Philips PM2519 - Page 25

2.3.2.4.
2.3.2.5.
27
Alternating current measurements
The ac input current ranges are shunted in the same way as the dc currents (refer to 2.3.2.3.). The voltage
from the shunts is supplied to the I2 input of the RMS converter.
Input I1 of the OO 00671 is earthed via resistance R1404.
INPUT SENSITIVITY INPUT SENSITIVITY
INPUT RNG G
RMS ADC
mA
mA
A
A
Resistance measurements
The unknown resistance is connected between the V, 2, mA and 0 input socket and supplied internally by
a constant-current source. This current results in a potential difference across the resistor that is proportional
to the resistance value, The measuring currents in the OQ 0063 are derived from a reference current source
{ref adjusted by R1510 in parallel with resistor R1511. The output current Irc of the reference current source
feeds the current multipliers, to give the currents Irx shown in the table, depending on the selected signal
RNG A, RNG B and RNG C,
As stated, the voltage Vx developed across Rx is applied to the ADC for measurements. However, the ADC
input resistance is finite (10 MQ) and the small input current drawn by the ADC has to be compensated to
avoid incorrect readings. This is achieved as follows: The voltage Vx across Rx is amplified by a factor of 2 in
the compensation amplifier (+Vin) the gain being determined by the equal value resistors R1505 and R1503.
The output voltage of 2 Vx appears at one end of R1506 and Vx is present on the other end. The voltage
across R1506 is therefore 2Vx - Vx = Vx. As R1506 is the input resistance of the ADC, the input current is
compensated. In this way, the load imposed by the ADC is compensated as Icomp = lade:
Irx’ = Irx + lade — Icomp
so Irx’ = Irx
Protection for the current source is afforded by the PTC resistors R1500 and R1501, zener diodes V1550,
V1553 and diodes V1551, V1552 and V1554.
in the event of a high voltage on the input terminals, the parallel network R1500//R1501 goes high resistance.
To prevent part of Irx leaking through the protection diodes, the anodes of V1554 and V1550 are connected
to buffered Vx. The leakage current is zero because the voltage over the protection diodes is zero.