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PGA870: PGA870 offset

Colin Prohasky
Prodigy 50 points
Part Number: PGA870

What is the valid range of DC offset which can be applied to the inverting input with gain at 0dB, and 20 dB ?

  • 0
    TI__Genius 12631 points
    Hello Colin,
    Are you referring to a common-mode offset? Since this is a differential input - differential output amplifier both noninverting and inverting inputs have to have the same common-mode offset. In the case of the PGA870, the common-mode offset is mid-supply. (See Device Operation - Input Attenuator, on page 14).
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 50 points

    I wasn't referring to CM offset.

    We want to purposely offset the output as seen by the ADC, and looking at how this can be best achieved.

    Input is AC coupled to non-inverting input; and the inverting input is grounded by same capacitor value.

    Can we simply apply an offset voltage (eg, DAC + buffer) to the inverting input?

    We understand there will be a dependency on gain setting.

  • 0 in reply to Colin Prohasky
    TI__Genius 12631 points
    Hello Colin,
    We haven't done any evaluation with the PGA in this mode so I cant say for sure one way or the other, but I don't see any reason for this not working. I will chat with some other engineers and let you know by tomorrow if they feel that this wont work.
    This is a strange way of configuring the amplifier - each output of the amp to the ADC will actually be riding on a different common-mode voltage. Is this by any chance because you have a unidirectional pulse output and you want to use the full dynamic range of the ADC?
    You could also purchase one of our EVMs and play around with your configurations to see if this method will work. If I have an EVM at work I will try myself. Can you please let me know:
    1. What gain configuration?
    2. Frequency and magnitude of the noninverting input signal.
    3. Voltage offset to be applied at the inverting input.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 50 points

    We are using the PGA870 to drive a fast ADC and also amplify a 1Vp-p signal of BW 220MHz which is AC coupled to the non-inverting input.

    Inverting input is grounded via the same capacitor value, therefore at AC ground potential.

    Is it possible to apply a DC potential to the inverting input via some form of DC coupling network to achieve a differential offset function at the differential output without affecting the common mode voltage?

  • 0 in reply to Colin Prohasky
    TI__Genius 12631 points
    Hello Colin,
    I spoke to some folks more familiar with the product and as long as you are okay with the output differential offset you should be able to drive the inverting input with a dc voltage from a low impedance source such as another wideband opamp.

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 50 points
    On Friday I tried the following, measuring Offset at ADC output...
    0dB gain:
    Tested with fixed resistor 680E to +5V and then to GND. Seems same differential change at input and output.
    However using 100E, offset appeared not to change, apart from an initial transient (and there was some evidence of transient with 680E).
    20dB gain:
    680E was able to offset between extremes of 500 and 15800 code.
    Proportionately less with higher resistors, so presumed linear.

    I will try with Lo Z voltage source.
  • 0 in reply to Colin Prohasky
    TI__Genius 12631 points
    Hello Colin,
    With a voltage source with a finite dc output impedance, there will be a voltage divider wrt to the 2.5V mid supplies. The internal resistor is approx. 75 ohms. Can you please check the voltage at the inverting input as well and check if it makes sense?

    -Samir