2635A
Users Manual
1-14
Caution
Do not exceed the specified input voltage levels or equipment
damage could result.
Measurement Connections

W Warning

To avoid electric shock:
• When the input module is installed, consider all channels
with connections as accessible terminals that may be
hazardous live.
• Disconnect the input module before touching or changing
external wiring.
Input connections include the front panel terminals (channel 0), rear panel connections
using the Universal Input Module (channels 1 through 20), and I/O functions using the
ALARM OUTPUTS and DIGITAL I/O connectors. The instrument is protected from
channel configuration errors. For example, accidentally applying 300V ac to a channel
configured for resistance measurements will not damage the instrument.

Using Shielded Wiring

Shielded wires and sensors (such as thermocouples) should be used in environments
where "noisy" voltage sources are present. When shielded wiring is used, the shield is
normally connected to the L (low) input terminals for each channel. Alternate
configurations should be examined for each equipment application.

Crosstalk

The instrument allows the mixing of various types of measurement. A phenomenon
known as crosstalk can cause one signal to interfere with another and thereby introduce
measurement errors. To reduce the effects of crosstalk in making measurement
connections, do the following:
• Keep any input wiring carrying ac volts signals physically separate from the input
wiring of other sensitive channels.
• Avoid connecting inputs with ac volts signals adjacent to sensitive channel inputs.
Leave unconnected channels between the inputs, if possible.
• Avoid connecting inputs with ac volts signals to any channel 10 numbers away from
a sensitive channel (i.e., 4-terminal input channels).
• Avoid tying L (low) or (especially) H (high) inputs of a sensitive channel to earth
(chassis) ground. This is very important in resistance measurements.
• Avoid high-source impedances on sensitive channels, or minimize the capacitance of
the sensitive channel to earth (chassis) ground for high impedance inputs.
• Whenever high ohms measurements (>10 kΩ) must be made accurately, avoid
connecting any inputs carrying ac volts signals.
Measurement errors introduced by crosstalk are discussed in Appendix B.

Universal Input Module Connections

For channels 1 through 20, use the H (high) and L (low) inputs on the rear panel
Universal Input Module, as shown in Figure 1-5. Perform the following procedure to
make connections to the Universal Input Module: