Philips PM2519 - Page 22

2-4
2.3. FUNCTIONAL DESCRIPTION
2.3.1. General
The automatic multimeter PM 2519 is designed around the microcomputer integrated circuit MAB 8440.
The microcomputer has 4k internal ROM and 128 bytes RAM. It also comprises 20 quasi-bidirectional I/O
parts, one serial 1/O line and an 8-bit timer/event counter.
In combination with the ADC interface, the microcomputer controls the timing and measuring functions of
the instrument. The communication between these devices is achieved by the aid of a serial bus, the so-called
12C-bus.
All the inputs are converted into d.c. signals and supplied to the ADC. The ADC. in combination with the ADC
interface converts these d.c. signals into digital logic signals and are sent via the 12C bus to the microcomputer.
2.3.2. Analog section
2.3.2.1. DC voltage measurements
The unknown voltage to be measured is passed to the d.c. attenuator where by means of resistors switched by
FET switches, the attenuation factor is changed (Fig. 2.3.}. Depending on the selection, the input voltage is
attenuated 11.11 or 1111.11 times. The table indicates the attenuation factor for each range, the ADC input
sensitivity and the range FETS.
The 11.11 attenuation is achieved with the resistors R1102, R103 and the Ri of the ADC, The 1111.11
attenuation, which is switched on by the signal RNG D, is achieved by the voltage division of R1102, R1103,
R1108 and the Ri of the ADC.
é i
Pad INPUT ADE
VQmA
x1002
vni00,
BF 568
Je
ZERO ADC
s14200
840713
Fig. 2.3. DC attenuator
RANGE
RANGE ATTENUATION anc INPUT ADG Ri PM 2579 |
RNG D
1.11
11.11
11.114
1111.11
7111.11
The 100 mV range is achieved by using a separate range. Attenuation is effected by means of R1110.and the
Ri of the ADC.