Calibrating The 3478A - HP 3478A Technical Manual

3478A
stant is then calculated and stored into the Calibration
RAM. The constant is used to calculate the correct
readings.

4-80. Calibrating the 3478A

4-81. The 3478A is calibrated by first applying a zero
input (INPUT Terminals shorted) and then applying a full
scale input (or in some cases a 1/3 scale input). An
example on how calibration is performed is in the
following steps. The calibration of the DC Volts
Function's 3V Range is chosen for the example. Other
ranges and functions are similar. It is suggested to go
through the example before using the calibration
procedures, to gain an understanding of how the 3478A
is calibrated.
a. Set the 3478A to the DC Volts Function and the 3V
Range. Make sure the instrument is not in Autorange.
b. Short the instrument's HI and LO INPUT Terminals
(zero input).
c. Using a small flat blade screwdriver, set the front
panel CAL ENABLE Switch to the calibration enable
position (the slot of the switch is in the vertical position,
as shown on the front panel).
d. Set the multimeter to the Single Trigger mode by
pressing the SGL/TRIG button.
e. A zero reading, or an approximate zero reading,
should now be displayed. In addition to the reading, "C:"
is also displayed to the right of the reading (e.g. " +
0.00002C:").
f. Press the blue Shift button and then the LOCAL
(CAL) button.
g. The 3478A then makes a measurement and
decides what type of input is applied to the instrument.
Since it is a zero input, the 3478A should now display +
0.00000? VDC. (The displayed "C:" is replaced by a
",?".)
h. Press the SGL/TRIG (Single Trigger) button only
once.
i. The 3478A will display "CALIBRATING" for about
five seconds and then "CAL FINISHED" for about two
seconds. The zero calibration constant for the DC Volts
Function in the 3V Range is now stored in the
instrument's Calibration RAM.
j. Remove the short from the 3478A and apply an
accurate + 3V to the HI and LO INPUT Terminals. For
best accuracy, use a + 3V calibration source of which its
exact value is known (e.g. +3.00015V, +3.00005V, etc.).
k. Press the blue Shift button and then the LOCAL
(CAL) button.
l. The 3478A again makes a measurement, and this
time decides that the applied input is + 3V. The display
should now show "+ 3.00000? VDC".
voltage is another value, like + IV for example, the
display will show the other value (" + 1.00000? VDC").
m. If the exact value of the calibration source is
unknown, but the source is accurate enough to calibrate
the multimeter, continue with step o.
n. If the exact value of the calibration source is known,
the 3478A can be calibrated to that known value. The
only thing to remember is that the input value cannot
deviate from the absolute full scale (or 1/3 scale) value
by more than + 7%. The calibration is performed as
follows:
1. After the CAL button is pressed and the 3478A
has decided that the applied voltage is within +
7%o of +3V, the instrument displays "+3.00000?
VDC". The reading on the display can now be
changed using the Uprange and Downrange
buttons until the applied value and displayed value
agree with each other.
2. If the source is above + 3V, press the 3478A's
Uprange button until the displayed reading agrees
with the source.
increment the reading by one count. To continue
increasing the displayed reading, press the button
repeatedly. For example, if the source is +
3.00015V, press the Uprange button 15 times, until
"+3.00015? VDC" is displayed. If the button was
pressed too long and the displayed reading is too
high, press the Downrange button until the values
agree.
3. If the source is below +3V, press the 3478A's
Downrange button until the displayed reading
agrees with the source.
same as using the Uprange button, except the
displayed reading is decreased rather than
increased. For example, if the source is +
2.99985V, press the Downrange button until
"+2.99985? VDC" is displayed.
o. Press the SGL/TRIG button only once.
p. The 3478A will display "CALIBRATING" for about
five seconds and then "CAL FINISHED" for about two
seconds. The 3V calibration constant (gain constant) for
the 3V Range in the DC Volts Function is now stored in
the instrument's memory.
q. If no more calibration is to be performed, set the
3478A's CAL ENABLE switch to the horizontal position.
The instrument is now ready for use.
4-12
TM 11-6625-3071-14
If the applied
Pressing the button once will
This operation is the