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INA188: Quiz in Precision lab

Zhihong Huang
Intellectual 1550 points
Part Number: INA188
Other Parts Discussed in Thread: INA819

Hi team,

In the Quiz, why the answer is C?

1. Answer B : I think the gain is set correctly from my calculation.

2. Answer C: I think It is very important to use a buffer because the input to the INA Ref pin is low impedance and errors will occur if you directly connect the voltage divider to the Ref input.

  • 0
    Guru 140200 points

    Hi Zhihong,

    the REF pin of INA188 must be driven very low ohmically. Otherwise R1 and R2 add to the internal 20k resistance of INA188. This would destroy the symmetry of differential amplifier stage of INA188 and make the INA188 work improperly. See also section 7.4.4 of datasheet.

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 48076 points

    Hi Zhihong and Kai,

    Kai, thanks much for the E2E support. 

    As Kai indicated, Ref pin in INA188 has to be a low impedance voltage source. The datasheet indicated that it has to be < 5 ohm. And it needs to be stable, low impedance and low noise voltage source without voltage drift over temperature changes and/or time. In the schematic drawing example above, it is the best to ground the Vref pin. If you have to elevate the Ref pin, you may use a buffer or stable voltage source to bias Vout to a mid point of voltage supply rail, if a single voltage rail is used. Please also check Figure 51 or 52 in INA819 datasheet. 

    Best,

    Raymond