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ADS7883: ADS7883SBDBV Double Read

Zach Bryant
Prodigy 10 points
Part Number: ADS7883

My circuit description:

+3.3V VDD

Input is a pulsed square wave of varying voltage, 300us pulse width.

The ADS7883SBDBV is used to measure the voltage of these pulses. I often get accurate measurements. Intermittently, I will get erroneous measurements, but if I make two measurements on the same pulse the second one is always very accurate. Only the first measurement will intermittently be inaccurate. This issue exists only when the input voltage is on the low end.

Question 1: Can you explain what we should expect to see in the measurement data on the first read after coming out of power down.  Less accurate measurement?  Random Noise? Zero? Other? Is there a reason for the first data not coming out correct?

Question 2: From the datasheet, it looks like the part needs to be powered down using the CS input between the first (invalid data) read and the second read. Is this correct? And if so, is there a minimum/maximum time that the part needs to be powered down for?

  • 0
    TI__Mastermind 22950 points

    Hi Zach,

    Is it possible to share a full schematic?

    1. I would expect to see normal results coming out of power down. However, the data sheet does recommend 0.8us of time after coming out of power-down mode. Here you can send some dummy conversion frames to satisfy this time. Data before 0.8us will be invalid so if you attempt to read this data, it would pretty much be garbage data. Have you tried measuring the input signal when the device has been fully powered and right after coming out of power-down mode to rule this out?

    Additionally, these bad measurements could be due to input settling. What exactly is driving the inputs? Do you have an input RC filter? With SAR converters, we typically have an input RC filter to act as a charge bucket to help dump charge into the input sample and hold circuit. 

    2.  The device enters power-down mode when CS goes high anytime after the 2nd SCLK falling edge to before the 10th SCLK falling edge. For a minimum power-down time, I don't think there is a requirement here. 

    Regards,
    Aaron Estrada