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INA138: need buffer at output side?

Part Number: INA138
Other Parts Discussed in Thread: ADS52J90

Dear team,

I would like to learn from you about why we need to add OP as buffer at OUT of INA138. (like datasheet Figure 10)

Customer plan to design current measured by INA138+ADS52J90.

Per my understood, the OP is buffering OUT and driving ADC. But looks like not all of ADC need to add buffer circuit.

Please let me know how to decide to add buffer at OUT. What's the benefit? 

Thanks

Regards,

Ben

  • Hi Ben,

    as being a current output the output impedance depends on the load resistor connected from the output to signal ground. When using a rather high load resistor of let's say 100k, the input bias current of the following stage can cause an unwantedly high voltage drop looking like a hugh offset voltage. Also, the output resistance of INA138 determined by the load resistor can result in a heavily increased settling time in combination with the obligatory charge kick back filter capacitance at the input of ADC.

    Kai

  • Ben,

    Kai is on the money here with his comments. 

    For more information on the ADC side, and how buffering choice intermingles with sampling requirements, charge bucket filtering, etc., check out this app note.