ADS1291 short inputs not 0

Hi Megan,

Welcome to the e2e forum! Your problem could be either HW or SW related. What code(s) do you see on DOUT when you short the pins together?

The output stream looks like this :

Hi Megan,

So the 0x7FFFF values represent full scale + input, what signal was applied during this test? Besides the non '0' values, are you able to communicate with the ADS1291? Have you been able to read andwrite to the registers to verify that your SPI interface is working correctly?

There was no signal applied during the test. The inputs were just exposed to air and shorted to each other. We thought the signal should go to 0 when the inputs are shorted, even if no signal is applied.

We are able to communicate correctly. When the register is written to, we read from it to check that the change was correct. If the electrode is worn for EMG, we get consistent signals that make sense. We are just seeing this weird behavior in "on the desk" situations when the electrode is not worn.

Hi Megan,

If the inputs are 'open' (no signal applied, no short to ground, not shorted together), I could see you getting a varying set of output data. When you short the pins together though, you should get near zero output. When you say you are shorting the pins, is that actually at the chip or is that at the electrode end of your input?

When we are shorting the pins, it is not quite at the chip level. There are 22k resistors (1%tolerance) between the pins and the electrode inputs. There are also bi-directional zener diodes from the line to ground for ESD