ADC saturating for no apparent reason

[joshdoe]

The problems with my DAQ never seem to end.

I've got an Apex Stratus X ND ADC and an RTD DM6620 DAC in a PC104 system. The DAC creates a 15kHz 1000-step triangle wave that provides modulation to an RF generator in an NMR system. The NMR signal goes through an amplifier, and then to the ADC using differential inputs. The ADC acquires the NMR signal at every step, generating a waveform that enables us to find the polarization of our material.

The ADC is saturating at all times, for every step, reading a value of 0, which is negative rail voltage. On an oscilloscope I see the signal fine (XY mode with triangle wave for X), even when checking the pins on the ADC circuit board itself. And the level is about 1V, much lower than the -10 to 10V scale the ADC is set to. When I feed the triangle wave directly into the ADC, everything is fine.

What could be the problem? Thanks for any input, I've really appreciated the members of this forum.

 

[melone]

Is the IC wired properly, i.e. + and - rails? Are the IC power pins decoupled?

 

[IRstuff]

Are you sure that you not asking for data too soon?

TTFN

 

[logbook]

Fishy. The ADC works when fed with a simple signal generator but fails to work with a similar signal level from another source. You have monitored the signal with a scope in both cases so you know the input to the ADC is correct. In this case it sounds like you have a digital problem. The thing you are connecting is somehow shorting the digital outputs and messing things up. Possibly an extra power supply is introduced that is also grounded but on a different rail so that the power rails get shorted. It sounds like this sort of problem.