AME AM452

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® carrier IC.
V CC = 6…35V
Differential input
± 400mV
AM452
VREF = 5/10V
IOUT = z.B.0/4...20mA
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® protocol communications
Modular signal conditioning with digital correction (Frame concept [1])
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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
1
FEATURES
3
SCHEMATIC
3
GENERAL DESCRIPTION
3
ELECTRICAL SPECIFICATIONS
4
BOUNDARY CONDITIONS
7
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
8
8
8
8
8
8
8
8
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
8
8
8
8
8
OPERATING AM452: IMPORTANT POINTS TO NOTE
8
DIMENSIONING
8
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
8
8
8
8
8
8
BLOCK DIAGRAM AND PINOUT
8
DELIVERY
8
PACKAGE DIMENSIONS
8
FURTHER READING
8
NOTES
8
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Analog Microelectronics GmbH
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February 2008 - Rev 1.2 - Page 2/21
AM452 – Voltage-to-current transducer IC
with a differential input
FEATURES
GENERAL DESCRIPTION
• 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® 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 -40°C...+85°C
• RoHS compliant
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
OP2
Input
12
15
16
OP2
Voltage Reference
10
VBG
Differential
input
voltages
3
11
SET Stage
2
9
V/I Converter
8
IA
AM452
ZERO Stage
13
Output
OP1
4
Offset
VCC
5
7
Amplifier
14
GND
Figure 1: Block diagram of AM452
Figure 1: Schematic of AM452
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February 2008 - Rev 1.2 - Page 3/21
AM452 – Voltage-to-current transducer IC
with a differential input
ELECTRICAL SPECIFICATIONS
Tamb = 25°C, VCC = 24V, VREF = 5V, IREF = 1mA (unless otherwise stated)
Parameter
Symbol
Conditions
Supply Voltage Range
VCC
VSET not connected
Quiescent Current
ICC
Tamb = – 40...+85°C, IREF = 0mA
Min.
Typ.
6
Max.
Unit
35
V
1.5
mA
Temperature Specifications
Operating
Tamb
–40
85
°C
Storage
Tst
–55
125
°C
Junction
TJ
150
°C
Voltage Reference
Voltage
VREF
VSET not connected
4.75
5.00
5.25
V
VREF
VSET = GND, VCC ≥ 11V
9.5
10.0
10.5
V
Current
IREF *
10.0
mA
VREF vs. Temperature
dVREF/dT
Tamb = - 40...+85°C
±90
±140
ppm/°C
Line Regulation
dVREF/dV
VCC = 6V...35V
30
80
ppm/V
dVREF/dV
VCC = 6V...35V, IREF ≈ 5mA
ppm/V
Load Regulation
60
150
0.05
0.10
%/mA
0.06
0.15
%/mA
1.9
2.2
5.0
µF
1.20
1.27
1.35
V
±60
±140
ppm/°C
dVREF/dI
dVREF/dI
Load Capacitance
0
IREF ≈ 5mA
CL
Current/Voltage Source OP2
Internal Reference
VBG
VBG vs. Temperature
dVBG/dT
Tamb = - 40...+85°C
Current Source: ICV = VBG/REXT
Adjustable Current Range*
ICV *
0
10
mA
Output Voltage
VCV
VCC < 19V
VBG
VCC – 4
V
VCV
VCC ≥ 19V
VBG
15
V
Voltage Source: VCV = VBG (REXT1 + REXT2) / REXT2
Adjustable Voltage Range
Output Current
Load Capacitance
VCV
VCC < 19V
0.4
VCC – 4
V
VCV
VCC ≥ 19V
0.4
15
V
ICV *
Source
ICV
Sink
CL
Source mode
0
1
10
mA
–100
µA
10
nF
* In 2-wire operation IS has to fulfill the condition ICC +IS < IOUTmin with IOUTmin = 4mA
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Analog Microelectronics GmbH
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February 2008 - Rev 1.2 - Page 4/21
AM452 – Voltage-to-current transducer IC
with a differential input
Parameter
Symbol
Conditions
Min.
Typ.
Max.
5
5.1
Unit
Instrumentation Amplifier (cont.)
Internal Gain
GIA
4.9
Differential Input Voltage Range
VIN
0
±400
Common Mode Input Range
CMIR
VCC < 9V, ICV < 2mA
1.5
VCC – 3
V
CMIR
VCC ≥ 9V, ICV < 2mA
1.5
6.0
V
Common Mode Rejection Ratio
CMRR
80
90
Power Supply Rejection Ratio
PSRR
80
90
Offset Voltage
VOS
-9.0
-1.5
VOS vs. Temperature
dVOS/dT
mV
dB
dB
+6.0
mV
µV/°C
±5
Input Bias Current
IB
–100
–250
nA
IB vs. Temperature
dIB/dT
–0.4
–0.9
nA/°C
Output Voltage
VOUTIA
VCC < 9V
VCC – 4
V
VOUTIA
VCC ≥ 9V
5
V
16
mV
250
pF
Minimum Output Voltage
VOUTIAmin
Load Capacitance
CL
4.5
Zero Adjust Stage
Internal Gain
GZA
Zero Adjust Voltage
VZA
Offset Voltage
VOS
VOS vs. Temperature
dVOS/dT
Input Bias Current
IB
IB vs. Temperature
dIB/dT
0.94
1
1.06
VZA ≤ VOUTIAmax – GIA ∆VIN ; Vcc<9V,
∆VIN=400mV, GIA=5
0
Vcc-6
V
VZA ≤ VOUTIAmax – GIA ∆VIN; Vcc≥9V,
∆VIN =400mV, GIA=5
0
3
V
±2.0
mV
±1.6
±5
µV/°C
47
120
nA
18
30
pA/°C
VCC – 5
V
±0.5
Operational Amplifier – Gain Stage (OP1)
Adjustable Gain
GGAIN
Input Range
IR
VCC < 10V
IR
VCC ≥ 10V
Power Supply Rejection Ratio
1
0
0
5
V
PSRR
80
90
Offset Voltage
VOS
-3.0
-1.0
1.0
mV
VOS vs. Temperature
dVOS/dT
±3
±7
µV/°C
Input Bias Current
IB
10
25
nA
IB vs. Temperature
dIB/dT
7
20
pA/°C
Output Voltage Limitation
VLIM
Output Voltage Range
VOP
VCC < 10V
0
VCC – 5
V
VOP
VCC ≥ 10V
0
VREF
V
250
pF
Load Capacitance
dB
VREF
CL
V
NB: The current in the IC is given as a negative quantity.
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February 2008 - Rev 1.2 - Page 5/21
AM452 – Voltage-to-current transducer IC
with a differential input
Parameter
Symbol
Conditions
Min.
Typ.
Max.
0.122
0.125
0.128
0.60
1.00
1.40
320
Unit
V/I Converter
Internal Gain
GVI
Trim Range
Adjustable by R0
Voltage Range at R0 FS
VR0FS
540
760
mV
Offset Voltage
VOS
βF ≥ 100
±2
±4
mV
VOS vs. Temperature
dVOS/dT
βF ≥ 100
±7
±14
µV/°C
Input Resistance
RIN
120
160
RIN vs. Temperature
dRIN/dT
0.2
0.3
Output Offset Current
IOUTOS
3-wire operation
–25
–35
µA
IOUTOS vs. Temperature
dIOUTOS/dT
3-wire operation
16
26
nA/°C
Output Offset Current
IOUTOS
2-wire operation
9.5
14
µA
IOUTOS vs. Temperature
dIOUTOS/dT
2-wire operation
6
8
nA/°C
Output Control Current
IOUTC
2-wire operation, VR0/100mV
6
8
µA
IOUTC vs. Temperature
dIOUTC/dT
2-wire operation
–10
–15
nA/°C
Output Voltage Range
VOUT
VOUT = RL IOUT, VCC < 18V
0
VCC – 6
V
VOUT
VOUT = RL IOUT, VCC ≥ 18V
0
12
V
Output Current Range FS
IOUTFS
IOUT = VR0/R0, 3-wire operation
Output Resistance
ROUT
Load Capacitance
CL
kΩ
kΩ/°C
20
0.5
mA
1.0
0
MΩ
500
nF
1.15
V
SET Stage
Internal Gain
GSET
Input Voltage
VSET
0
0.5
Offset Voltage
VOS
-4.0
VOS vs. Temperature
dVOS/dT
Input Bias Current
IB vs. Temperature
-1.0
+2.0
mV
±1.6
±5
µV/°C
IB
8
20
nA
dIB/dT
7
18
pA/°C
690
mV
Ground vs. VS vs. VOUT
35
V
Ground vs. VS vs. IOUT
35
Protective Functions
Voltage Limitation at R0
VLIMR0
VR0 = VIN GI, SET = GND
VLIMR0
VIN = 0, VR0 = VSET/2
Protection against reverse polarity
Current in event of reverse polarity
VREF/8
580
635
Ground = 35V, VS = IOUT = 0
4.5
Ideal input
0.05
mV
V
mA
System Parameters
Nonlinearity
0.15
%FS
3-dB-frequency
f3db
RL = 600Ω, C2 = 1nF
5
kHz
Statistical output impedance
Rstat.
RL =600Ω, C2 = 1nF,
4·103
MΩ
Dynamical output impedance
Rdyn.
For f= 2,2kHz, RL =600Ω,
3·103
Ω
C2 = 1nF,
Table 1: Specifications
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February 2008 - Rev 1.2 - Page 6/21
AM452 – Voltage-to-current transducer IC
with a differential input
BOUNDARY CONDITIONS
Parameter
Symbol
Sense Resistor
Stabilization Resistor
Load Resistance
Sum Gain Resistors
Conditions
R0
IOUTFS = 20mA
R0
c = 20mA/IOUTFS
R5
IOUTFS = 20mA
R5
c = 20mA/IOUTFS
RL
Only for 3-wire operation
R1 + R2
Min.
Typ.
Max.
Unit
16
27
38
Ω
c ⋅ 16
c ⋅ 27
c ⋅ 38
Ω
35
40
45
Ω
c ⋅ 35
c ⋅ 40
c ⋅ 45
Ω
0
600
Ω
20
200
kΩ
Sum Offset Resistors
R3 + R4
20
200
kΩ
Sum IA Offset Resistor
R6 + R7
20
200
kΩ
VREF Capacitance
C1
Min. value for Tamb 85°C
1.9
2.2
5.0
µF
Output Capacitance
C2
Only for 2-wire operation
90
100
250
nF
D1 Breakdown Voltage
VBR
35
50
T1 Forward Current Gain
βF
50
150
e.g. BCX54/55/56
V
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 τ = RL C2.
VC VREF
1
AM452
VC VSET
VSET
VREF
C1
R3
15
12
R4
GSET SET Stage
2
VIN-
11
Voltage Reference
OP2
VIN+
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.
VS
16
R0
10
VBG
GS ET
9
GVI V/I Converter
3
IA
8
GV I
OP1
4
T1
D1
ZERO Stage
VR EF
VZA
R6
13
7
5
14
GND
R7
R2
R1
VOP
R5
IOUT
Ground
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:
R

VOP = VOUTIA ⋅ GGAIN with GGAIN =  1 + 1
 R2

(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
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February 2008 - Rev 1.2 - Page 8/21
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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Analog Microelectronics GmbH
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February 2008 - Rev 1.2 - Page 9/21
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
2-wire system
signal source and
conditioning IC
GND ≠ Ground
VCC ≠ VS
(6)
VCC
IOUT
GND
RL
VS
3-wire system
signal source and
conditioning IC
IOUT
RL
GND = Ground
VCC = VS
Ground
VCC = VS
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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February 2008 - Rev 1.2 - Page 10/21
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
SET Stage
C2
11
9
V/I Converter
9
8
R0
10
R0
10
V/I Converter
11
SET Stage
T1
8
D1
T1
R5
D1
2-wire
connection
14
14
R5
GND
VS
GND
IOUT
IOUT
RL
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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February 2008 - Rev 1.2 - Page 11/21
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 = 600Ω due to the condition:
VOUT max = 12V @ VCC ≥ 18V.
Equation 7 is also valid for the 2-wire version; here, however, the RLmax = 600Ω limitation
does not apply. Here, load resistor RLmax = 900Ω when VS = 24V.
RL [Ω]
RL ≤
VCCmin = 6V
RLmax = 600Ω
VS − VCCmin
IOUTmax
IOUTmax = 20mA
600
300
Operating range
0
0
6
12
18
24
35
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).
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 600Ω 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.
C1
1
AM452
15
12
VS
16
11
G SET SET Stage
2
OP2
R0
10
Voltage Reference
9
VBG
RSET
3-wire-connection
R4
R3
GVI
V/I Converter
IA
D1
OP1
4
T1
8
3
R5
ZERO Stage
13
7
5
R2
IOUT
14
RL
R1
GND
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.
Ground
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

mit GI = GIA GGAIN = 5 1 +

(11)
V
R4
R1 
 und I SET = REF ⋅
R2 
2 R0 R3 + R 4
(12)
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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 10kΩ chosen for R2 and 5kΩ 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 +
0.004 A =
8 ⋅ R0
R1
)
10kΩ + (
5V
5kΩ
⋅
2 ⋅ R0 ( R3 + 5kΩ)
)
5V
5kΩ
⋅
2 ⋅ R0 ( R3 + 5kΩ)
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 = 27Ω
R3 = 110.74kΩ
RL = 0...600Ω
R1 = 59.12kΩ
R4 = 5kΩ
C1 = 2.2µF
R2 = 10kΩ
R5 = 39Ω
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).
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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 10kΩ has been chosen for R2, with 5kΩ selected for R4. VREF = 5V. The
value of R0 has been set to 33kΩ. Applying Equations (12) and (13) the values for R1 and R3
are as follows:
0.02 A =
R1
)
5
5kΩ
10kΩ + (
⋅
8 ⋅ 33Ω
2 ⋅ 33Ω ( R3 + 5kΩ)
(0.1V ⋅ 5 ⋅ (1 +
5
5 kΩ
⋅
2 ⋅ 33Ω ( R3 + 5kΩ)
0.004 A =
C1
AM452
1
16
VS
2
R SET
R4
R3
15
12
GSET SET Stage
OP2
)
C2
Voltage Reference
11
VBG
R0
10
3
9
GVI V/I Converter
IA
OP1
4
T1
8
D1
ZERO Stage
13
7
5
R2
2-wire
connection
14
R1
R5
IOUT
GND
IC ground: GND
System ground: Ground
}
different
potentials!
RL
Ground
Figure 7: Typical 2-wire application for differential input signals
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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 = 33Ω
R3 = 89.7kΩ
RL = 0...900Ω
R1 = 74.48kΩ
R4 = 5kΩ
C1 = 2.2µF
R2 = 10kΩ
R5 = 39Ω
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
5kΩ
⋅
⋅
= 3,9mA =
2 R0 R3 + R4
66Ω R3 + 5kΩ
Once R3 has been put through the equation R3 = 92.125kΩ instead of 89.7kΩ. 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
OP2 als Spannungsreferenz
verschaltet
VC VREF
IS
AM452
µP
2
RS
1
OP2
VBG
Figure 8: OP2 as a constant current source
AM452
R6
2
R7
1
OP2
VBG
Figure 9: OP2 as a voltage reference
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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 Ω
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 = 10kΩ and R6 = 16kΩ are accrued for the resistors.
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AM452 – Voltage-to-current transducer IC
with a differential input
BLOCK DIAGRAM AND PINOUT
CVREF
AM452
CVSET
VSET VREF
1
SET
16
15
12
GS ET
2
OP2
Voltage Reference
V/I Converter
VBG
IN+
GVI
3
IA
11
RS+
10
VCC
9
RS-
8
IOUT
OP1
4
IN13
ZA
7
5
GAIN
Figure 10: Simplified block
VOP
diagram
14
GND
Figure 10: Simplified block diagram
NAME
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
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
AM 452
PIN
16
15
14
13
12
11
10
9
SET
VREF
GND
ZA
VSET
RS+
VCC
RS-
Figure 11: Pinout
Table 3: Pinout
Values which can be measured at the pins have indices; the pin name is written in capital
letters.
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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]
Technical article: PR1012 – AM462 Voltage-to-current converter IC for 2-wire current
loop applications
[3]
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.
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February 2008 - Rev 1.2 - Page 21/21