Electrical – Is buffer needed for very long coaxial cable carrying DC analog signals coupled to an ADC with a huge input impedance

Someone else will most likely pop in with a better answer but as far as I know, the RC circuit has two components.

1) the SAR needs to see a stiff input when it samples the input. That could be either a capacitor or an op-amp that is stable with a heavy capacitive load.

2) most o-amps are NOT stable with heavy capacitive loads. Especially fast op-amps.

So the R isolates the op-amp from the heavy capacitive load presented by the SAR. But now the input to the SAR will sag because of the resistance.

The capacitor is added so that the input signal seen by the SAR doesn't sag very much when the signal is sampled.

In other words, even though it looks like a RC low-pass filter, that is not the intended result. Just consider it to be a freebie bonus!

Buffering may not necessarily be the issue. I would double check that the chip selects aren't being asserted for multiple sensors at the same time. If more than one is asserted, this would describe the exact problem you are having where the data coming back is bad, and removing one device fixes it. This is due to bus contention (each sensor is trying to drive MISO, whereas only one should).

You mentioned using a multiplexer to select the chip selects, I think you meant to say decoder? (like a 74HC138 3 to 8 decoder). That's what you need for a circuit like this.

If you are sure you need to buffer the MISO outputs, then you can use a multiple-buffer part like the 74HC125 and have each sensor's MISO output run to an independent buffer on the '125. The decoder producing the chip selects would enable the buffer output such that only one device has an enabled buffer at any time.

Also try running things at a low speed first. You can always crank up the SPI clock rate after you've verified operation in the KHz range.