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PGA870: How to correct output offset voltage

Shinichi Inoue
Guru 21951 points
Part Number: PGA870
Other Parts Discussed in Thread: TINA-TI, LMH6515

Tool/software:

Dear Specialists,

My customer is considering PGA870 and has a question.

I would be grateful if you could advise.

---

According to the datasheet, output offset voltage (Vos) is specified –30mVmax ±5mVtyp 30mVmax.

When used with DC coupling, it affects the output.

Could you please let us know how to correct offset voltage.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Expert 8700 points

    Hi Shinichi, 

    How are the inputs being connected to the device? If the inputs are driven single-ended and are DC coupled you will need to make sure both input pins are terminated to the same common-mode voltage. Otherwise the part will have a permanent offset at the output. 

    Regards, 

    Jacob 

  • 0 in reply to Jacob Freet
    Guru 21951 points

    Hi Jacob,

    Thank you for your reply.

    ---

    When using the single-ended circuit of PGA870 with DC coupling, I understand that there is a need to apply a common mode voltage (midsupply VS/2)  to the IN+ and IN- pins.

    Please provide an example of a single-ended DC-coupled circuit.

    How much offset voltage will there be in this case? 

    Is it possible to think +/-5mVtyp and +/-30mVmax Output offset voltage described in the datasheet P.3

    Could you please provide the circuit example of a single-ended DC-coupled circuit.

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    TI__Expert 8700 points

    Hi Shinichi, 

    The TINA-TI reference design that is downloadable from the PGA870 product page shows the device DC coupled in the circuit below. 

    This circuit has the part in split supply configurations with the inputs both referenced to ground through the 100k resistors. The common mode of the input signal is also 0V. 

    If you were to use a single 5V supply then the common mode would be 2.5V. 

    Best, 

    Jacob 

  • 0 in reply to Jacob Freet
    Guru 21951 points

    Hi Jacob,

    Thank you for your reply.

    When using single-ended DC coupling, is it possible to inject a common mode voltage into each of the differential pins (+/-)?

    Is it able to input an offset voltage into an unused input pin?

    I created an example circuit based on the other thread you mentioned.

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1454222/lmh6515-connection-of-single-end-and-dc-coupled/5581992#5581992

    Could you please see and advise?

    PGA870 single end DC coupled input.pdf

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21951 points

    Hi Jacob,

    Could you please advise this question?

    The offset voltage of the PGA870 can be adjusted by adding a correction voltage to the non-signal input, just like the LMH6515. Is that correct?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi