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ADS1262: CHOP mode available if PGA is bypassed?

Josh Heiner
Intellectual 860 points
Part Number: ADS1262

Hi,

I'm designing the ADS1262 into a new project.  I want to take advantage of the benefits of CHOP mode, but as I'm planning to bypass the PGA for all inputs, I wanted to be sure I can use this mode.  The source of my confusion is section 9.4.12 of the datasheet, which says "The device uses a chopper-stabilized PGA and modulator..."  This makes it sound like the chopping circuitry is baked into the PGA itself.

However, in figure 120 just below this paragraph, the chopping circuit is depicted as a discrete block that appears to be separate from the PGA.  The bypass switches aren't shown in this diagram, so I'm unsure of where they are relative to the chopping circuit.  In looking through the registers, the CHOP enable doesn't seem to have any dependency I can see on enabling the PGA, so I'm hopeful I can set things up the way I'd like.

I'd be grateful for a confirmation from TI before moving forward.

Thanks,

Josh

  • 0
    Intellectual 860 points

    My attempt to paste in figure 120 failed, so I'm trying again with a different method.

  • 0
    Intellectual 860 points

    Dear TI -

    Sorry, trying to be patient, but my whole project is hung up on this one point.  I really need to know ASAP whether I can design in this ADC and I would appreciate a quick answer from someone at TI if possible.

    Thanks,

    Josh

  • 0 in reply to Josh Heiner
    TI__Mastermind 35570 points

    Hi Josh,

    The PGA itself consists of two chopper-stabilized type amplifiers which can provide a low input voltage offset and drift inside PGA, please see the PGA block diagram in Figure 71 of datasheet. However, the Chop mode is an additional structure designed with specific control and switch circuit to reduce the offset and offset drift to very low levels in the whole signal chain including the error from the ADC core and filter which is different from the chopper-stabilized amplifiers of internal PGA.

    The internal PGA on the ADC can be bypassed if you really do not want to use it or you can set the gain of PGA to 1, see the picture below for the offset voltage vs. temperature under different PGA gain configurations(bypass, 1,2...32) when the Chop mode is on. The higher gain, the lower voltage offset and drift.

    I apologized for missing your post, thanks for your patience.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    Intellectual 860 points

    I see Dale - if I'm understanding correctly then, it sounds like there are two independent chopper blocks, and I'm glad to hear I can still enable and use the one that is external to the PGA.

    Thanks very much for the answer!  This is a big relief.

    Josh