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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

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

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

seconds before

the next prompt appears. (The

8840A

automatically uses

this input to calibrate the offset for all ranges.) While the

8840A

is executing

calibration step, it ignores all of the

front panel buttons and delays execution of dl remote

commands.

6-1

INITIAL

PROCEDURE

Always begin the calibration procedure as follows:

Allow the

8840A

to stabilize in an environment with

ambient temperature of

18°C

28OC

and relative

humidity less than

75%.

Turn

the

8840A

on and allow it to warm up for at

least

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

pressed.

The following functions are inappropriate during calibra-

tion, and are therefore unavailable:

Offset

Autoranging

External Trigger

Figure

6-2.

First

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

performed in its entirety, and may not

per-

formed in part. If the

AID

calibration is discontinued prior

to completion, the last complete set of

calibration

constants will be retained unchanged.

perform

Calibration, proceed

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

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

near

the

value prompted,

and

displays

ERROR

it exceeds

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

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

Converter has undergone

repair, the

AID

Calibration procedure must

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

high

and low input for every range of

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Brand	Fluke
Model	8840A
Category	Multimeter
Language	English

Fluke 8840A User Manual

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Fluke 8840A Specifications

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