HP E1563A User Manual page 18

3-Wire and 2-Wire
The HP E1563A and E1564A Digitizers provide a three-terminal input
system (High, Low and Guard). An unavoidable and undesirable current is
Input Cabling
injected from chassis ground to the Guard terminal. Dependent on whether
Considerations
you measure on a low-voltage range or a high-voltage range, the way you
connect the Guard terminal may or may not introduce a measurement error
due to this current. This section describes some considerations you must
take to use the Guard terminal properly to minimize measurement error.
Digitizer Input Model
The input model for the digitizer is shown below. Maximum voltage
between Low and Guard is 5V.
Note
Maximum voltage between Low and Guard is 5V. Exceeding this
limitation will not damage your digitizer but will generate invalid data for
any measurement taken.
Three-Wire Connection
This section shows two examples of connecting the input using a three-wire
connection. Both examples can be constructed using shielded twisted pair.
The first example shows making connections for a bridge measurement
where the L-to-G voltage is ≤ 5V and where the L-to-G voltage exceeds 5V.
A "Wagner ground" is used to satisfy the L-to-G restriction of ≤ 5V and to
make a Guard connection point that minimizes measurement error due to the
digitizer's injected current. A capacitor is added to the Wagner ground to
provide a signal path to ground to minimize common mode voltages. The
second example shows measuring the voltage across a small current sensing
resistor where the input to the digitizer is switched through a multiplexer
switch module.
3-Wire Cable Connection Example 1
18 Digitizer Module Set-up