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INA128, Adding -9V offset with reference pin

YILMAZ KIRÇİÇEK
Intellectual 905 points
Other Parts Discussed in Thread: INA128, TINA-TI

Hi,

There is a bipolar (-10V/+10V) ADC on my circuit. And want to measure 0V to 5V signal with high empedance circuitry. To not loose ADC resolution I want to add a negative offset in INA128 circuit without using second amplifier. (0V to 5V  input;  -9V to 8.5V output) Theroticaly and experimentaly (using TINA-TI)  applying -9V to INA128 reference input solve my problem. Input and output voltage seems to be within specified limits but what about internal node voltages.

According to my calculation; when input is 5V and output is 8.5V,  A2 output node should be 11.25V.  But it seems difficult the reach this level with 12V supply. (Is it RRO)

Could you please clearify and make me sure for these ?

1.) Reference input is just intended for applying small offset nulling voltages or can I use it to apply higher offset ?

2. ) Using -9V offset is adequate for INA128 ? If yes how does it effect the CMRR ?

3.) Using 15V positive supply for INA128 allows me to apply -9V offset to reference pin if 12V supply is not enough?

 Best regards.

 

( Diodes are BAV199 but not found in TINA-TI library so I used BAV99 instead. )

 

  • 0
    TI__Mastermind 25795 points

    Hello Yilmaz,

    I am looking at your post and will get back with you shortly.

  • 0
    TI__Mastermind 25795 points

    Hello Yilmaz,

    Here are my responses:

    1.) Reference input is just intended for applying small offset nulling voltages or can I use it to apply higher offset ? 

    No, you can apply -9V to the ref pin under these conditions.

    2. ) Using -9V offset is adequate for INA128 ? If yes how does it effect the CMRR ?

    That is fine.  Concerning CMRR, ensure that the -9V source is from an low-impedance source as discussed in the PDS.

    3.) Using 15V positive supply for INA128 allows me to apply -9V offset to reference pin if 12V supply is not enough? 

    I don't fully understand this question.  A +15V/-12V (27V) supply does not violate the operating conditions of the device.

  • 0 in reply to Pete%20Semig
    Intellectual 905 points

    Hi Pete,

    I thank you for your answers. But still I have doubt about internal node voltages. I have no idea about internal structure of INA128 but if  I try to constitude INA128 with 3 opamps and use in the circuit above for given conditions (Vin=5V, Vo=8.5V, reference =-9V) output of the A2 opamp should be 11.25V. It is only possible if A2 is RRO with 12V supply. So I need to ask third question. Using 15V allow to ouput of A2 to be 11.25V even if it is not a RRO opamp. Output of INA128 is not RRO and I could not to be sure.

    Best Regards.

  • 0 in reply to Pete%20Semig
    Intellectual 905 points

    Hi Pete,

    I thank you for your answers. But still I have doubt about internal node voltages. I have no idea about internal structure of INA128 but if  I try to constitude INA128 with 3 opamps and use in the circuit above for given conditions (Vin=5V, Vo=8.5V, reference =-9V) output of the A2 opamp should be 11.25V. It is only possible if A2 is RRO with 12V supply. So I need to ask third question. Using 15V allow to ouput of A2 to be 11.25V even if it is not a RRO opamp. Output of INA128 is not RRO and I could not to be sure.

    Best Regards.

  • 0 in reply to YILMAZ KIRÇİÇEK
    TI__Mastermind 25795 points

    Hello Yilmaz,

    Thank you for the clarification.  The output of A2 can swing to within ~0.7V of the supplies.  Please be aware, however, that the output of the input stage amplifiers is 0.7V less than what you would calculate.  This is because the INA128/9 devices utilize a current-feedback topology.  It is mentioned in the note for Figure 8.