I'm using the LTC2308 device to digitize 6 input channels. I've noticed some error on two of my channels. As a test I set my FPGA to read out channels 0-5 (so read and digitize the next channel) so I'm recursively running through 0-5 in order of 0-1-2-3-4-5. I have taken ch1 - disconnected it from my circuit and connected it directly to ground via a 113k resistor so there is no external circuitry connected to channel 1 (second channel). I see the below voltage spike on the input. I've also proved the clock and it looks like it happens as soon as the 6'th bit in the serial link kicks off the sample and hold/digitize function. Per the data sheet, after the 6th bit the sample and hold cap is enabled. This looks to line up perfectly. It also looks like this spike lines up with every 6th transmission which would sync up with the 6 channel loop. Said differently,it looks like every time I start the sample/conversion for channel 1 I get this spike.
Here is a snap shot of the LTC2308 circuit. Note - I have channel 1 grounded with a 113k resistor and thats it.
Does anyone know where this voltage spike may be coming from? An adjacent channel? Channel 0 has no activity and looks like this:
This current sense circuit (for this example) is undriven. The output of the amp U14 is at 0 volts. When I measure the point/net that connects R54 and C27 (the "CUR_TOTAL") node I get ~76mV when nothing is running but the ADC. When I measure the output of the amp u14 I see 0v. It looks like the LTC2308 INPUTS are actually outputting a voltage.
I think I have this isolated down though for this test example where channel 1 isn't even connected to any circuitry and I still see this spike.
Any help would be greatly appreciated!