Fluke 8845A, User Manual

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D-
1
Appendix D
Analog Filter Applications
Introduction
The Meter’s analog filter is intended to reduce the presence of external ac when making
dc measurements. Most applications do not require the use of this filter, but in some
situations it can be utilized to improve DC measurements. A good example of this is
using the filter when measuring the DC value of a signal with AC content, such as a DC
power supply voltage that has significant AC line ripple present.
The analog filter is not intended to reduce the internal noise within the DMM, and usually
has no affect in reducing noise when measuring an open circuit in DCI, measuring a short
circuit in DCV or Resistance, or measuring the output of a precision DC calibrator. In
fact, the analog filter may actually add to the noise in these situations, and often times
offsets the reading. Because of the latter phenomena, when the analog filter is used, the
DMM must first be Zeroed in the range, NPLC setting, and triggering approach used. If
your application makes it difficult to first ZERO the instrument, the offset can be
characterized, and the typical errors are shown in Tables D-1 through D-3 can be used.
For those ranges and NPLCs not shown, there are usually no additional errors associated
with using the analog filter.
Table D-1. DC Volts Analog Filter Errors
Range NPLC Additional Analog Filter Error
1, 10
1.5
μ
V
0.2 12
μ
V
100 mVdc
0.02 40
μ
V
10, 100 0.0002 V
100 Vdc
<10 0.001 V