How is multiplexing accomplished on the NI-6221?

[amifer]

Hi, I am using the NI-6221 to acquire signals from 8 sensor probes (Ion Selective Electrodes, input impedence ~100Kohm - 10Mohm).  I am seeing what seems to be carry-over between data acquisition channels, where all 8 are set up as double-ended analog inputs, and typical inputs are <500mV.  To diagnose what is going on, I have a few questions:

1) In what order are the inputs sampled?

2) The instruction manual indicates that the input impedence ( for AI+ to AI_GND) is 10Gohm when "device is on" and 820ohm when "device is off".  Do these numbers apply similarly to input impedence for "AI+ to AI-" connections?

3) Referring to #2, does "device is on" mean "when the dataq board is powered" or "when the in-amp for that specific channel is powered"?  i.e. Does the method of multiplexing (switching on/off the in-amp?) mean that the input impedence seen at pins "AI+ to AI-" switches between 10Gohm ("on") and 820ohm ("off")?

Thanks in advance for any help!

-Amy 

[John_P1]

Hi Amy,

Welcome to the forums! 

It sounds like you are experiencing what we refer to as ghosting.  Do you mean that the output impedance of your electrodes is between 100K and 10 M ohm?  If this is the case, then this would explain the behavior you are seeing.  You can refer to the link I posted for more information, but essentially the high source imepdance will increase the settling time of the instrumentation amplifier to the point where the charge built up from the previous sample does not discharge in time for the next sample.

To answer your specific questions:

1.  The inputs are sampled in whatever order you specify.  For example, the following Physical Channel String will sample (in order) ai0, ai3, ai2:

Dev1/ai0, Dev1/ai3:2

2.  I'd have to verify this one to find an exact value, but the impedance between AI+ and AI- in differential mode should be of the same order as that between AI+ and AI GND.

3.  "Device is on" means that the device is powered.  If you were to connect a signal to your DAQ card with your computer off then you would expect to see the 820 ohm impedance.

The other steps mentioned in my first link might help with the problem somewhat, but the best recommendation I can give is to implement a Voltage Follower to reduce the source impedance before inputting the signal to the DAQ device.

Best Regards,