Circuit Note
CN-0122
Circuit Designs Using Analog Devices Products
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Devices Connected/Referenced
AD8271/AD8274 Difference Amplifier
ADA4627-1
High Speed JFET Input Operational
Amplifier
AD8599 Low Noise Operational Amplifier
High Speed Instrumentation Amplifier Using the AD8271 Difference Amplifier and
the ADA4627-1 JFET Input Op Amp
Rev.A
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CIRCUIT FUNCTION AND BENEFITS
A traditional method for building an instrumentation amplifier is
to use three op amps and seven resistors as shown in Figure 1.
This approach requires four precision matched resistors for a good
common-mode rejection ratio (CMRR). Errors in matching will
produce errors at the final output. An imbalance of one or two
picofarads on certain nodes will drastically degrade the high
frequency CMRR, a fact often overlooked.
This circuit uses a monolithic difference amplifier with laser
trimmed thin film resistors for the output amplifier, thereby
providing good dc and ac accuracy with fewer components
than the traditional approach.
CIRCUIT DESCRIPTION
This circuit utilizes the AD8271 difference amplifier and two
ADA4627-1 amplifiers, which have low noise, low drift, low
offset, and high speed. For high impedance sources, the
ADA4627-1 is an ideal choice for the input stage amplifiers due
to the extremely low input bias current of their JFET inputs.
The op amps selected for the input stage must also have low
offset voltage and low offset voltage drift with temperature. They
also need to have good drive characteristics. This allows the use
of low value resistors to minimize resistor thermal noise.
Headroom issues relating to the op amp must be considered in
this circuit for proper operation.
When working with any op amp having a gain-bandwidth
product greater than a few MHz, careful layout and bypassing
are essential. A typical decoupling network consists of a 1 µF
to 10 µF electrolytic capacitor in parallel with a 0.01 µF to
0.1 µF low inductance ceramic MLCC type.
For the lowest noise with low impedance sources only, low
voltage noise is important. The AD8599 has lower noise, lower
offset voltage drift, and lower supply current; but the input bias
currents are much higher, and the bandwidth will be lower than
that obtained with the ADA4627-1. The measured −3 dB points
–IN
+IN
10k
10k
10k
10k
ADA4627-1
ADA4627-1
R
G
20
R
F1
R
F2
2k
AD8271
OUT
2k
V
S
= ±15V
08517-001
+V
S
+V
S
–V
S
+V
S
–V
S
–V
S
NOTES
1. 10kΩ THIN FILM TRIMMED RESISTOR
ARE INTERNAL TO THE AD8271.
Figure 1. In Amp with Gain = 201
(Simplified Schematic: Decoupling and All Connections Not Shown)
are 56.6 kHz and 87.6 kHz for the AD8599 and ADA4627-1,
respectively. (See Figure 2).
With high impedance sources, the input bias current and the
input noise current of a bipolar op amp can result in errors. The
bias current creates an I × R drop, which will be multiplied by
the overall circuit gain. This can result in several volts of offset
at the output. The input noise current is also multiplied by the
source impedances, creating an additional noise voltage. To
avoid this, a JFET input op amp, such as the ADA4627-1,
should be used. Even though the voltage noise is slightly higher
than the AD8599, the current noise is significantly lower,
resulting in lower overall noise when used with high impedance
sources.
As Figure 3 and Figure 4 show, the AD8599 is the proper choice
with low source impedances, and the ADA4627-1 is better with
higher source impedances. There is a trade-off: the input
capacitance of JFET op amps is higher than bipolar op amps, so
the RC time constant must be considered.
CN-0122 Circuit Note
Rev. A | Page 2 of 3
50
20
25
30
35
40
45
100 1k 10k 100k 1M
GAIN (dB)
FREQUENCY (Hz)
08517-002
ADA4627-1
AD8599
Figure 2. Bandwidth of Circuit Shown in Figure 1 Comparing the ADA4627-1
to the AD8599 as the Input Stage.
10
0.1
1
0.02
0.1 1 10 100 200
RTO NOISE (µV/√Hz)
FREQUENCY (kHz)
08517-003
ADA4627-1
R
SOURCE
= 0
AD8599
Figure 3. Noise Spectral Density (RTO) of Circuit Shown in Figure 1
Comparing the ADA4627-1 to the AD8599 as the Input Stage:
Low Impedance Source (0 Ω)
10k
1k
100
10
1
0.02 0.1 1 10 100 200
RTO NOISE (µV/√Hz)
FREQUENCY (kHz)
08517-004
AD8599
ADA4627-1
R
SOURCE
= 66k
Figure 4. Noise Spectral Density (RTO) of Circuit Shown in Figure 1
Comparing the ADA4627-1 to the AD8599 as the Input Stage:
High Impedance Source (66 kΩ)
COMMON VARIATIONS
The AD8271 or AD8274 can be used with a variety of op amps
to optimize the overall performance with respect to supply
current, signal bandwidth, temperature drift, and noise.
For the lowest possible drift over temperature, one of the auto-
zero amplifiers, such as the AD8539, can be used, but the band-
width will be reduced and wideband noise increased. This would
be an excellent choice for bandwidths less than 10 Hz, however.
When selecting op amp and difference amplifier combinations
for this circuit, always ensure that the input common-mode
voltage range of each amplifier is not violated. This is com-
monly overlooked but is the subject of a fair number of
application questions.
If the first stage gain is greater than about five, consider using a
decompensated op amp, such as the OP37, to get a higher slew rate
and signal bandwidth with less supply current. To avoid common-
mode oscillation, the circuit must be modified slightly as described
in "Phase Compensation of the Three Op Amp Instrumentation
Amplifier." White, D. Rod. IEEE Transactions on Instrumentation
and Measurement. Vol. IM-36, No. 3, Sept. 1987.
With microvolt-level input signals and a gain of 1000, the first
stage can be operated on ±2.5 V, saving power and giving more
choices of op amps, such as the AD8539 auto-zero amplifier.
However, if the input common-mode voltage range is high, an op
amp with a higher supply voltage must be chosen for the first stage.
LEARN MORE
Jung, Walter G. 2005. Op Amp Applications Handbook.
Elsevier/Newnes. 2005. ISBN 0-7506-7844-5. Chapter 2.
Kitchin, Charles and Lew Counts. 2006. A Designer’s Guide to
Instrumentation Amplifiers, 3rd Edition. Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
MT-055 Tutorial, Chopper Stabilized (Auto-Zero) Precision
Op Amps. Analog Devices.
MT-061 Tutorial, Instrumentation Amplifiers (In-Amp) Basics.
Analog Devices.
MT-063 Tutorial, Basic Three Op Amp In-Amp Configuration.
Analog Devices.
MT-064 Tutorial, In-Amp DC Error Sources. Analog Devices.
MT-065 Tutorial, In-Amp Noise. Analog Devices.
MT-068 Tutorial, Difference Amplifiers. Analog Devices.
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
White, D. Rod, "Phase Compensation of the Three Op Amp
Instrumentation Amplifier." IEEE Transactions on
Instrumentation and Measurement. Vol IM-36, No. 3,
Sept. 1987.