In a specs of a data acq. device the input impedance is shown as "20 MΩ differential". Does that mean the measured impedance in diff input case? If so how can I calculate the input impedance for single input case? Here is the device: http://www.mccdaq.com/PDFs/manuals/USB-1616HS-BNC.pdf
It depends on how the inputs are designed. The instrumental amplifiers have very high input impedance (almost infinity) so, these 20MOhms are result of the input dividers. And what will be the impedance, if you connect one of the differential inputs to the ground, depends on the schematic.
There are generally two variants, simplified shown on the below schematic. If you ground the negative input, the left schematic will have a half of the differential impedance - i.e. 10MOhm, but the right one will have the full 20MOhm.
In this case the 20Mohm is the (single ended) input impedance of any of the BNC's as measured between the pseudo-floating shield and center conductor.
The word "differential" is probably left in there because that is what it said in the ADC chip datasheet and no-one cared too much about this. And you have inputs that are pseudo-floating in that you can have different voltages on the shields of two BNC's (within the common mode range - 6V or 10.5V).
Does that help you?