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Original question:

PCM1794A: The I/V converter for PCM1794A

PCM1794A: PCM1794A Vbias question

Jahongir Dodokhonov
Prodigy 130 points
Part Number: PCM1794A
Other Parts Discussed in Thread: TPS7A39

Dear Ti Support Team,

I followed the original thread regarding applying Vbias oo noninverting inputs of Transimpedance Amplifier and would like to confirm this again.

I copied the original schematic provided on "Figure 3. Amplifiers Schematic (±10-V Power Supply)" of the sboa237 document for my new PCM1794A DAC based PCB design, except for the noninverting inputs of  Transimpedance Amplifier being grounded as in original PCM1794A datasheet. 

My question is, If I plan to supply the OPAMPs of both stage  with ±12V or even a bit higher, then will I have to add any Vbias to the noninverting inputs of Transimpedance Amplifier in order to reduce the common mode voltage of the differential amp to nearly 0V? 

 Many thanks and best regards,

  • 0
    TI__Guru 52683 points

    Hi Jahongir,

    We will take a look at this tomorrow after the US holiday. 

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    TI__Guru 52683 points

    Hi Jahongir,

    Figure 3 actually has an error that makes it compatible with the PCM1794A.  In the figure, there is a bias of 5.77V.  The supply of the PCM1794A is only 5V, so it will not actually be able to source current into the TIA.  I generally do not recommend a bias voltage of greater than 2.5V on the PCM1794A.  Instead of increasing the bias, you could instead reduce the negative supply lower than -12V.  This circuit does not require symmetrical supplies, so you could implement a +5V/-18V for the first stage, if you wanted.

    Thanks,

    Paul

  • 0
    Prodigy 130 points

    Hi Paul,

    But I'm planning to use symmetrical supplies for both OPAMPs, so what do I do? 

    I will be using TPS7A39 to supply the OPAMPs used and I would like to go with symmetrical supplies. And if I am to go with 12V/-12V for the first and second stage, what amount of Vbias should be used then?

    Thanks,

  • +1 in reply to Jahongir Dodokhonov
    TI__Guru 52683 points

    The Vbias should not exceed 2.5V.  You may have to scale your gain of the TIA stage to ensure that the output does not clip on the negative supply.