AD8271
Rev. 0 | Page 18 of 20
Many signal gains have more than one configuration choice, which
allows freedom in choosing the op amp closed-loop gain. In
general, for designs that need to be stable with a large capacitive
load on the output, choose a configuration with high loop gain.
Otherwise, choose a configuration with low loop gain, because
these configurations typically have lower noise, lower offset,
and higher bandwidth.
The AD8271 Specifications section and Typical Performance
Characteristics section show the performance of the part primarily
when it is in the difference amplifier configuration. To estimate
the performance of the part in a single-ended configuration, refer
to the difference amplifier configuration with the corresponding
closed-loop gain (see Table 10).
Table 10. Closed-Loop Gain of the Difference Amplifiers
Difference Amplifier Gain Closed-Loop Gain
0.5 1.5
1 2
2 3
Gain of 1 Configuration
The AD8271 is designed to be stable for loop gains of 1.5 and
greater. Because a typical voltage follower configuration has
a loop gain of 1, it may be unstable. Several stable configurations
for gain of 1 are listed in Table 9.
KELVIN MEASUREMENT
In the case where the output load is located remotely or at
a distance from the AD8271, as shown in Figure 51, wire
resistance can actually cause significant errors at the load.
07363-149
–IN
10kΩ
10k
10kΩ
10kΩ
+IN
R
W
(WIRE RESISTANCE)
R
L
1kΩ
Figure 51. Wire Resistance Causes Errors at Load Voltage
Since the output of the AD8271 is not internally tied to any of
the feedback resistors, Kelvin type measurements are possible
because the op amp output and feedback can both be connected
closer to the load (Figure 52). The Kelvin sensing on the feedback
minimizes error at the load caused by voltage drops across the
wire resistance. This technique is most effective in reducing errors
for loads less than 10 k. As the load resistance increases, the
error due to the wire resistance becomes less significant.
Because it adds the sense wire resistance to the feedback resistor, a
trade-off of the Kelvin connection is that it can degrade common-
mode rejection, especially over temperature. For sense wire
resistance less than 1 , it is typically not an issue. If common-
mode performance is critical, two amplifier stages can be used:
the first stage removes common-mode interference, and the
second stage performs the Kelvin drive.
10kΩ R
w
R
w
SENSE
FORCE
07363-150
–IN
10kΩ
10kΩ
10kΩ
+IN
R
L
1kΩ
Figure 52. Connecting Both the Output and Feedback at the Load Minimizes
Error Due to Wire Resistance
INSTRUMENTATION AMPLIFIER
The AD8271 can be used as a building block for high performance
instrumentation amplifiers. For example, Figure 53 shows how
to build an ultralow noise instrumentation amplifier using the
AD8599 dual op amp. External resistors R
G
and R
Fx
provide gain;
therefore, the output is
()
()
8271
2
1
AD
G
Fx
ININ
OUT
G
R
R
VVV
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+−=
−+
–IN
+IN
10kΩ
10kΩ
10kΩ
10kΩ
REF
AD8599
A2
D8599
A2
R
G
20Ω
R
F1
R
F2
2kΩ
AD8271
OUT
2kΩ
V
S
= ±15V
07363-153
Figure 53.Ultralow Noise Instrumentation Amplifier Using AD8599
Configured for Gain = 201
For optimal noise performance, it is desirable to have a high
gain at the input stage using low value gain-setting resistors, as
shown in this particular example. With less than 2 nV/√Hz
input-referred noise (see Figure 54) at ~10 mA supply current,
the AD8271 and AD8599 combination offers an in-amp with a
fine balance of critical specifications: a gain bandwidth product
of 10 MHz, low bias current, low offset drift, high CMRR, and
high slew rate.
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
1 10 100 1k 10k 100k
VOLTAGE NOISE SPECTRAL DENSITY (nV/√Hz)
FREQUENCY (Hz)
07363-151
G = 201
BANDWIDTH
LIMIT
Figure 54. Ultralow Noise In-Amp Voltage Noise Spectral Density vs.
Frequency, Referred to Input
