Multiplexing Time Delay on the MCC USB-1608GX

[Marc Pearlman]

I am running the MCC USB-1608GX with DAQami.

As a test, I have the same sine wave connected to 4 different analog input channels (DE).   I'm noticing phase differences in the signals being read from the four channels.   These phase differences also seem to vary over time.

(As one example, I'm seeing this with a 30 kHz sine wave,  with all analog input channel sampling at 62.5 ksps)

I suspect this is due to the ADC multiplexing.   Is that the case?

Is there any way to minimize this behavior, and/or adequately account and correct for it?

Thanks!

[JRys]

Hello,

The USB-1608GX has a single ADC with multiplexed channels and because of this, there will be a phase difference from channel to channel. The amount of time between each channel depends on the aggregate sample rate. For instance, four channels times 62.5k S/s is 250k S/s. To figure the time between each channel, take the inverse or 1 / 250k or 4uS. At 500k S/s, it's 2uS. The USB-1608GX can run in one of two modes: Burst and non-Burst. Burst Mode forces the device to rapidly sample the channels irrespective of the sample rate. The only way to take advantage of Burst Mode is to create a C or Python program using the Universal Library. MCC's higher-level programs, like DAQami, TracerDAQ, DASYLab, or LabVIEW, cannot enable it.

There is a downside to Burst Mode. Switching from channel to channel at a high rate reduces the channel settling time. If the connected signals have an output impedance of 100 ohms or less, it is usually not a problem. However, large output impedance sensors, such as resistor dividers, can cause crosstalk between channels because they require more settling time.

Best regards,

John