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D-1
Appendix D
Making Mixed Measurements
Introduction
This appendix augments the discussion of ac signal effects on other channels (cross talk)
found in Chapter 5 ("Making Mixed Measurements"). Effects on each measurement
function are discussed below. These numbers should only be considered as references.
Since cross talk can be introduced into a measurement system in many places, each setup
must be considered individually.
The effect of cross talk could be much better than shown for "Typical"; in extreme cases,
the effect could be worse than the "Worst Case" numbers. In general, the "Worst Case"
information assumes that none of the guidelines for minimizing cross talk (Chapter 5)
have been followed; the "Typical" information assumes that the guidelines have been
followed where reasonable.
These numbers assume that input L (low) is tied to earth ground; refer to "Using
Shielded Wiring" in Chapter 5. For dc volts and thermocouple temperature
measurements, a source impedance of 1 k in series with the H (high) input is assumed
(except where otherwise noted.)
Signal Cross Talk in a DC Voltage Channel
DCV Error Ratio (CTRR) =
VDC error
VACrms
()
Worst case Typical
50, 60 Hz, ±0.1%: 1.1
× 10
-7
2.0 × 10
-8
Other Frequencies: 3.8 × 10
-6
8.6 × 10
-7
For example, to find the typical effect of a 300V ac signal at 60 Hz on another channel
for the 300 mV range, you would calculate: 300
× 2.0 × 10
-8
= 0.01 mV.