USB-6229 analog input coupling of channels

[slee6229]

Hi,

I have a new USB-6229 DAQ that I'm currently testing out.  I'm testing out the analog inputs.

I created multiple analog input channels in the NI MAX software.  Using a process calibrator, when I inject a voltage signal (eg. 1 V) to the first channel, the rest of the channels also show up with 1V (they are not connected to any signal).  When I inject a second voltage signal to the next channel (different voltage value eg. 1.5 V), the value of the first channel isn't 1V anymore, but slightly higher.  For some reason the signal from neighbouring channels influence each other.

I can get correct signal readings if I don't use neighbouring channels, but if I skip every other channel.  This will work for me, but it makes my USB DAQ only 50% functional. 

Is it the DAQ? Am I doing anything wrong?

Cheers, 

[johnsold]

This is known as ghosting. The input multiplexer which switches various inputs to the single A/D converter in the device has capacitance which can charge up  and "carry" an input voltage to another channel.

The solution is to drive the inputs from low impedance sources, such as an op amp. Any unused inputs should be connected to ground or some low impedance source within the common mode range of the input.

Lynn

[slee6229]

Thanks for the reply.

Is this ghosting common to some or older NI DAQs?  I understand that the USB-6229 is an older DAQ.  Are newer ones less prone to ghosting?

I assume when you say operational amplifier, you mean something like a signal conditioner, right?

Just searching through the forums on ghosting, seems like some people suggest to delay the time it reads from channel to channel to minimize the ghosting.  Sorry to go off topic abit, but I can't seem to find the option to change the timing of switching between channels in the MAX. 

[johnsold]

Ghosting may occur on any DAQ device which multiplexes inputs. That generally includes all multichannel devices which are not "simultaneous sampling" devices.

A signal conditioner can be used to minimize ghosting. 

I do not have MAX so I cannot say how (or whether you can) change the interchannel delays. Many datasheets for DAQ devices have graphs showing the effects of settling time as a function of source impedance. The device manuals may also have information about setting the switching times.

Lynn