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Amplifier OPA890 vs OPA836 effects on ADC INL errors

makdam ASFOUR
Prodigy 110 points
Other Parts Discussed in Thread: OPA890, OPA836, OPA684, OPA690, OPA683, OPA830

Hello , 

I 'm driving a 16bits SAR ADC (from ADI) using the OPA890. The driver consists of 2 single OPA890 to generate at the ADC inputs two signals in opposite phase with respect to the ADC common mode voltage. The RC filter between the amplifier output and the ADC input is R=10R and C=620pF. The ADC is running at 10MSPS. And the amplifier power supplies are +6V and -2.5V 

When measuring the ADC INL, I found in particular sharpe INL discontinuty at 16000 , 32000 and 48000 (i.e at Fs/4, FS/2, and 3*FS/4). (red line trace in figure here below)

Same resultats are noticed for a similaire amplifier from AD (AD8014). (bleu trace)

However , when using the OPA836, the  ADC INL is very nice ( no more shapes discontinuty are presents): the amplifier is here power supplied with 4,7V and -0.7 V. (green data)

Can you help me to understand this?  . What is the amplifier specification to take into-account or to be considered here in order to be able to chosse the right amplifier . (I cannot keep the OPA836 since its power supplies are not forseen  on already avaiable boards)  I 'm planning  to do these tests using your OPA683/OPA684 or OPA690 . 

  • 0
    TI__Genius 12631 points

    Hello,

       Are you measuring all codes and are you measuring DC INL? Can you please provide a schematic. Can you please bump up the series resistor from 10Ohm to 25Ohm and remeasure the INL?

    When testing with the OPA836, what power supplies are you using?

    Thanks,

    Samir

  • 0 in reply to Samir Cherian
    Prodigy 110 points
    Hello Samir ,
    Thanks for replying

    Yes , the input signal to the amplifier is a DC one and The ADC runs at 10MSPS
    I have already tried to bump up the resistor value to 22R and tried differente values for RC filter components . but no significant difference can be noticed
    I will ask my manger if I can provide schematic , but the amplifier is in inverter configuration (G=-1) with an offset shift

    The OPA836 is power supplied with VDD=4.7V and VSS=-0.7V

    regards
  • 0 in reply to makdam ASFOUR
    TI__Genius 12631 points
    Hello,
    In my experience, what is happening here is that when the major bits are flipping the ADC requires a large amount of transient current. I believe the OPA890 is incapable of providing this inrush current which is resulting in the dc error around the major carries.
    The OPA836 has higher Slew rate and output drive current. I think these are the key factors that are showing the difference in performance. The OPA830 could be another option that will suit your needs. It has higher Slew and output current drive relative to the OPA890 and OPA836.
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 110 points
    Helllo ,
    Of course , I agree with your feeling . I suspected the amplifier output driving current . But the amplifier I tested (AD8014) has an ouput current of 50mA (which is similaire to OPA836? ) and a slew rate as high as 2500V/us .

    Today i will performe these tests with the OPA638 (since i need a sot23-6 with disable) .
    Regards
  • 0 in reply to makdam ASFOUR
    TI__Genius 12631 points
    Hello, I looked through the AD8014 datasheet briefly and noticed that it is a current feedback amplifier (CFB). Do you have a feedback cap (in parallel with your feedback resistor) when you test INL with the AD8014? If so, then you should remove that cap. CFB performance tends to degrade when you have the feedback cap.
    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 110 points
    Hello ,
    I have no FB capacitor in my circuit .
    Today , i tested OPA638 (CFA) and OPA690 (VFB), the resulats are same as OPA890. For instance, only the OPA836 is good , and i can not know why?

    Thanks