Download CIRCUIT FUNCTION AND BENEFITS

CIRCUIT FUNCTION AND BENEFITS
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.
+VS
–IN
ADA4627-1
+VS
AD8271
–VS
RF1
2kΩ
RG
20Ω
10kΩ
10kΩ
OUT
10kΩ
RF2
+VS
2kΩ
10kΩ
–VS
+IN
ADA4627-1
VS = ±15V
–VS
NOTES
1. 10kΩ THIN FILM TRIMMED RESISTOR
ARE INTERNAL TO THE AD8271.
08517-001
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.
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).
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.
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.
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
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.
Headroom issues relating to the op amp must be considered in
this circuit for proper operation.
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50
COMMON VARIATIONS
45
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.
ADA4627-1
GAIN (dB)
40
For the lowest possible drift over temperature, one of the autozero amplifiers, such as the AD8539, can be used, but the bandwidth will be reduced and wideband noise increased. This would
be an excellent choice for bandwidths less than 10 Hz, however.
35
AD8599
30
20
100
1k
10k
100k
1M
08517-002
25
FREQUENCY (Hz)
Figure 2. Bandwidth of Circuit Shown in Figure 1 Comparing the ADA4627-1
to the AD8599 as the Input Stage.
ADA4627-1
1
AD8599
RSOURCE = 0Ω
0.1
0.02
0.1
1
10
100 200
FREQUENCY (kHz)
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)
Jung, Walter G. 2005. Op Amp Applications Handbook.
Elsevier/Newnes. 2005. ISBN 0-7506-7844-5. Chapter 2.
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.
1k
MT-061 Tutorial, Instrumentation Amplifiers (In-Amp) Basics.
Analog Devices.
AD8599
100
MT-063 Tutorial, Basic Three Op Amp In-Amp Configuration.
Analog Devices.
ADA4627-1
MT-064 Tutorial, In-Amp DC Error Sources. Analog Devices.
RSOURCE = 66kΩ
10
MT-065 Tutorial, In-Amp Noise. Analog Devices.
0.1
1
10
100 200
FREQUENCY (kHz)
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)
08517-004
RTO NOISE (µV/√Hz)
LEARN MORE
Kitchin, Charles and Lew Counts. 2006. A Designer’s Guide to
Instrumentation Amplifiers, 3rd Edition. Analog Devices.
10k
1
0.02
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 commonmode 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.
08517-003
RTO NOISE (µV/√Hz)
10
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 commonly overlooked but is the subject of a fair number of
application questions.
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.
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Data Sheets
AD8271 Data Sheet
AD8274 Data Sheet
AD8539 Data Sheet
AD8599 Data Sheet
ADA4627-1 Data Sheet
OP37 Data Sheet
REVISION HISTORY
2/10—Rev. 0 to Rev. A
Changes to Common Variations Section ....................................... 2
Changes to Learn More Section ...................................................... 2
10/09—Revision 0: Initial Version
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