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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

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  • Hi, Gary.

    We have a wide range of ADCs in our portfolio with SE inputs, differential inputs, and others configurable for both. Deciding how to drive it will depend on your application. I understand that you chose to use a differential ADC to take advantage of its rejection of common mode interference and your sensor accepts single ended signals only. We have newer SAR ADCs in our portfolio, may I ask for the reason you selected the AD7684?

    How was your programmable gain stage implemented - PGA, or discrete? High-order filtering should not be performed at the ADC driver as this would limit bandwidth at the ADC input and prevent adequate settling of artifacts resulting from the switching of the cap DAC input of the ADC.  The ADA4941 is a suitable choice for converting the signal from SE to differential, and you could add gain at this stage, taking note that there is a tradeoff between gain and bandwidth.

    Regards,

    Karen

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  • Hi, Gary.

    We have a wide range of ADCs in our portfolio with SE inputs, differential inputs, and others configurable for both. Deciding how to drive it will depend on your application. I understand that you chose to use a differential ADC to take advantage of its rejection of common mode interference and your sensor accepts single ended signals only. We have newer SAR ADCs in our portfolio, may I ask for the reason you selected the AD7684?

    How was your programmable gain stage implemented - PGA, or discrete? High-order filtering should not be performed at the ADC driver as this would limit bandwidth at the ADC input and prevent adequate settling of artifacts resulting from the switching of the cap DAC input of the ADC.  The ADA4941 is a suitable choice for converting the signal from SE to differential, and you could add gain at this stage, taking note that there is a tradeoff between gain and bandwidth.

    Regards,

    Karen

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