Minimum sample rate for 10KHz square wave with MIO

[Steverhnm]

Need to display a 10KHz square wave input to a 16E MIO. Presently sampling 1 channel at 400KS/s. Gives me 2.5uS/sample. Even at 800KS/s-same thing. How come I can't get a decent picture of the waveform. Presently I see a ramp up-down akin to an RC time constant. At 1KHz I get a decent waveform. I'm wondering if I have something set wrong in my program. Did I miss something in determining the proper sample rate?

[Ben]

Hi Steve,

Are you sure you are sampling at 400 and 800 KS/s?

I thought those boards only go up to about 100KS/s.

(show your point on your plot. Are you getting 40 samples per period?)

If you really are sampling that fast, then you have partially answered your own question. There is a small capacitance at teh input to the board. A high impeadance source will combine the input capacitance of the board to give you the charge ramp you mentioned. Only solution to this situation is ussing a low impeadance output buffer between your signal source and the AI.

Ben

[RIO-Todd]

Hey Steve,

The MIO-16E-1 has a maximum sampling rate of 1.25 MS/s, so if you're using that device you'll be able to sample that fast.

Source impedance is usually an issue when acquiring from multiple channels, but it could be causing problems here. If your source has a somewhat large impedance (around 1 Kohm or greater) and you are sampling at high rates, then you might be running into charge injection of the instrumentation amplifier. I would take a look at the document linked below that describe using a non-inverting buffer to decrease the source impedance dramatically.

Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device
http://zone.ni.com/devzone/conceptd.nsf/webmain/CD57A73721E0612586256BAE0055CDD9?opendocument

Regards,

Todd D.
NI Appl
ications Engineer