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6.4.
6.4.1,
6.4.2,
6.4.3,
45
LOCAL CONTROL
Function selection
'The measuring functions are set by pressing the FUNCTION push-buttons, When a function is selected the
corresponding LED in the push-button lights. Vpeak measurements can be made only if the peak voltage
option PM9259 installed in the PM2528.
Vhf measurements can be made only if the Vhf option PM9258 is installed.
Zeroing the offset voltage and the input current
Input offset voltage
Due to a thermal emf at the input sockets an offset voltage can arise. This offset voltage is compensated in the
following way. Short-circuit the input terminals 0 and V-Q (Q interconnected to GUARD).
Select the HIGH RES made, INT START mode, AUTO RANGING and FUNCTION
V = -
The displayed value is the offset voltage. By pressing the push-button OFFSET twice the offset voltage is
compensated. When the push-button OFFSET is pressed twice again (under the above-mentioned conditions)
the offset voltage, which can be changed due to warming up, is displayed. This offset can be compensated by
pressing the push-button OFFSET twice again. The compensation is maintained for ali measurements.
The maximum offset voltage which can be compensated is 80xV.
The LED in the OFFSET push-button is not lighted during zeroing of the input offset voltage.
input current {only in the 200M
22 and 2000M Q range)
Whenever the input current of the PM2528 is very low it can influence the measuring result at very high
resistance measurements (200M Q and 2000M Q range}.
An input current of 10pA wil! cause an additional error of 1% in the 2000M Q range and an additional error
of 0.1% in the 200M Q range.
In the other ranges and functions the error caused by the input current is negligible.
To reach a very high measuring-accuracy in the 200M & and 2000M Q range the input current can be
adjusted, For zeroing the input current proceed as follows:
— Select the function V = and the modes AUTO RANGING, HIGH RES and INT START.
= Short-circuit the 0 and V-Q terminal and zero the input offset voltage as described above.
—
Remove the short-circuit. The displayed value will increase or decrease contiiuously, The reason is that the
input capacitance is charged by the input current. Therefore the input voltage slopes up or slopes down.
The input current can be adjusted with the potentiometer mounted behind a hole in the front panei of the
PM2528. This hole is situated between the 2.5 A FUSE and the V .= push-button,
—
Turn the potentiometer clockwise when the displayed value decreases (becomes more negative) or counter
clockwise when the displayed value increases (becomes more positive).
Continue untii the displayed value neither increases nor decreases (average) more than 1mV per
measurement.
Relative reference mode
In the relative reference mode the difference between a predetermined value and a measured value is displayed.
This mode can be used in the functions V =, 22W, 24W, A = and °C only, within one range (autoranging
and manual UP and DOWN ranging are not permitted).
Supply the predetermined value (offset value} to the input terminals (V=< and Q2W to the 0 and V-Q socket,
A = to the 0 and A socket, Q24W and °C to the PROBE input), Press push-button
OF FSET (LED must ignite, when
LED is already on, switch it off before by pressing OFFSET). After one measuring cycle, which can be
started internally, manually or externally, the predetermined value is stored in the memory of the PM2528.
At the following measuring cycles, the value of the input signal is compared to the predetermined (offset)
value. The difference is displayed, including its polarity. When the input value is higher than the offset value
a+
is displayed, otherwise a — is displayed. The relative reference mode is switched off by pressing one of the
OFFSET, RANGING or FUNCTION push-buttons. By using the relative reference mode, the ranges are not
extended. Overload occurs when the input signal exceeds end of normal range.
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