USB-1608-GX - scattering on analog signal

[Matthias M]

Hello,
i am using the digilent USB-1608-GX. On the analog inputs (differential mode) i noticed a scattering of approximately +-6V, when there should be 0 V. The resistance of 100kΩ between CH0L and AGND (recommended by the manual) does not solve the problem. If i put the resistance between CH0H and AGND, the value shows about 0V, like it should be. 
Is there any risk to use the resistance between CH0H and AGND?
Could it be that the Single Ended Mode is used although the Instacal software noted Differential Mode?

Kind Regards
 

[JRys]

The USB-1608GX is designed to work with low-impedance signal sources. Inputs that are unconnected (open) are high-impedance and will charge up to various stray voltages. A reference resistor from the channel's low side is only needed if the connected signal is isolated from the ground. If you want to verify the input is in differential mode, connect a 1.5-volt battery between the channel high side and the analog ground. This should produce an inaccurate measurement. Then, connect the channel's low side to the ground, and the measurement should be accurate. You could also connect the battery between the channel's high and low. In this case, the battery is an isolated source, so you must add the reference resistor between the low channel and the ground.