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Multiple USB-1808 analog In offset based on USB hub?



We have a setup of 6 USB-1808 devices on the same computer measuring the same analog input on the first channel (CH0) of each device. We are using two USB hubs where each of them has 3 devices connected. The problem is that there is a voltage difference of about 10mv between the devices connected on different USB hubs.

Here are the setup details:

  • 6x USB-1808 on the same machine (Ubuntu 20.04).
  • Two USB hubs (Anker A7516), each of them has 3 of the 6 devices connected.
  • Input signal (DC 1V) is connected to CH0 of each device (CH0H, CH0L and AGND).
  • Devices are configured to measure with a frequency of 10kHz and an input range of BIP10VOLTS.

Here is a plot of the data from a 5 seconds measurement. The USB1808_1, USB1808_2 and USB1808_3 are connected to the same USB hub whereas the remaining 3 devices are connected to the other USB hub.6xUSB1808_CH0_3.thumb.png.859103cf6c63914edeb4cd2978e395f7.png

As you can see there is about 10mV offset in the measurements of the devices connected to different hubs. There is also a visible difference in the noise. Additionally, we would expect the average voltage to be around 1V but both hubs show about +-5mV offset which seems quite high for 18bit resolution. The only difference in our setup is that the devices are connected to different hubs.

What we have tried:

  • Different input signal sources -> no difference
  • Connect only on device per hub -> no difference
  • Switch the devices on the hubs -> no difference, measured values for each hub stay the same regardless of which devices is connected

It seems that all devices connected to the same hub measure the same voltage and there is a difference between the hubs.

Any help would be appreciated!

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Hello @flomich.

The Anker A7516 is an USB powered hub.  First, test each USB-1808 connected directly to the target system, so you have a baseline.  What are the system specs?  What are the results of each test?  Next, use an externally powered USB hub for your tests.



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