Fluke 8840A - Initial Calibration Procedure Steps; A;D Converter Calibration; Offset and Gain Calibration

Maintenance
CALIBRATION
installing the True RMS AC option, it may be necessary
only to perform Offset and Gain Calibration for the ac
functions, followed by High-Frequency
AC Calibration.
But if the
A/D Calibration is performed, it must be fol-
lowed by a complete Offset and Gain Calibration for all
functions and then by High-Frequency
AC Calibration.
Some of the calibration calculations are complex and take
the
8840A some time to execute. For example, when you
store the zero input during the Offset and Gain Calibration
for the
VDC function, it takes around
22 seconds before
the next prompt appears. (The
8840A automatically uses
this input to calibrate the offset for all ranges.) While the
8840A is executing a calibration step, it ignores all of the
front panel buttons and delays execution of dl remote
commands.
6-1 1. INITIAL PROCEDURE
Always begin the calibration procedure as follows:
Allow the
8840A to stabilize in an environment with
ambient temperature of
18°C to 28OC and relative
humidity less than
75%.
Turn the 8840A on and allow it to warm up for at
least
1 hour.
Enable the calibration mode by pressing the
CAL
ENABLE switch with a small screw-driver or other
suitable instrument.
(The CAL ENABLE switch is
located on the right side of the display and is normally
covered by a calibration seal.)
When the calibration mode is enabled, the
CAL annuncia-
tor lights up, and the
8840A displays the first prompt for
the
A/D Calibration procedure (Figure 6-2). To exit the
calibration mode, press the
CAL ENABLE switch again.
In the calibration mode, the front panel controls assume
the functions described in Figure
6-3. Some of these
functions are advanced features and are not required for
the basic calibration procedure. The display blanks briefly
when a button
is pressed.
The following functions are inappropriate during calibra-
tion, and are therefore unavailable:
Offset
- Autoranging
External Trigger
- -- --
Figure 6-2. First ND Calibration Prompt
Front Panel Trigger
Front panel SRQ (under local control)
Diagnostic self-tests
6-12. A/D CALIBRATION
The A/D Calibration procedure calibrates the analog-to-
digital converter for offset, gain and linearity. The
8840A
automatically selects the A/D calibration procedure when
the CAL
ENABLE switch is first pressed. The procedure
must
be performed in its entirety, and may not be per-
formed in part. If the
AID calibration is discontinued prior
to completion, the last complete set of
AD calibration
constants will be retained unchanged.
perform
AD Calibration, proceed as follows:
Ensure the Initial Procedure has been completed. The
8840A then displays the prompt for the first reference
source, zero volts (i-e., a short).
Each time the
8840A prompts you for a reference
source, apply the requested source to the HI
and LO
INPUT terminals, and press the STORE button. When
STORE is pressed, the numeric display field blanks
while the
8840A performs the necessary calculations.
(Do not change the reference source while the display
is blank.) The
8840A then displays the next prompt.
For reference, all prompts are shown in Table
6-8.
NOTE
The 8840A automatically checks that the refer-
ence input
is near the value prompted, and
displays ERROR 41 if it exceeds a specific
tolerance. (See Advanced Features
and Special
Considerations, later in
this section.)
After the last input is stored, the 8840A begins taking
readings in the
2V range of the VDC function. (The
CAL annunciator remains on.) Verify the
A/D calibra-
tion using the test points in Table
6-9. If you wish to
repeat the
AID Calibration procedure, press the AID
button.
The
A/D Calibration procedure is an iterative process.
Each pass through the procedure uses the constants stored
previously and improves them. Normally, one pass is
adexpate. However, if the calibration memory
has been
erased or replaced, or the
AD Converter has undergone
repair, the
AID Calibration procedure must be performed
tw~ce.
Because the
A/D Calibration procedure is iterative, the
instrument's performance can be slightly enhanced by
going through the procedure more than twice. However,
this is not necessary to meet the published specifications.
6-13. OFFSET AND GAIN CALIBRATION
This procedure calibrates the instrument's offsets and
gains by applying
a high and low input for every range of
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