DrDAQ accuracy error when loading the 5 V USB Bus

Glovisol · Post by **Glovisol** » Thu Nov 12, 2015 4:03 pm

There seems to be an accuracy error in the EXT. channels with the DrDAQ working with Picolog when the + 5V USB bus is loaded with an external load. This error amounts to 5 mV for every 100 mA of extra load to the bus.

To explain in detail.

The EXT. 1 channel is measuring a fixed voltage of +1,250.0 mV. The scaling equation multiplies this value by 4 and the display on Picolog shows 5,000.0 mV. Applying a load of 100 mA (with a simple resistor) the reading becomes 5,005.0 mV, while an external monitoring DVM checks that the 1,250 mV input to EXT.1 does not change. With a bus load of 200 mA the Picolog reading becomes 5,010.0 mV. Note that we are still well within the USB +5V specification with these loads.

I wonder where the fault is: in my measurement, in the DrDAQ, in Picolog or perhaps in the Picolog - PC interface?

I am asking the staff to verify this problem.