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AD7684 Pre-Processing-Best Methods Approach

Hi,

I have a couple of questions on this part, and the best way to pre-process the incoming signal.

I have a low frequency AC sensor that is single ended and requires AC coupling. With the present circuit, I'm using an ADA4096 (due to it's excellent OVP) as a buffer. I then go to a AD8616 Bessel-Butterworth LPF. From there, it goes to another programmable gain stage, and then into a 12 bit micro ADC. Total gain is about 80dB.

I want to change this to take advantage of any common mode noise from the sensor by going to a differential circuit, then to a 16 bit ADC. After reading through AD data sheets, it appears the best device for the frequencies I am working at is the AD7684.

I realize that at some point I need to go from single-ended to differential. However, I still want to have the OVP from the ADA4096 in the circuit, at least as a buffer for the sensor.

So...at which point does it make sense to go from single-ended, to differential, to take advantage of the benefits of differential input?

Where should the filtering and gain stage be, and should it be differential? I have a requirement for filtering off 60Hz signals, although I am working below that. So if the filter were differential, it would need to be order 8 to 10 to provide the same transition I presently have.

The data sheet for the ADA4941 makes it an ideal candidate for a pre-ADC buffer and gain amplifier.

What will be lost in using differential if I were to input buffer, filter, gain and then go single-ended to differential using the ADA4941, vs differential from the input right through - keeping in mind I still need to AC couple and have OVP.

Thanks for any advice or opinions on this!

Regards,

Gary

Parents
  • Regarding the ADC choice, what do you recommend? How about the AD7916?

    The "programmable" gain stage is pretty complex - an ADG708 multiplexer selecting the gain resistor in the op-amp feedback loop.

    You said: "High-order filtering should not be performed at the ADC driver..."

    What's high order filtering? 2nd, 3rd....8th? Can you be more specific?

    Sorry, my question still isn't answered, and I was hoping to really get that resolved. I understand best performance can be achieved using differential amplifier and ADC (or pseudo-diff ADC)

    Recap of objectives and problem:

    1 - Single ended sensor (present configuration) that must be AC coupled

    2 - Need to LPF 60Hz by at least 60dB (stop band), all signals of interest 5 to 30Hz

    3 - Need front end gain of 80 to 90dB

    4 - Would like to have 16 bits resolution to discriminate small signals

    Since I'm starting out with single ended, AC coupled sensor, where in the chain, keeping the above in mind, should I introduce the single-ended to differential stage?

    Thanks...

    Gary

Reply
  • Regarding the ADC choice, what do you recommend? How about the AD7916?

    The "programmable" gain stage is pretty complex - an ADG708 multiplexer selecting the gain resistor in the op-amp feedback loop.

    You said: "High-order filtering should not be performed at the ADC driver..."

    What's high order filtering? 2nd, 3rd....8th? Can you be more specific?

    Sorry, my question still isn't answered, and I was hoping to really get that resolved. I understand best performance can be achieved using differential amplifier and ADC (or pseudo-diff ADC)

    Recap of objectives and problem:

    1 - Single ended sensor (present configuration) that must be AC coupled

    2 - Need to LPF 60Hz by at least 60dB (stop band), all signals of interest 5 to 30Hz

    3 - Need front end gain of 80 to 90dB

    4 - Would like to have 16 bits resolution to discriminate small signals

    Since I'm starting out with single ended, AC coupled sensor, where in the chain, keeping the above in mind, should I introduce the single-ended to differential stage?

    Thanks...

    Gary

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