DAQami software cannot get the right data from DAQ (USB-1608GX-2AO).

[QiwenSheng]

I used DAQami to record data with a sampling rate of 20K. The DAQ is USB-1608GX-2AO. I used two channels CH0 and CH1 with Single-Ended mode. However, I cannot get data correctly. Attached are the original data and the data exported from DAQ.

Could you please provide me with any suggestions to get the right data? Thanks

[Fausto]

Hello @QiwenSheng.

The MCC USB-1608GX-2AO uses analog multiplexing to gate each channel, one by one, to the A/D converter. Internally, the input multiplexer has a capacitance between each channel. When it switches, the voltage difference from one channel to the next appears as an AC signal and passes through to the next channel. However, as long as the input signal has a low impedance, the charge dissipates, allowing the input signal to charge the input to the correct value before the following A/D conversion.

Recommend not scanning any open channels, which is not the issue here, and also recommend signal sources with an impedance of 100 Ohm or less.

What are the specifications of your input signal?

If not using all analog input channels, skip a channel between the connected signals.  Add a short wire between the channel high screw terminal (CH#H) and AGND, but do not add the skipped channel to the acquisition's channel list.

You may also want to check two other channels in case channels 0 and 1 are bad.

Regards,

Fausto

[QiwenSheng]

Hi Fausto,

Thanks for your help. Should "Add a short wire between the channel high screw terminal (CH#H) and AGND" be connected in the skipped channels or the tested channels? I am using the SE channel mode.

Edited March 19 by QiwenSheng

[Fausto]

The skipped channels.

[QiwenSheng]

Hi Fausto,

I added the short wires. I also tried different channels with SE mode. I still cannot get the right results. It is same with what I got before. Is this DAQ out of order? Thanks.

Qiwen

[Fausto]

Hello @QiwenSheng.

Thanks for your response.  From your original post, which channel is the blue data line and which is the red data line?  The blue data line appears to be correct.  Would you agree?

 

How was the 'Original data' created?  Did you use this USB-1608GX-2AO device or a different device?

Please provide a schematic of your wire connections, include your signal source(s).

Also, please provide screen captures of each channel setting in DAQami.

Lastly, disconnect your existing wires and connect the +5V terminal to the CH0 and CH1 terminals.  Launch DAQami and set these channels to single-ended and a sampling rate of 20 kHz.  Is the data shown for both channels at the +5V level or not?  If not, please reply with a screen capture.

 

 

Regards,

Fausto

[QiwenSheng]

Hi Fausto,

Thanks for your quick response.

1. The blue signal is from the channel 0, and the red signal is from the channel 1. I agree with you that the channel 0 shows correct data.

2. "Original data" is created by oscilloscope.

3. I connected the +5V terminal to the CH0 and CH1 terminals. The data obtained from these two channels are both around +5V. I think both channels are good. I also connected CH0 and CH1 to a function generator. The obtained data is consistent with the function generator output. The issue may come from the red signal source. The CH1 terminal was directly connected with a photodetector (Thorlabs - DET01CFC 1.2 GHz InGaAs FC/PC-Coupled Photodetector, 800 - 1700 nm, 8-32 Tap) via a sma cable. When I switch to the photodetector, the obtained signal shows unexpected increase. I can't understand why this does happen. Is there possible way to solve this issue? Thanks a lot.

 

Edited March 22 by QiwenSheng

[Fausto]

Hello @QiwenSheng.

Thank you for your responses and confirming the USB-1608GX-2AO device is working.

If the Thorlabs photodetector is connected to CH1, what's connected to CH0?

16 minutes ago, QiwenSheng said:

When I switch to the photodetector

Please elaborate what you are doing when you "switch to the photodetector".  Is there a physical inline switch?  Are you acquiring data from two different sources to CH0 and CH1?

The photodetector's manual indicates a high impedance output signal from the photodector.  MCC daq devices were designed to work with low impedance signal sources, 100 Ohm or less.  Use an op amp configuration to convert your high impedance signal to low impedance. 

If you confirmed that the output signal has a low impedance, then have you tried connecting the output signals to the USB-1608GX-2AO analog input channels in differential mode, as shown in Figure 6 of the user manual?  Recommend adding the reference resistor between the low channel (CH#L) and the ground (AGND) terminals.

 

Regards,

Fausto

[QiwenSheng]

Hi Fausto,

Thanks for your response.

Yes, I have two signal sources; one is connected to a function generator, and the other one is connected to a photodetector.

"Switch to the photodetector" refers that the acquired data from DAQ is correct when "CH0" and "CH1" are both connected to a function generator, but when I connect "CH1" to a photodetector, the acquired data shows incorrect value.

I think I may need to get an operational amplifier to make my DAQ work properly. Thanks for you assistance.

Qiwen Sheng