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AD7684 Pre-Processing-Best Methods Approach

Question asked by groger on Sep 5, 2015
Latest reply on Sep 21, 2015 by KarenNE

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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