ZHCSQ83A March 2022 – October 2022 INA851
PRODUCTION DATA
Low offset voltage is one of the key parameters for an instrumentation amplifier (INA). In a current-feedback INA, this error source is classified in three stages: input, output, and intermediate. The input-stage dc offset (VOSI) is mainly caused by the mismatch of the input transistors Q1 and Q2, (see Figure 8-1). The output-stage dc offset (VOSO) is caused partially by the mismatch of the output amplifier A3. In the INA851, A3 is a fully-differential amplifier and gained up by the noise gain of the circuit (1 + R5 / R3). An additional intermediate stage offset contribution error adds to VOSO that is caused by the mismatch of the current mirrors in the front end (through R1 and R2).
Unlike typical instrumentation amplifiers that incorporate a difference amplifier (A3) with a fixed output gain, the INA851 has two different output gain stages that subsequently contribute differently to VOSO; see Gaussian distributions for G = GOUT = 1 V/V in Figure 7-3 and for G = GOUT = 0.2 V/V in Figure 7-4.
The following equation calculates the total offset voltage error referred to the input: