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PGA870: Can PGA870 VMID2 Output be Used?

David Hand
Intellectual 925 points
Part Number: PGA870
Other Parts Discussed in Thread: THS4535, THS4151, REF35

Tool/software:

We are planning on using a PGA870 with VS+ at 5V. So VMID2 output should be at 2.5V.

We would like to use VMID2 as the VOCM input to an opamp that feeds it. The input to that stage is bipolar and we need to offset it be be from 0 to 5V.

But the PGA870 datasheet makes the following statement:

So it says "don't use it" but then go ahead as long as you don't draw too much current.

So can we safely use it to drive the VOCM pin of an opamp, which will draw much less than 200 uA?

If the VMID2 pin is not intended to be used, why does it exist?

  • +1
    TI__Mastermind 24529 points

    Hello David,

    Great questions!  The VMID2 pin is bonded-out to the package for external connection to a bypass capacitor, as seen in the paragraph you provided above.  Even though the function and buffer of VMID2 are internal to the device, bypassing to GND with a capacitor is important to prevent noise coupling into the node.  If this pin was kept internal, there would be potential concerns for the accuracy and reliability of this buffer signal.

    As you mentioned above, the VMID2 can be used as an external reference as long as 200uA or less are sourced from this node.  If you can assert current limiting or a low-current sink which uses the VMID2 reference, you would not violate the VMID2 pin's integrity of operation.

    I can help review your design if you have concerns about the circuit using PGA870 VMID2 as VOCM.  For one of our own TI FDAs, we do typically provide specifications or data curves which help characterize VOCM.

    I will add that for level-shifting or single-to-differential conversion we do have a new FDA which will be released before the end of the year, THS4535.  While not perhaps a best-fit if you are using higher-frequency signals, this high-speed amplifier is still one I like to make known to our customers.

    Thank you for working with TI.  I am glad to support you on your design David.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Intellectual 925 points

    Alec,

    Thanks for the great help. I think that tells us what we need to know.

    Just to explain what we are doing, the input to the PGA870 will be a THS4151 which has a bipolar input. So we need to shift the THS4151 output to be around a common mode of the PGA870 (ie 2.5V).

    Looking at Fig 26 of the THS4151 datasheet, the VOCM impedance is well under 14K and dropping (we go to about 2 MHz). So at 2.5V I guess we would have 180uA of current. That's too close for comfort.

    We'll just use a 2.5V reference such as the REF35. It's so small, can't complain too much.

    Regards,

    Dave

  • +1 in reply to David Hand
    TI__Mastermind 24529 points

    Hello David,

    Thank you for the update; I agree with your conclusion.  For robustness it is good to use a dedicated REF IC.

    Please let me know if you have additional questions.

    Best,

    Alec