AN-1206: Variable Gain Inverting Amplifier Using the AD5292 Digital Potentiometer and the OP184 Op Amp (Rev. C) PDF

AN-1206
APPLICATION NOTE
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Variable Gain Inverting Amplifier Using the AD5292 Digital Potentiometer
and the OP184 Op Amp
CIRCUIT FUNCTION AND BENEFITS
VDD
R3
1kΩ
This circuit provides a low cost, high voltage, variable gain
inverting amplifier using the AD5292 digital potentiometer
in conjunction with the OP184 operational amplifier.
+15V/+30V
V+
OP184
VOUT
V–
–15V/GND
R2
4.99kΩ ± 1%
C1
10pF
The circuit offers 1024 different gains, controllable through an
SPI-compatible serial digital interface. The ±1% resistor tolerance
performance of the AD5292 provides low gain error over the
full resistor range, as shown in Figure 2.
VSS
VDD
+15V/+30V
RAW
RAB
20kΩ
AD5292
The circuit supports input and output rail to rail for both single
supply operation at +30 V and dual supply operation at ±15 V;
and is capable of delivering up to ±6.5 mA output current.
VSS
–15V/GND
SERIAL
INTERFACE
08426-001
VIN
Figure 1. Variable Gain Inverting Amplifier (Simplified Schematic:
Decoupling and All Connections Not Shown)
In addition, the AD5292 has an internal 20-times programmable
memory that allows a customized gain setting at power-up. The
circuit provides accuracy, low noise, low THD, and is well suited
for signal instrumentation conditioning.
The circuit gain equation is
CIRCUIT DESCRIPTION
where D is the code loaded in the digital potentiometer.
Table 1. Devices Connected/Referenced
When the circuit input is an ac signal, the parasitic capacitances
of the digital potentiometer can cause undesirable oscillation in
the output. This can be avoided, however, by connecting a small
capacitor, C1, between the inverter input and its output. A value
of 10 pF was used for the gain and phase plots shown in Figure 3.
Product
AD5292
OP184
Description
10-bit, 1% resistor tolerance digital potentiometer
Rail-to-rail input and output, low noise, high slew
rate operational amplifier
This circuit employs the AD5292 digital potentiometer in conjunction with the OP184 operational amplifier, providing a low
cost variable gain noninverting amplifier.
G=−
G=−
G=
RAB
R
⇒ R2 = − AB
R2
G
(1)
1024
(3)
1024 – D
D
(4)
where D is the code loaded in the digital potentiometer. A gain
plot versus code is shown in Figure 5.
The circuit gain is defined in Equation 4.
The AD5292 has a 20-times programmable memory, which
allows presetting the output voltage in a specific value at power-up.
The maximum current through the AD5292 is ±3 mA, which
limits the maximum input voltage, VIN, based on the circuit
gain, as Equation 2 describes.
VIN ≤ 0.003 × R2
R2
A simple modification of the circuit provides a logarithmic
gain function, as shown in Figure 4. In this case, the digital
potentiometer is configured in the ratiometric mode.
The input signal VIN is amplified by the OP184. The op amp
offers low noise, high slew rate, and rail-to-rail input and output.
The maximum circuit gain is defined in Equation 1.
(1024 − D) × RAB
(2)
When the input signal connected to VIN is higher than the
theoretical maximum value from Equation 2, R2 should be
increased, and the new gain can be recalculated using Equation 1.
The ±1% internal resistor tolerance of the AD5292 ensures a
low gain error, as shown in Figure 2.
Excellent layout, grounding, and decoupling techniques must be
utilized in order to achieve the desired performance from the
circuits discussed in this note (see MT-031 Tutorial and
MT-101 Tutorial). As a minimum, a 4-layer PCB should be
used with one ground plane layer, one power plane layer,
and two signal layers.
Rev. C | Page 1 of 2
AN-1206
Application Note
10k
Excellent layout, grounding, and decoupling techniques must be
utilized in order to achieve the desired performance from the
circuits discussed in this note (see MT-031 Tutorial and MT-101
Tutorial). As a minimum, a 4-layer PCB should be used with one
ground plane layer, one power plane layer, and two signal layers.
GAIN
100
COMMON VARIATIONS
The AD5291 (8-bits with 20-times programmable power-up
memory) and the AD5293 (10-bits, no power-up memory) are
both ±1% tolerance digital potentiometers that are suitable for
this application.
3
2
2
0.01
0.0001
0
500
1000
CODE (Decimal)
08426-005
3
1
Figure 5. Logarithmic Gain Function
GAIN
ERROR (%)
LEARN MORE
1
0
0
–1
–1
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
ERROR (%)
GAIN
1
MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp, Analog
Devices.
GAIN
–2
–2
–3
–3
0
200
400
600
–4
1000
800
MT-091 Tutorial, Digital Potentiometers, Analog Devices.
08426-002
–4
MT-087 Tutorial, Voltage References, Analog Devices.
CODE (Decimal)
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Figure 2. Gain and Gain Error vs. Decimal Code
PHASE, RAW = 100Ω
AD5292 Evaluation Board.
225
200
–20
175
150
–30
125
100
GAIN, RAW = 100Ω
–40
AD5293 Data Sheet.
OP184 Data Sheet.
REVISION HISTORY
75
50
25
–50
–60
AD5292 Data Sheet.
275
250
600
1k
10k
100k
0
200k
FREQUENCY (Hz)
Figure 3. Gain and Phase vs Frequency for AC Input Signal
R3
1kΩ
Changes to Circuit Function and Benefits Section .......................1
VOUT
12/09—Rev. 0 to Rev. A
Corrected trademark.........................................................................1
VSS
8/09—Revision 0: Initial Version
SERIAL
INTERFACE
20kΩ
AD5292
D × RAB
1024
(1024 – D) × RAB
1024
08426-004
RAB
VIN
Changed Document Name from CN-0113 to
AN-1206 .............................................................................. Universal
4/10—Rev. A to Rev. B
VDD
OP184
C1
10pF
4/13—Rev. B to Rev. C
08426-003
GAIN (dBV)
GAIN, RAW = 10kΩ
GAIN, RAW = 20kΩ
325
300
PHASE (Degrees)
PHASE, RAW = 10kΩ
–10
AD5291 Data Sheet.
350
10
0
Data Sheets and Evaluation Boards
400
375
PHASE, RAW = 20kΩ
20
Figure 4. Logarithmic Gain Circuit
©2009–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN08432-0-4/13(C)
Rev. C | Page 2 of 2
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