Philips PM2519 - Page 29

2-13
2.3.4. Analog-to-digital converter
The ADC converts the analog signal into a digital signal by the delta modulation principle. Basically, the delta
modulation ADC counts the difference in the time taken to charge and to discharge a capacitor about a fixed
level, over a fixed period of time.
The number of charge/discharge cycles within this fixed time depends on the charge/discharge current
which is made proportional to the unknown input voltage to the ADC, Therefore, the number of pulses
counted within a fixed measuring period is proportional to the unknown voltage Vx. The obtained data signal
is fed to the ADC interface D1703 where it is counted.
To obtain automatic zero i.e. counteract drift and internal offset, one complete measurement consist of two
fixed measuring periods {two AZC periods).
One complete measurement is used to update the bargraph or for automatic ranging. However, a display result
consist of two complete measurements.
During the first period of a measurement the AZC input is low and the ADC interface counts up on each
clock-edge the logical state of the data signal. The value is kept in a register. During the second period, the
data signal is inverted by the ADC interface and on each clock-edge the logical state of the input signal, the register
is counted down. Also the input of the ADC is inverted so that offset in the result is compensated.
The ADC has two input sensitivities 90 mV and 900 mV, selected by the signal RNG E. This signal selects either
R1211 or R1216 as conversion resistor.
A1200
OQ 0067A
C1204
390p
1
x [> ss
TP i014 = R1201
C1213
2 CLOCK IN
A7p
' cols CLOCK OUT
| TRACK SHIELDING
j
|
AZC AXH13 AZC
CONTROL 2% iz
vob —a+9V
Tenzos
On
1209 8
B35k Tk: QUAL
C12% | crz096 We} RANGE 14 RNGE
GP Sp CONVERTER CONTROL
R1210 " ——_—} vsso! -9v
eae
1
1
©1208
10n
1200 ic1207 devs
1p4t10p 3p9 Ge
ST4407
840713
Fig. 2.10. ADC