AD7124-8 input circuit strong disturbance in Single Conversion Mode

Hi,

I've recently encountered a strange phenomenon in Single Conversion Mode. At the beginning of the new conversion strong interference appears at both pins of the first logical channel used. The input circuit is typical - differential mode anti-alias filter 2x10k + 10nF and 2x3n3 pair to system analog GND. The interference is in the form of positive pulse of about 100mV amplitude and about 50us of time and it's present at both pins of the logical channel (because of this, differential measurement is disturbed to a less extent). The disturbance comes from the inside of the IC, it's not present anywhere beyond the input circuit. The disturbance appears only at the beginning of the first conversion after the Standby phase after previous single conversion. If the ADC is configured to take some consecutive measurements, the first one, and only the first one is disturbed, no matter what logical channel is used. The disturbance is independent of the hardware channels used, one can "move" the disturbance to any pin pair, just by reconfiguring the first logical channel used in the sequence. The distrurbance is independent of the conversion config (filter, data rate etc). Burnouts are off.

Any idea how to get rid of this? Switching to continuous conversion (or conversion in pairs - with the first conversion "lost") would be a problem.

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  • Hi,

    In my understanding, it is only the first channel (Channel_0) has the issue? when you tried the second channel (Channel_1) or third channel (Channel_2) in the sequence with the same analog input pins you cannot see the disturbance? Can you clarify this? When you are not taking measurements, or does not select any channel, are you still seeing this issue? If you could share the register settings and as well as your measurement setup (block diagram or schematic) it would be helpful. Do you also have a scope shots of the above scenarios so I can easily visualize the issue? Are you using the evaluation board or your own design board? Have you check the supply/reference decoupling and grounding connections? Can you also perform noise test by shorting the inputs together and bias to some voltage (usually mid supply).

    Thanks,

    Jellenie

Reply
  • Hi,

    In my understanding, it is only the first channel (Channel_0) has the issue? when you tried the second channel (Channel_1) or third channel (Channel_2) in the sequence with the same analog input pins you cannot see the disturbance? Can you clarify this? When you are not taking measurements, or does not select any channel, are you still seeing this issue? If you could share the register settings and as well as your measurement setup (block diagram or schematic) it would be helpful. Do you also have a scope shots of the above scenarios so I can easily visualize the issue? Are you using the evaluation board or your own design board? Have you check the supply/reference decoupling and grounding connections? Can you also perform noise test by shorting the inputs together and bias to some voltage (usually mid supply).

    Thanks,

    Jellenie

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