AN-1218: Compact, Low Cost, 5 V, Variable Gain, Noninverting Amplifier Using the AD5270/AD5272 Digital Rheostat and AD8615 Op Amp (Rev. A) PDF

AN-1218
APPLICATION NOTE
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Compact, Low Cost, 5 V, Variable Gain, Noninverting Amplifier Using the
AD5270/AD5272 Digital Rheostat and AD8615 Op Amp
This circuit shown in Figure 1 provides a compact, low cost, low
voltage, variable gain noninverting amplifier using the AD5270/
AD5272 digital rheostat in conjunction with the AD8615
operational amplifier. The small package sizes of the AD5270/
AD5272 (10-lead 3 mm × 3 mm × 0.8 mm LFCSP) and the
AD8615 (5-lead TSOT-23), as well as their low cost, present an
industry leading solution to a common analog signal processing
circuit.
The circuit offers 1024 different gains, controllable through an
SPI (AD5270) or I2C-compatible (AD5272) serial digital interface.
The ±1% resistor tolerance performance of the AD5270/AD5272
provides low gain error over the full resistor range, as shown in
Figure 2.
The input signal, VIN, is amplified by the AD8615. The op amp
offers low noise, high slew rate, and rail-to-rail inputs and outputs.
The maximum circuit gain is defined in Equation 1.
G = 1+
RAW
R
→ R2 = AW
R2
G–1
VDD
+2.5V/+5V
R1
VIN
3.32kΩ ±1%
R2
4.99kQ ±1%
VOUT
–2.5V/GND
C1
VSS
10pF
+2.5V/+5V
VDD
In addition, the AD5270/AD5272 has an internal 50-times
programmable memory that allows a customized gain setting
at power-up.
RAW
20kΩ
AD5270/AD5272
VSS
–2.5V/GND
The circuit provides accuracy, low noise, and low THD and is
well suited for signal instrumentation conditioning.
CIRCUIT DESCRIPTION
Table 1. Devices Connected/Referenced
Description
10-bit, 1% resistor tolerance digital rheostat
Precision, 20 MHz, CMOS, rail-to-rail
input/output CMOS op amp
The circuit employs the AD5270/AD5272 digital rheostat in
conjunction with the AD8615 CMOS operational amplifier,
providing a low cost, compact, variable gain noninverting
amplifier.
V+
AD8615
AD8615
V–
The circuit supports rail-to-rail inputs and outputs for both singlesupply operation at +5 V and dual-supply operation at ±2.5 V
and is capable of delivering up to ±150 mA output current.
Product
AD5270/AD5272
AD8615
(1)
SERIAL
INTERFACE
09051-001
CIRCUIT FUNCTION AND BENEFITS
Figure 1. Variable Gain Noninverting Amplifier (Simplified Schematic:
Decoupling and All Connections Not Shown)
The maximum allowable current through the AD5270/AD5272
(RAW = 20 kΩ version) is ±3 mA, which limits the maximum
input voltage, VIN, based on the circuit gain as described in
Equation 2.
|VIN| ≤ 0.003 × R2
(2)
When the input signal, 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.
On the other hand, the minimum gain should be calculated to
reduce the error due to the leakage current in the AD5270/
AD5272. To assume a negligible leakage current error, the current
through R2 should be at least 100 times the worst-case leakage
specification of 50 nA . Therefore, the minimum current through
R2 should be 5 µA, which defines the minimum value for R2, as
in Equation 3.
|VIN| ≥ 5 × 10−6 × R2
Rev. A | Page 1 of 3
(3)
AN-1218
Application Note
90
15
10
PHASE, RAW = 100Ω
100k
5
GAIN (dBV)
RESISTANCE (Ω)
1k
R2 MAX
100
0
PHASE, RAW = 10kΩ
–5
GAIN, RAW = 20kΩ
–10
GAIN, RAW = 10kΩ
–90
–15
10
–180
GAIN, RAW = 100Ω
–20
1
R2 MIN
–25
600
0.1
PHASE (Degrees)
0
PHASE, RAW = 20kΩ
10k
1k
10k
100k
–270
200k
FREQUENCY (Hz)
1
10
100
Figure 4. Gain and Phase vs. Frequency for the AC Input Signal (Vertical Scale
Compressed to Show All Gain Curves)
09051-002
0.01
0.1
1k
INPUT VOLTAGE (mV)
The AD5270/AD5272 have a 50-times programmable memory,
which allows presetting the output voltage in a specific value at
power-up.
Figure 2. R2 Value Range vs. Minimum Input Signal
The ±1% internal resistor tolerance of the AD5270/AD5272
ensures a low gain error, as shown in Figure 3.
The circuit gain equation is
G =1+
(1024 – D ) × RAW 1024
(4)
R2
where D is the code loaded in the digital potentiometer.
GAIN
ERROR (%)
GAIN
6
COMMON VARIATIONS
0.6
The AD5271/AD5274 (8-bits with 50-times programmable
power-up memory) are both ±1% tolerance digital rheostats
that are suitable for this application if 10-bit resolution is not
required.
5
0.5
4
0.4
3
0.3
The same basic circuit shown in Figure 1 can be adapted to
operate on a 30 V supply using higher voltage devices as
described in the CN-0112 Circuit Note.
2
0.2
1
0.1
0
200
400
600
800
CODE (Decimal)
0
1023
09051-003
GAIN
0
Excellent layout, grounding, and decoupling techniques must be
used to achieve the desired performance from the circuits
discussed in this note (see Tutorial MT-031, Grounding Data
Converters and Solving the Mystery of “AGND” and “DGND”
and Tutorial MT-101, Decoupling Techniques). As a minimum,
a 4-layer PCB should be used with one ground plane layer, one
power plane layer, and two signal layers.
0.7
ERROR (%)
7
09051-004
Figure 2 shows the possible R2 value range based on the input
voltage to the op amp, assuming these conditions.
Figure 3. Gain and Gain Error vs. Decimal Code
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 4.
Rev. A | Page 2 of 3
Application Note
AN-1218
LEARN MORE
Data Sheets and Evaluation Boards
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Analog Devices.
AD5270 Data Sheet
MT-032 Tutorial, Ideal Voltage Feedback (VFB) Op Amp,
Analog Devices.
AD5270 Evaluation Board
MT-087 Tutorial, Voltage References, Analog Devices.
MT-091 Tutorial, Digital Potentiometers, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
AD5272 Data Sheet
AD5272 Evaluation Board
AD5271 Data Sheet
AD5274 Data Sheet
AD8615 Data Sheet
REVISION HISTORY
4/13—Rev. 0 to Rev. A
Document Title Changed from CN-0161 to AN-1218....... Universal
7/10—Revision 0: Initial Version
©2010-2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D09051-0-4/13(A)
Rev. A | Page 3 of 3
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