Hello all,
I've begun using the NI 6225 DAQ (USB) a couple of weeks ago, and noticed some peculiar ghosting issues.
I'm using my DAQ to measure roughly 50 single ended analog signals.
I'm attempting this through Labview, using the DAQ assistant. All my inputs are configured as NRSE (non referenced single ended).
Scenario 1:
I measure 8 analog inputs. One input is connected to a low impedance 5V source and the others are open circuited. The result is that I see 5V across all inputs. This sort of ghosting makes sense because the DAQ's internal capacitor in the analog input circuitry does not have a discharge path.
Even if I remove the 5V source, I still see 5V across all inputs. This still makes sense because there is no discharge path.
The inputs are
AI 0 (AI 0+) --connected to 5V
AI 1 (AI 1+) --open circuit
...
AI 7 (AI 7+) --open circuit
Scenario 2:
Measuring 9 analog inputs. One of them is connected to a low impedance 5V source and the others are open circuited. The result is that I see 5V across all inputs. HOWEVER this time around, if I remove the 5V source, the inputs all read 0V.
The inputs are
AI 0 (AI 0+) --connected to 5V
AI 1 (AI 1+) --open circuit
...
AI 7 (AI 7+) --open circuit
AI 8 (AI 0- ) -- Notice that this channel is the negative differential input for differential measurements.
Does anyone understand what is going on with the DAQs internal circuitry that would make AI 8 alter the measurements in such a way?
Aside:
In my completed application, most channels will not be left open circuited. But it is a possible scenario,
so I intend measure a grounded dummy signal after every 'real' measurment, to discharge the capacitor.
