Description of the problem
Why and when should the 100k resistor between IN- (Pin7) and GND (Pin4) be used?
Description of the solution
This is to avoid the effects of common-mode voltage and it also works as a high impedance ground connection in case your signals are floating.
If non-isolated sensors would be connected to separate channels without the resistor between GND and IN- (4 to 7) it could cause different potentials on the GND connection and this could damage the device.
When the measurement is in single-ended mode connection type. Better than 100R resistor is a short between the In- and GND as shown in the images from the manual.
Example:
If the single-ended measurement is used, the Sirius will do a short between the GND and IN- which is the same as shown on the picture.
The DEWE-43 doesn't have this feature, which is why the BNC adapters were made with 100R resistor (IN- -> GND).
From our experience, the measurements have shown better if there is a lower resistance between IN- and GND regarding the noise and interference. 100kR connection is high impedance which can cause interference on the measurement and can cause more noise.