Section 8. Operation
Note This section contains advanced information not required for normal
operation of the CR800.
Summary
• Voltage input limits for measurement are ±5 Vdc. Input Limits is the
specification listed in Specifications
(p. 91).
• Common-mode range is not a fixed number. It varies with respect to
the magnitude of the input voltage.
• The CR800 has features that help mitigate some of the effects of
signals that exceed the Input Limits specification or the common-mode
range.
With reference to the figure PGIA with Input-Signal Decomposition (p. 348), the
PGIA processes the voltage difference between V+ and V–. It ignores the
common-mode voltage, or voltages that are common to both inputs. The figure
shows the applied input voltage decomposed into a common-mode voltage (V
cm
)
and the differential-mode component (V
dm
) of a voltage signal. V
cm
is the
average of the voltages on the V+ and V– inputs. So, V
cm
= (V+ + V–)/2 or the
voltage remaining on the inputs when V
dm
= 0. The total voltage on the V+ and
V– inputs is given as V+ = V
cm
+ V
dm
/2, and V– = V
cm
– V
dm
/2, respectively.
The PGIA ignores or rejects common-mode voltages as long as voltages at V+
and V– are within the Input Limits specification, which for the CR6 is ±5 Vdc
relative to ground. Input voltages wherein V+ or V–, or both, are beyond the ±5
Vdc limit may suffer from undetected measurement errors. The Common-Mode
Range defines the range of common-mode voltages that are not expected to
induce undetected measurement errors. Common-Mode Range is different than
Input Limits when the differential mode voltage in non-negligible. The following
relationship is derived from the PGIA figure as:
Common-Mode Range = ±5 Vdc – |V
dm
/2|.
The conclusion follows that the common-mode range is not a fixed number, but
instead decreases with increasing differential voltage. For differential voltages
that are small compared to the input limits, common-mode range is essentially
equivalent to Input Limits. Yet for a 5000 mV differential signal, the common-
mode range is reduced to ±2.5 Vdc, whereas Input Limits are always ±5 Vdc.
Consequently, the term Input Limits is used to specify the valid voltage range of
the V+ and V– inputs into the PGIA.
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