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Pete Semig

Feb 28, 2014

INA826

INA322

Low
Supply Voltage Amplifier (<=2.7V)

INA321

fully
differential amplifier

Low
Input Bias Current Amp (<=10pA)

INA331

Difference
Amplifier

Low
Offset Voltage Amplifier (<=500uV)

INA332

General
Purpose Amplifier

Zero
Drift Precision Amplifier

Low
Power Amp (<=500uA)

Precision
Amplifier

Low
Noise Amp (<=10nV/rtHz)

High
Supply Voltage Amplifier (>=30V)

High
Output Current Amplifier (>=50mA)

instrumentation
amplifier

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Related

Why doesn’t my INA CMRR change with gain?

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My esteemed colleague, John Caldwell (not to be confused with ‘John’ the intern), authored a couple excellent blogs
about instrumentation amplifier (INA) power supply and
common-mode rejection ratio (PSRR & CMRR). (See “Dealing with rejection:
Instrumentation amplifier PSRR and CMRR,” Part I and Part II)

He correctly noted that the CMRR & PSRR performance of most INA devices
changes with gain. But what about the handful of INAs whose CMRR does not change
with gain?

Figure 1 shows the CMRR vs. Frequency datasheet plot for the low-power,
single-supply INA331, which exhibits this behavior.

Figure 1: INA331 CMRR vs. Frequency

Although CMRR is measured according to Equation 1 below, Equation 2 depicts
the academic definition of CMRR, where Adm is the differential gain
and Acm is the common-mode gain.

Differential amplifiers, including INAs, attempt to reject common-mode
signals and amplify the differential signal. Therefore, according to Equation 2,
increasing Adm will increase CMRR. The 36-V, low-power, rail-to-rail
output (RRO) INA826 depicts this behavior, as shown in Figure 2.

Figure 2: INA826 CMRR

So, why do the INA331 and INA826 behave differently when you change the device’s
gain?

Figure 3 shows simplified schematics of the devices. They both have three op
amps, differential inputs and single-ended outputs, and the gain is set using
external resistor(s).

Figure 3: INA826 (left) vs. INA331 (right)

Even the gain equations, shown by Equations 3 and 4, look similar.

Examining the input and output
stages of both devices will lead us to the answer.

Figure 4 shows the input (A1-A2) and output (A3) stages of the INA826. RG determines the overall gain of the
device, because the gain of the output stage is fixed due to the integrated
resistors. Since the input stage is differential, RG controls the
differential gain of the device. Therefore, as the device’s gain changes, so
does the CMRR performance.

Figure 4: INA826 simplified schematic

Figure 5 shows the output stage (A3) of the
INA331.
R1 and R2 are external resistors that set the
device’s gain according to Equation 4. Notice the output stage is
single-ended.

Figure 5: INA331 output stage, external resistors

Figure 6 depicts a redrawn version of the INA331 input stage (A1-A2). Equation 5 is the transfer
function for the circuit in Figure 6. The input stage of the INA331 performs three functions. It converts inputs from
differential to single-ended signals, applies differential gain and adds the
reference voltage.

All of the resistors in this stage are internal
to the device. Therefore, the differential gain of the INA331 is fixed by the ratio of Rf to
Rg!

Figure 6: INA331 input stage, all resistors internal

In summary, even though R1 and R2 change the gain of
the INA331, they do not change the differential gain. The
integrated resistors of the input stage fix the differential gain. This is why
the CMRR of the device does not change with gain!

Other INAs that exhibit the same behavior are the single-supply, low-power,
CMOS INA321, INA322 and INA332.

So, next time you’re changing the gain of an INA, be sure to understand which
gain you’re changing…the CMRR depends on it!

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