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РМ 2522
11
IV. Principle of operation
INPUT CIRCUIT
(Fig. 8, page 44)
The purpose of the input circuit is to supply a direct voltage of 2 V to the ADC input, at end
of range values.
The analog sections translate all input signals viz. d.c. and a.c. voltages, d.c. and a.c.
currents and resistances to this signal of 2 V.
For the d.c. and a.c. voltages the same divider is used.
The attenuated signal is supplied to an 1 x or 10 x amplifier with an output of 2 V d.c. or
2 Vr m.s»
In the case of a.c. measurements the output signal of the amplifier is supplied to an AC/DC
converter, which is switched off for d. c. measurements.
Currents supplied to input A are converted into voltages by shunts.
These voltages are also supplied to the 1 x or 10 x amplifier and then supplied to the ADC di-
rectly or via the AC/DC converter for a.c. currents.
For resistance
measurements,
a constant
current passes through the unknown
resistance.
The following currents are used in the different ranges:
Measuring voltage
RESI
саи
(at end of range)
200
©
1 mA
200 mV
2 КО
!
1 тА
2
N
20 kQ
10 pA
200 тУ
200
kQ
10 pA
М
2 МО
100 nA
200 mV
20 MO
100 nA
2
W
The measuring voltage across the unknown resistance is supplied to the ADC via the 1 x or 10 x
amplifier.
DIGITAL SECTION
The analog to digital converter of PM 2522 is based on the principle of delta-pulse modulation.
This integrating system ensures good linearity and series mode rejection.
Furthermore the circuit contains a minimum of critical elements, as the accuracy of the reference
voltage is only important for the accuracy of the ADC.
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