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Driving a buffered SAR ADC
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SethGI:
Hi all,

I've been working on a 4 channel data acquisition system, and am designing around the analog devices LTC2357-16 ADC. It's a four-channel, SAR, simultaneous sampling (important for me), buffered ADC. I know in general SAR ADCs can be a bit tricky to drive, so my original plan included a buffer (LT6018) which is specifically marketed for driving SAR ADCs. However, since the ADC itself is buffered, can I get around that requirement?

For reference, here's my overall system plan:

piezoelectric hydrophone -> charge preamplifier (recommended by manufacturer to prevent any discrepancies in cable length from affecting system) -> active butterworth bandpass filter -> PGA (LTC6915, fairly important for my application) -> driver? -> ADC

My question is can I swip that last driver step due to the buffered ADC.

Thanks, and sorry if this is a super basic question!

-Seth
Kleinstein:
The ADC internal  buffer should not need a special driver before the ADC.

The choice of PGA is add: the LTC6915 is a very slow INA with good CMRR - more something for low voltage DC of very low frequency. Its odd to combine it with a LTC2357 and than caring about simultaneous sampling.
I would more expect a faster PGA, with no special need for very good INA. Ideally one may get a fully differential one.
The ADC is also a odd choice here as it's specially made for a high input voltage range. The more normal ADCs are more like +-2 V input range.  Some ADCs also include some PGA function.

A charge amplifier can still get effected by cable capacitance and sound pick-up by the cable. At least a long cable will add to the noise. Ideally the first amplifier would be directly at the sensor, possibly powered through the same cable as the signal.
SethGI:
Hi! Thanks for the reply. As to the charge amplifier, I know it doesn't totally eliminate the noise, but the manufacturer of the sensor recommended we use it. So, I'll oblige. I totally agree with the other feedback. What if I were to remove the PGA and buffer steps all in one and use ADAR7251 instead. It has a LNA followed by PGA, then samples at the desired rate. I would then switch my charge amplifier and filters to be fully differential for better CMRR.

Any thoughts on that approach? That ADC (ADAR7251) seems a bit harder to work with, but overall better. As I have like six months for this project, I'm fine with that. Plus, I'm much stronger on the digital side of things than analog so complications in digital-land I'm fine with.

Thanks again for your help and patience!

So it would be input -> differential charge amp -> differential 4th-order butterworth filter -> ADAR7251


 
--- Quote from: Kleinstein on August 10, 2019, 09:49:11 pm ---The ADC internal  buffer should not need a special driver before the ADC.

The choice of PGA is add: the LTC6915 is a very slow INA with good CMRR - more something for low voltage DC of very low frequency. Its odd to combine it with a LTC2357 and than caring about simultaneous sampling.
I would more expect a faster PGA, with no special need for very good INA. Ideally one may get a fully differential one.
The ADC is also a odd choice here as it's specially made for a high input voltage range. The more normal ADCs are more like +-2 V input range.  Some ADCs also include some PGA function.

A charge amplifier can still get effected by cable capacitance and sound pick-up by the cable. At least a long cable will add to the noise. Ideally the first amplifier would be directly at the sensor, possibly powered through the same cable as the signal.

--- End quote ---
Kleinstein:
The ADAR7251 is a sigma delta ADC and thus does not need much anti aliasing filtering at the input. For the frequency in question here (some 10-50 MHz) the filter could be just simple RC, maybe LC filter, if at all. Just the limited BW of the amplifiers may be enough.

Even with not so high sampling rate (e.g. 300 kSPS) the digital interface is quite fast. This requites a good board layout to keep interference away from the analog side. This also applies to all comparable ADCs - 4 fast 16 ADCs just produce a lot of data and thus need a fast interface.  Chances are the ADC would be coupled to an FPGA or maybe DSP, that can also produce EMI.

An important question is which frequency range is really of interest.
ogden:

--- Quote from: Kleinstein on August 11, 2019, 08:58:42 am ---The ADAR7251 is a sigma delta ADC and thus does not need much anti aliasing filtering at the input.

--- End quote ---

I would put it this way: every ADC needs anti aliasing buffer at the input, not every ADC needs low impedance signal source.
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