What kind of multiplexer is on the USB-6281 card?

[kontor]

Hello Everybody,

I am performing some accurate measurements based on a complex voltage ratios. I am using NI USB 6281 card. I need to know what kind of multiplexer is used on that card. Especially I wish to know what is on resistance, off capacitance or even the value of the on resistance against the input voltage. Maybe somebody can tell me a name of the counterpart which I can find on analog.com or similar website.

Best Regards,

Kontor

[Henrik_Volkers]

The problem you have is the not buffered input stage with this multiplexed card. See settle time error over source impedance in the spec.....  BTDT

You can measure the complex input impedance with a AC source and at least  two different, known source impedances.  But you need to do that for each samplerate & channel setting , and related to the absolute values of both channels. And you need to check if this values are stable over temperature and time.......

If you stick with this card, use good input buffers with low output impedance. (if you can't avoid the error, try to make it as small as possible 😉 )

If you want it precise, avoid the multiplexer, use real synchronious sampled channels, do the measurement twice with swapped channels and use good, low EMF relays for that.... my two cent 😉

[kontor]

Hello Everybody,

I agree with you Henrik. Multiplexers inputs are buffered, in other words, voltage follower is added on each multiplexers input. The thing is that on resistance of the multiplexer with the off capacitance of the multiplexer forms a voltage divider. The voltage applied to the off channel is added in little part to the measured signal applied to the on channel. I can see it in measurements. I want to make some corrections and to do that I need to know these parameters.

Best Regards,

Kontor

[Henrik_Volkers]

well, if you have a model (and it seems like you have one , since you asked for some internal values) how about applying known signals and do a parameter fit?

What type of signals do you use for your ratio measurements? 

[kontor]

Hello Henrik,

There is no possibility to measure accurately the on resistance of the multiplexer because it is in series with big impedance of the amplifier at the output of the multiplexer. Off capacitance may be measured more accurately.

I measure voltages in the impedance bridge to determine the impedance ratio. I wish to obtain as accurate results as it is possible with this card. No other equipment aside from voltage follower.

Best Regards,

Kontor

[Henrik_Volkers]

If you can't measure it, it doesn't hurt you.
Again: If you have a model of our input it's just Kirchhoff and some linear algebra.

If you have a input buffer before the mux, tau build buy mux on resistor and C is what you want. A common way to measure tau is to apply a step and measure with different sample rates . The settle error ... See spec.