Philips PM2528 Operating Manual page 39

4.1.1,
39
Standard measurements
V .= measurements
Direct voltages {V = ) are attenuated by a dc attenuator or amplified by a chopper stabilised dc amplifier.
V~ and V~ measurements
Alternating voltages can be measured excluding or including a de component. The input voltage is attenuated
by an ac attenuator or amplified by an ac amplifier, The rms value is then determined by the rms converter,
Q2W and QAW measurements
The unknown resistance is connected to a current source via the V-Q socket ( {2 2W) or via the PROBE input
{Q 4W). The current depends on the selected range. The voltage across the unknown resistance is measured via
the ADC.
A = and A~ measurements
In the ranges 2000mA, 200mA and 20mA the current is converted into a voltage via shunt resistors. This
voltage is measured like direct voltages (V =: ) or alternating voltages including de component (V~). In the
ranges up to 20mA the current is supplied to the virtual ground of the de amplifier. As the output of the
amplifier compensates this current, the output voltage is proportional to the input current,
Both, A=. and A~ are measured in this way.
For alternating currents (A~), the rms value is also determined.
°C measurements
Temperatures are measured with a Pt-100 resistor — e.g, the temperature probe PM9249 — via the PROBE
input, The resistance of the Pt-100 element is measured in the {24W configuration.
This resistance is proportional to the temperature of the probe tip.
Linearisation and conversion into °C takes piace in the control section, via a software routine,
Offset function (relative reference).
A correction register in the PM2528 is loaded with the value of an input voltage. Measurements are corrected
with the contents of the correction register,
Optional measurements
Vpeak measurements
In the function Vpeak, the input voltage is supplied via the ac amplifier to a peak detector. The peak detector
determines the maximum vaiue (V") or the minimum value (Vy ) of the input signal. The peak-peak value
{V ¢) is computed by the microcomputer in the control section. It adds the result of a V* and V y
measurement.
Vhf measurements
In the function Vhf, a high frequency vottage can be measured according to the compensation principle, with
the high frequency probe PM9211. The hf voltage is rectified in the probe. A 100kHz signal is generated on
the hf plug-in card in the PM2528. This signal is supplied to the probe and also rectified. The difference
between the two rectified signals is supplied to the de amplifier input. The output voltage of the de amplifier
controls the amplitude of the 100kHz oscillator voltage. This 100kHz signal is controlled until it compensates
the hf voltage. The output voltage of the osciilator is then proportional to the hf input voltage and is measured
via the ac amplifier and rms converter.