Many industrial and medical applications use instrumentation
amplifiers to condition small signals in the presence
of large common-mode voltages. INAs have complete
closed-loop operational amplifiers with feedback components
included. Under normal conditions, an INA is easy
to use, as long as you pay attention to the input stage and
the output of the first stage. The quick and inexpensive solutions described
here can help you avoid the INA circuit’s input-stage pitfalls.
In a classic, three-op-amp INA
(Figure 1), the input stage has two op
amps in an adjustable-gain configuration
and provides high input impedance on
both the inverting (VIN−) and noninverting
(VIN+) inputs. The output stage has
four matched resistors around a single
op amp. When the circuit designer exercises
proper precautions with the input
pins, this configuration rejects external
common-mode voltage and noise. The
output stage has a reference voltage that
level-shifts the output with respect to
ground. The level shift is convenient in
single-supply applications. As with many
INAs, you can program the gain found in
Figure 1 with a single resistor, RG.
Depending on the INA’s silicon process,
VIN+ and VIN− connect to a bipolar
transistor base, FET gate, or CMOS gate.
All inputs to the INA require a current-return
path to ground and a bias-voltage
reference. Without the current-return
path and the bias-voltage reference, the
INA input stage saturates or floats to an
undesirable voltage. Either condition creates an invalid output voltage.
The floating thermocouple circuit
in Figure 2a does not provide a
current path to ground or a bias-voltage
reference for the INA’s input pins. Thus,
the input-current leakages are not dissipated,
and the two inputs can float
to any undefined voltage. That situation,
in turn, causes the INA’s output
to change to an invalid output voltage,
usually in the middle of the INA’s output
range. The invalid voltage can appear
to be legitimate, making it difficult to
detect the correctness of the INA input
implementation. Figure 2b illustrates
a correct thermocouple connection to
an INA; both inputs have a path to
ground—in this case, through a 10-kΩ
resistor, biased to a voltage within the
INA’s input range, or ground.
Circuit designers often misapply a
thermocouple or even a two-wire microphone
to the INA’s input circuit; the
problem arises when the INA inputs are
connected without proper consideration
to current paths or biasing. Heed those
considerations for the INA’s input stage,
and you can be confident that the INA’s
output voltage is representative of the
But wait—there may be a problem
with the output values of A1 and A2
(Figure 1). How would you solve this
problem? Comment below. I’d really like to hear
Kugelstadt, Thomas, “Getting themost out of your instrumentationamplifier design,” Analog Applications
Journal, Texas Instruments, 4Q 2005.
- Trump, Bruce, “Instrumentation amplifiers—avoiding a common pitfall,”
TI E2E Community, March 27, 2012.
- Trump, Bruce, “Instrumentation amplifiers—they’re not op amps butwhat are they?” TI E2E Community,
April 2, 2012.
Bonnie Baker is a senior applications engineer
at Texas Instruments.