AD6645 overshoot

We design a data acquisition system and noticed an overshoot with a ~3us time constant in the ADC data. It is a small effect but it biases our data quality. I spent some time in identifying the source of the time constant.

The source of the time constant seems to be the ADC AD6645.

Test Setup:

AD8138 configured as a single ended to differential amplifier drives the AD6645 sampling at 100Ms/s

The signal source is a pulse generator with 50ns rise time (our application is in the time domain).

I have 2 plots showing the signal traces. One is an overview and the second one is zoomed in on the overshoot.

x axis: time in 200ps/step (scope). The AD6645 is stretched to match the scope time base.

y axis: is in Volt scaled and offset to the AD6645 input

The traces are:

1. single ended input to the AD8138 with a differential probe between the signal and ground.

2. input to the AD6645 with a differential probe between AIN and AINbar.

3. data read from the AD6645.

As the plots show the AD6645 data has an overshoot which is not present at the analog input. The time constant seens pretty long for an ADC (10000 x axis tics = 2us).

Is there something we might do wrong in the operation of the ADC or is this intrinsic to the ADC?

Thank you,

Thorsten

attachments.zip
  • 0
    •  Analog Employees 
    on May 18, 2015 10:37 PM

    Hi Thorsten,

    I'd like a little more info about your AD8138 & AD6645 interface configuration.

    1. Are you AC or DC coupling the AD8138 outputs into the AD6645 Ains?

    2. Are you driving the AD8138 VOCM input using the 2.4V VREF out from the AD6645 to insure the Amp Output and ADC Input Common Modes are aligned/balanced?

    3. What units do the Y-Axis amplitude labels represent? Is the Y-Axis reflecting the differential voltage swing around the AD6645's internally fixed ~2.4V input common mode voltage?

    Thanks,

    Tony M


  • Hello Tony,

    good questions.

    1. Are you AC or DC coupling the AD8138 outputs into the AD6645 Ains?

    ->  The system is DC coupled all they way.


    2. Are you driving the AD8138 VOCM input using the 2.4V VREF out from the AD6645 to insure the Amp Output and ADC Input Common Modes are aligned/balanced?

    -> The  AD8138 VOCM is driven from the AD664 Vref output.


    3. What units do the Y-Axis amplitude labels represent? Is the Y-Axis reflecting the differential voltage swing around the AD6645's internally fixed ~2.4V input common mode voltage?

    ->The Y-axis is in Volt with all the traces normalized to the AD6645 differential input level.

    The 3 traces are

    red: scope measurement with a differential probe between the "single ended" AD8138 input and ground

    green: scope measurement with the differential probe between the two analog inputs of the AD6645. As you correctly asked it is the differential swing around the ~2.4V common mode voltage Vref.

    blue: AD6645 digital output scaled to the analog input voltage.

     

    Thank you,

    Thorsten

  • 0
    •  Analog Employees 
    on May 22, 2015 1:05 AM

    Hi Thorsten,

    I am contacting you directly via e-mail to discuss your configuration further.

    Best Regards,

    Tony M