HP E1563A User Manual page 19

3-Wire Cable Connection Example 2
Two-Wire Connection
When Low and Guard are simply connected together at the digitizer's input
on a low-voltage range (4V and below), the injected current is directed to
flow through the source impedance (in a floating source) and the resultant
voltage drop will introduce a measurement error. The resultant voltage drop
through the source impedance can be a significant error on low-voltage
ranges where the voltage of interest is small. It is not as significant an error
on high-voltage ranges simply because the error introduced is not a
significant part of a larger voltage; the percent of error is less significant.
Measurement error can increase significantly when you connect Low to
Guard at the digitizer's input AND use switches to switch input signals to
the digitizer. Some switches have input protection resistors (usually 100Ω)
in series with the switch. The digitizer's injected current now generates a
voltage drop across this resistor in addition to the voltage drop generated
across the source impedance. Even with a grounded source, an error voltage
is generated across the switches current limiting resistor.
2-Wire Cable Connection Example
Digitizer Module Set-up 19