to pin 11 (RS+). Supply current ICC then flows directly into AM452 (power consumption). In 2-wire operation (see Figure 4, left) system supply voltage VS is connected to pin 11 (RS+) and pin 10 (VCC) to pin 9 (RS-). The overall current including the supply current then flows via R0, enabling the relevant voltage drop to be used to regulate transistor T1. The IC's connection to ground pin 14 (GND) is contacted to the node between resistor R5 and load resistor RL (current output IOUT). IC ground GND is thus not the same as the ground of the system (Ground). The output signal is tapped via load resistor RL which links system output IOUT to the ground of the system.
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AM452 - Voltage-to-current transducer IC with a differential input
Selecting the supply voltage
"System" supply voltage VS needed to operate AM452 is dependent on the selected mode of operation. The word "system" here refers to the IC plus its external circuitry. When using current output pin 8 (IOUT) in conjunction with the external transistor VS is dependent on the relevant load resistor RL used by the application. The following is then applicable to the minimum system supply voltage VS:
VS I OUT max RL + VCC min
(7)
Here, IOUTmax stands for the maximum output current and VCCmin for the minimum IC supply voltage which is dependent on the selected reference voltage:
VCC min VREF + 1V
(8)
For the 3-wire version the load resistance is limited to RLmax = 600 due to the condition: VOUT max = 12V @ VCC 18V. Equation 7 is also valid for the 2-wire version; here, however, the RLmax = 600 limitation does not apply. Here, load resistor RLmax = 900 when VS = 24V.
RL [] 600
RL
VS - VCCmin IOUTmax
VCCmin = 6V RLmax = 600 IOUTmax = 20mA
300 Operating range 0
In Equation (7) of Figure 5 the ohmic resistance of power supply lines RR is not taken into consideration. This is entered as an additive quantity (IOUT max RR) to the calculation of VS in Equation (7).
0
6
12
18
24
35
VS [V]
Figure 5: Working range in conjunction with the load resistor in 3-wire operation
Setting the offset and output current range for VIN = 0
When adjusting AM452 a preset should first be made. To this end the offset of the output current is compensated for, in which the two IA inputs are first short-circuited (VIN = 0) and then both set to a permitted potential (c.f. CMIR in ELECTRICAL SPECIFICATIONS). With the short-circuit at the input the following is derived from Equations (3) and (4) when the voltage divider from R3 and R4 is taken into account for reference voltage VREF (see Figure 2, for example):
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AM452 - Voltage-to-current transducer IC with a differential input
I OUT (V IN = 0) = I SET with I SET =
R VREF VREF R4 3= -1 2R0 R3 + R4 R4 2 R0 I SET
(9)
The output current range (e.g. 16mA) is set by the selection of external resistors R1 and R2 (or fine adjustment with R0 ). Output current IOUT is then calculated as:
I OUT = VIN G I GVI + I SET with G I = G IA GGAIN and VZA = 0 R0
(10)
If the offset of the AM452 signal source and input amplifier (IA) is such that it cannot be ignored, when setting the output current range (gain) I OUT (VIN = 0 ) also changes. This shift must possibly be accounted for by making a fine adjustment to R3 and R4. If the offset of the signal source and input amplifier is not relevant to the required degree of precision, Equations (9) and (10) continue to apply.
OPERATING AM452: IMPORTANT POINTS TO NOTE
1. When using AM452 it is imperative that external capacitor C1 (a ceramic capacitor) is always connected. Care must be taken that the value of the capacitor does not exceed the range of values given in the boundary conditions - also within the temperature range (see Table 2). In 2-wire operation ceramic capacitor C2 must also be used. 2. All AM452 function blocks not required by the application (OP2 or VREF) must be placed in a defined (and allowed) operating state. 3. The voltages at the IA inputs (pins IN+ and IN-) must always lie within input voltage range CMIR. 4. At the current output a load resistance of 600 maximum is permissible for 3-wire operation. 5. The values of external resistors R0, R1, R2, R3, R4, R5, R6 and R7 must be selected within the permissible range given in the boundary conditions. 6. The tolerances of the resistors and their temperature coefficients are entered into the overall error. 7. In order to avoid temperature gradients it is imperative that the transistor is placed far enough away from IC AM452 and that a sufficient temperature outlet is ensured. 8. In a 2-wire setup with a minimum output (offset) current of IOUT min the current balance (the total domestic power supply across a temperature range of < IOUT min) of the IC and all connected components (such as sensors) must be taken into account.
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AM452 - Voltage-to-current transducer IC with a differential input
9. For applications where IOUT min > 0mA (e.g. 4mA) in both 2- and 3-wire applications the following condition applies to the IA input values:
VFS .IA I OUT max , where VFS.IA is the maximum input signal and VOS.IA the VOS .IA I OUT min
positive offset at the input IA.
10. If signal source and/or input amplifier IA have a negative offset this can be compensated for using corrective voltage VZA and a suitable voltage divider (R6 and R7; see Figure 2).
DIMENSIONING
Two possible dimensioning methods are suggested here. Dimensioning the external components according to the equations given in the data sheet Dimensioning according to the equations given in the data sheet enables all modules to be used, making it possible for the setup to be adapted to suit the most diverse application requirements. As a rule the offset of the AM452 input signal must be taken into account. If an input signal offset is present and an offset current of IOUT min > 0mA is required, the following boundary condition then applies:
VFS .IA I OUT max , where VFS.IA is the maximum input signal and VOS .IA I OUT min
VOS.IA the positive offset at the input IA (see: chapter before). Should the signal source have a negative offset, the offset can be set via pin 13 (ZA) and voltage divider R6 and R7 (see Figure 2). Equation (1) forms the basis for all other equations in this particular case. If the offset is negligible, Equations (9) and (10) apply. See the following applications for further details.
Dimensioning AM452's external components using an Excel spreadsheet
AM452's external components can also be dimensioned with the help of Excel spreadsheet Kali_AM452.xls when the input signal is positive (see [3]). The algorithm is such that the offset output current of 4mA can only be set via pin 13 (ZA) of the zero adjust stage. The SET stage is not active. The full-scale output current is set to 20mA using the OP1 gain setting. The calibration process is also based on the condition that the output signal should be a 4...20mA current loop signal in 2-wire operation.
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AM452 - Voltage-to-current transducer IC with a differential input
Due to the algorithm upon which it is based the program also accounts for the spec. tolerances of the IC and the components connected to it.
APPLICATIONS
Typical 3-wire application with a differential input signal
The aim of the calculation is the dimensioning of resistors R0 to R5. In 3-wire operation (see Figure 6) the IC's connection to ground (pin 14 or GND) is connected to the system ground (Ground) which is applied externally. System supply voltage VS is connected to pin 10 or VCC and pin VCC to pin 11 or RS+. In this configuration AM452's quiescent current does then not flow via resistor R0. Figure 6 depicts the 3-wire application in which the differential output signal of a measuring bridge supplied with current is amplified and converted. Power is supplied to the measuring bridge by operational amplifier OP2 (c.f.: Using OP2 as a current source). It is assumed in this application that no negative differential input voltages occur. Pin 13 (ZA) is thus connected to the IC ground GND.
3-wire-connection
C1
1
R3
15 16
R4
VS
11 10 9
AM452
2
12
G SET SET Stage OP2 Voltage Reference VBG
R0
RSET
3
GVI
IA OP1
V/I Converter
8
T1 D1
4
ZERO Stage
R5
7 14
13
5
IOUT RL GND Ground
R2
R1
Figure 6 illustrates a typical 3-wire circuit with a positive, differential input signal which can be used for calibrated sensing elements, for example. The offset current is set using the SET stage and the full scale via the gain at OP1.
Figure 6: 3-wire application for differential input signals According to Equations (9) and (10) the following applies to output current IOUT:
I OUT = VIN GI + I SET mit VZA = 0 8R0
(11) (12)
mit GI = GIA GGAIN = 5 1 +
V R4 R1 und I SET = REF R2 2 R0 R3 + R 4
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 15/21
AM452 - Voltage-to-current transducer IC with a differential input
Here, GI is the overall gain of the instrumentation amplifier (IA) and the back-end operational amplifier (OP1). ISET is the additional offset current which is set using a voltage at the SET pin and which can raise the output current of the VI/ converter by a constant value. 1) Example 1: VIN = 0...100mV differential, IOUT = 4...20mA (3-wire) For a measuring bridge with a signal of VIN = 0...100mV (without an offset) at the IA input the external components should be dimensioned in such a way that output current IOUT is 4...20mA. If the input signal offset is negligible, resistors R0, R1, R2, R3 and R4 must be determined. With the two voltage dividers it is sufficient to calculate just one of the two resistors; the other can be selected within the stipulations given by the boundary conditions. In this example a value of 5V has been selected for VREF, with 10k chosen for R2 and 5k for R4. With a current of 20mA the voltage should drop by a typical value of 540mV at resistor R0. The following is accrued: R0 0.02 A = 0.54V (13)
With reference to Equations (11) and (12) and the values given in Example 1 the following is obtained: 0.02 A = 0.1V 5 (1 + 8 R0 R1 ) 10k + ( 5V 5k 2 R0 ( R3 + 5k)
)
0.004 A =
5V 5k 2 R0 ( R3 + 5k)
By solving the above system of equations and taking the given defaults into account, the following values are computed for the 3-wire, 4-20mA current interface: R0 = 27 R3 = 110.74k RL = 0...600 R1 = 59.12k R4 = 5k C1 = 2.2F R2 = 10k R5 = 39
If the offset output current is not exactly 4mA due to component tolerances and deviates from this value, the voltage can be adjusted at pin 16 (SET) using voltage divider R3 and R4 (see Figure 7) and the output value thus corrected (c.f.: Offset compensation using a voltage divider at SET).
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 16/21
AM452 - Voltage-to-current transducer IC with a differential input
Typical 2-wire application with a differential input signal
2) Example 2: VIN = 0...100mV differential, IOUT = 4...20mA (2-wire)
In order to determine the system resistors R0 to R5 must first be determined. For a measuring bridge with a signal of VIN = 0...100mV (without an offset) at the IA input the external components in the AM452 circuitry should be dimensioned in such a way that the output current range is 4...20mA. AM452 is configured in such a way that the entire current, including the chip's quiescent current, flows through R0 (example for the 2-wire application). As in Example 1, R2 and R4 can be freely selected within the boundary conditions. In this example a value of 10k has been chosen for R2, with 5k selected for R4. VREF = 5V. The value of R0 has been set to 33k. Applying Equations (12) and (13) the values for R1 and R3 are as follows:
0.02 A = (0.1V 5 (1 + R1 ) 5 5k 10k + ( 8 33 2 33 ( R3 + 5k)
)
0.004 A =
5 5 k 2 33 ( R3 + 5k)
R3
15 16
C1
R4 VS
GSET SET Stage
AM452
2
1
12
OP2
Voltage Reference
C2
11 10
R SET
3
VBG
R0
IA
4 ZERO Stage 13 5
GVI V/I Converter OP1
9 8
T1 D1
2-wire connection
7
14
R2
R1 GND
IC ground: GND System ground: Ground
R5
IOUT
RL
}
different potentials!
Ground
Figure 7: Typical 2-wire application for differential input signals
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 17/21
AM452 - Voltage-to-current transducer IC with a differential input
By solving the above system of equations and taking the given defaults for the external components into account, the following values are computed: R0 = 33 R3 = 89.7k RL = 0...900 R1 = 74.48k R4 = 5k C1 = 2.2F R2 = 10k R5 = 39 C2 = 100nF
In the 2-wire application particular attention must be paid to the overall power consumption which may not exceed a value of 4mA across the entire temperature range.
Here it is also possible to correct the offset output current using the voltage divider at the SET stage input.
Offset compensation using a voltage divider at SET stage
The offset value of the output current can be adjusted at the SET pin (pin 16) via voltage divider R3 and R4 (see Figure 7). If, due to internal offsets and parasites, the output current is too great by 0.1mA, for example (4.1mA and 20.1mA), the current must be reduced by this amount, i.e. ISET may only be 3.9mA. In this example and if VREF = 5V Equation (9) yields the following:
I SET = VREF R4 5V 5k = 3,9mA = 2 R0 R3 + R4 66 R3 + 5k
Once R3 has been put through the equation R3 = 92.125k instead of 89.7k. The voltage at SET (pin 16) is then just 257.4mV instead of 264mV and the output current has been reduced by 0.1mA.
OP2 als Stromquelle verschaltet
VC VREF
1
OP2 als Spannungsreferenz verschaltet
IS
AM452
2
P
OP2
VBG
R6
2
AM452
OP2
VBG
1
RS
R7
Figure 8: OP2 as a constant current source
Figure 9: OP2 as a voltage reference
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 18/21
AM452 - Voltage-to-current transducer IC with a differential input
Using OP2 as a current source
The additional operational amplifier OP2 can easily be configured as a constant current source. Using the circuitry shown in Figure 8 the following equation is generated:
IS = V BG 1 .27 V = RS RS
(14)
The bridge symbol is supposed to represent the component to be supplied with current (such as a piezoresistive sensing element or a temperature sensor, for example).
Example: A supply current of IS = 1mA is to be set. Using Equation (14) the below value is accrued for external resistor RS, which determines the quantity of current:
RS = V BG 1 .27 V = = 1 .27 k 1mA IS
Using OP2 as a voltage source
In addition to the integrated voltage reference OP2 can also be used to supply voltage to external components such as A/D converters or microprocessors, for example. This permits lower supply voltages of 3.3V, for example, to be generated. The following is derived from the circuitry in Figure 9:
R V CVREF = V BG 1 + 6 = 1 .27 V R7 R 1 + 6 R7
(15)
Example: A voltage of VCVREF = 3.3V is to be set. Using Equation (15) the following ratio is
provided for external resistors R6 and R7:
R 6 V CVREF = - 1 2 .6 - 1 = 1 .6 R7 V BG
Example values of R7 = 10k and R6 = 16k are accrued for the resistors.
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 19/21
AM452 - Voltage-to-current transducer IC with a differential input
BLOCK DIAGRAM AND PINOUT
CVREF
CVSET 2
VSET VREF
1 12 15 16
SET
11
AM452
OP2
RS+ VCC RSIOUT
Voltage Reference
GS ET
V/I Converter
10 9 8
IN+
3 4
VBG
GVI
IA OP1
5 7 14
IN13
Figure 10: Simplified block
ZA
GAIN
VOP diagram
GND
Figure 10: Simplified block diagram
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
NAME CVREF CVSET IN+ IN- GAIN NC VOP IOUT RS- VCC RS+ VSET ZA GND VREF SET
EXPLANATION Current/Voltage reference Current/Voltage reference set Positive input Negative input Gain set Not connected OP1 output Current output Sensing resistor - Supply voltage Sensing resistor + Reference voltage source set Offset set IC ground Reference voltage source output Output offset current set
CVREF CVSET IN+ INGAIN NC VOP IOUT
1 2 3 4 5 6 7 8
16 15 14 13 12 11 10 9
SET VREF GND ZA VSET RS+ VCC RS-
Figure 11: Pinout
AM 452
Table 3: Pinout
Values which can be measured at the pins have indices; the pin name is written in capital letters.
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 20/21
AM452 - Voltage-to-current transducer IC with a differential input
DELIVERY
AM452 is available as an: * SO16(n)
PACKAGE DIMENSIONS
Please see the data sheet on our website: package.pdf
FURTHER READING
[1] The Frame ASIC concept: http://www.Frame-ASIC.de/ The following links refer to the Analog Microelectronics website: http://www.analogmicro.de/ [2] [3] Technical article: PR1012 - AM462 Voltage-to-current converter IC for 2-wire current loop applications Download: Kali_AM452.xls
NOTES
Analog Microelectronics reserves the right to make amendments to any dimensions, technical data or other information herein without further notice.
Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Analog Microelectronics GmbH An der Fahrt 13, D - 55124 Mainz February 2008 - Rev 1.2 - Page 21/21

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