analog generation discontinuity

[Denes]

Hi, it was suggested that I post this question here as opposed to on the Labview board where it was originally posted. 

I am working with an NI USB-6221 BNC DAQ and trying to output a continuous sine wave from the analog out.  Eventually it will be output to an external device, but I'm testing it right now and am getting a discontinuity in the received signal on the same device with the analog input.  I am using the sample programs Acq&Graph Voltage-Int Clk.vi & Cont Gen Voltage Wfm-Int Clk.vi, and am outputing a 1 kHz waveform at a sampling rate of 200 kHz, well within the range of the board. and sending one complete cycle to the buffer.  Using only one channel on the recording I am inputting at 192 kHz, which I understand shouldn't be a problem since I am not sharing the input A-D with other channels (since this is my only task).  The problem is noticeable when the amplitude drops below 80mV.  

In response to the questions on the other post, I took a shorter recording to show individual cycles on the input with the sporadic spikes.  As can be seen in the pictures, the spikes are occuring at the zero crossings of the waveform.  To hopefully rule out external interference, I disconnected the BNC cable from the output while still generating and recorded, and had a flatline (as would be expected), I also set the output amplitude to 0V, and again recorded a flatline.  Any help would be greatly appreciated

Thanks,

Sam

[Ben]

For a quick test check to see if if changing the output to 10 complete cycles changes the freq of the spikes.

It almost looks like the are gaps in the output.

Ben

[Chris_W. · Accepted solution]

Hi Sam,

The behavior you are seeing is likely to be a known and specified behavior for the anlog output of this board. This looks like the dissipation of charge that accumulates in the DAC during normal operation. This behavior is specified under "Glitch Energy" HERE in the specifications for the NI 6221. Since it is specified as 100mV, it makes sense that you are only seeing this during generation of smaller voltages, since the SNR is much less. One option here is to implement a low-pass filter to try to get rid of that glitch on the output of your 6221.

I also found another forum addressing the same issue HERE.

I hope this is useful. 

Chris W