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Is it acceptable to connect Vocm1 and Vocm2 together?

Category: Datasheet/Specs
Product Number: ADA4940-2

I am designing a fully differential active bandpass filter (around 35-45KHz) so reasonably low frequency.

The circuit is powered from a single 3v3 supply.


I have shown them connected to a low impedance 1v65 supply.

I don't care what the ultimate common mode output voltage is.

However, I was wondering if it is acceptable to just connect Vocm1 and Vocm2 together and bypass with 100n? That is simplest.

OR...


Would a single resistive divider (2k2 + 2k2, say) to generate the 1v65 mid supply for both be acceptable (or even needed if I can just connect Vocm1 and Vocm2.

Thanks.

Derek

  • I said “I didn’t care what the ultimate output common mode voltage was.” What i should have said is that I just want it approximately mid-rail. Derek

  • Hi Derek,

    The SETTING THE OUTPUT COMMON-MODE VOLTAGE section of the datasheet says: The VOCM pin of the ADA4940-1/ADA4940-2 is internally biased at a voltage approximately equal to the midsupply point, [(+VS) + (−VS)]/2. Relying on this internal bias results in an output common-mode voltage that is within approximately 100 mV of the expected value.

    So only 100nF capacitors needed at the Vocm inputs, as you can see on Fig. 71, for example.

    Zoltan

  • Thanks Zoltan,

    I’ve seen that. I suppose I was trying to figure out if in the circuit configuration I gave, it would be preferable if the common mode voltages were as similar as possible and whether that would be achieved if the two Vocm pins were tired together?

    FYI, the circuit simulates ok with 1.65V applied to both Vocm pins. I’m wondering if it would be better with the Vocm pins tied together, worse or just not needed. 

    Derek

  • Hi Derek,

    I don't see any reason to have exactly the same common  mode for the two amps, since the signals are differential. As a matter of fact, even if you connect Vocm1 and Vocm2 to the same voltage, the output common mode will be slightly different (by up to 1% according to the specs). So I think you can use 100nF capacitors only or you can connect Vocm1 and Vocm2 to a common voltage source, both will be OK:

    Zoltan

  • Thanks for your comments Zoltan - from your comments, I don't see any harm in connecting them together and it can only really bring the outputs closer together - and on average bring them closer to the mid-point. I was looking out for any downsides and no downsides to connecting them together seem apparent.