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ADS7924: Using an Operational Amplifier Between Multiplexer Output and ADC Input

Tim De Bondt
Prodigy 110 points
Part Number: ADS7924
Other Parts Discussed in Thread: TLV9001

Tool/software:

Hello,

I have a question regarding "Using an Operational Amplifier Between Multiplexer Output and ADC Input".

When testing we noticed two "issues". 


First of all the AVDD is lower then DVDD. This is solved by shorting VREF and VIN on U38.

Second of all, we measured ADCIN and it is 0.035V while pin 3 of the opamp (U10) is the expected 1.5V. Do you have any idea of why the opamp would not be following?

Greetings,

Tim

  • 0
    TI__Guru**** 181751 points

    Hi Tim,

    OK on the power rails.  As for the output of the OPA, are you measuring that with a DMM or an oscilloscope?  If you are using a meter, could you probe AIN with an o'scope and verify that the TLV9001 is not oscillating?  We normally recommend the R/C between the OPA and the ADC since the analog input is actually looking ito a swithed capacitor which could cause the amplifier to become oscillatory.

  • 0 in reply to Tom Hendrick
    Prodigy 110 points

    Hi Tom,

    I measured the ADCIN/TLV9001Output with an olliscope and it is stable (+-0V). Do you have any other theories I could try?


    Greetings,

    Tim

  • 0 in reply to Tim De Bondt
    TI__Guru**** 181751 points

    Can you double check the PWRCON pin?  That is meant to drive a shut down or enable pin, it's not really meant to power an external amplifier.

  • 0 in reply to Tom Hendrick
    Prodigy 110 points

    Hi Tom,

    Yes this was also our initial check. With the osciloscope it looked like a stable 3.3V. To further check this theorie we also tied the opamp power directly to 3.3V (and also shorting PWRCON to 3.3V). 

    P.S. you might want to change Figure 57. in the datasheet, this can confuse some designers (like me :) )a bit:

     

    Greetings,

    Tim

  • 0 in reply to Tim De Bondt
    TI__Guru**** 181751 points

    OK - power is good, the output is not oscillating.  Does this happen on multiple boards?  If so, you could remove the OPA and verify the voltages at CHx in to MUX out.  You could then short MUXout to ADCin and verify the ADC is working.

  • 0 in reply to Tom Hendrick
    Prodigy 110 points

    We have tested 3 boards, and all of them have the same issue.

    On one of the boards we have removed the OPA and directly shorted the MUXOUT to the ADCin and confirmed it was working.
    (The voltages of CHx are around 1.2V)


    Greetings,

    Tim

  • +1 in reply to Tim De Bondt
    TI__Guru**** 181751 points

    Hey Tim,

    Can you double check the TLV9001 part number?  The TLV9001UIDBV has a different footprint versus the non U version.

  • 0 in reply to Tom Hendrick
    Prodigy 110 points

    Hey Tom,

    Yes this seems to be the issue! The one in schematics is the TLV9001U (same as the one actually placed on the board), but the altium part actually uses the TLV9001T pindef/pinout.

    I will change the part manually to the TLV9001U and confirm if the PWRCON can actually directly drive this signal or not.

    Thank you for you solution!

    Greetings,

    Tim