8.3 Feature Description

8.3.1 Gain-Setting

The gain equation of INA351ABS can be given by Equation 1:

G = 1+180 kΩRG

(1)

The value of the internal gain resistor RG for INA351ABS can then be derived from the gain equation:

RG = 180 kΩG − 1 

(2)

Similarly The gain equation of INA351CDS can be given by Equation 1:

G = 1+290 kΩRG

(3)

The value of the internal gain resistor RG for INA351CDS can then be derived from the gain equation:

RG = 290 kΩG − 1 

(4)

Gain selection table below outlines how to choose different gain options across INA351ABS and INA351CDS.

The 60-kΩ, 90-kΩ, and 145-kΩ resistors mentioned are all typical values of the on-chip resistors.

Table 8-1. Gain Selection Table

DEVICE

GAIN SELECT (GS)

SELECTED GAIN

INA351ABS

High or No Connect

20

Low

10

INA351CDS

High or No Connect

50

Low

30

8.3.1.1 Gain Error and Drift

Gain error in INA351 is limited by the mismatch of the integrated precision resistors and is specified based on

characterization results. Gain error of maximum 0.1% can be expected for all gains of 10, 20, 30 and 50. Gain

drift in INA351 is limited by the slight mismatch of the temperature coefficient of the integrated resistors. Since

these integrated resistors are precision matched with low temperature coefficient resistors to begin with, the

overall gain drift is much better in comparison to discrete implementation of the instrumentation amplifiers built

using external resistors.

8.3.2 Input Common-Mode Voltage Range

INA351 has two gain stages, the first stage has a common-mode gain of 1 and a differential gain set by the GS

pin. The second stage is configured in a difference-amplifier configuration with differential gain of 1 and ideally

rejects all of the input common mode completely. The second stage also provides a gain of 1 from REF pin to set

the output common-mode voltage.

The linear input voltage range of the INA351, even for a rail-to-rail first stage, is dictated by both the signal swing

at output of the first stage as well as the input common-mode voltage range output swing of the second stage. In

order to maximize performance, it is critical to keep the INA351 within its linear range for a given combination of

gain, reference, and input common-mode voltage for a particular input differential. Input common-mode voltage

(VCM) vs output voltage graphs (VOUT) in this section show a particular reference voltage and gain configuration

to outline the linear performance region of INA351. A good common-mode rejection can be expected when

operating with in the limits of the VCM vs VOUT graph. Note that the INA351 linear input voltage cannot be close

to or extend beyond the supply rails, as the output of the first stage will be driven into saturation.

The common-mode range for the most common operating conditions is outlined below. Figure 8-1 shows the

region of operation where a minimum of 86 dB can be achieved. Figure 8-2 has much wider region of operation

INA351

SBOSAD5 – DECEMBER 2022

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