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THEORY
OF OPERATION
GENERAL
€
This section in conjunction with the block diagram Figure 5 and
waveforms
Figure 6, contains a discussion of the theory of oper-
ation of the Model 1292 Digital Panel Meter.
Basically,
the Model
1292 is a single range,
high input impedance,
bi-polar digital panel meter that is capable of being constructed as
a voltmeter or current meter with the addition of an appropriate
attenuator or shunt.
Standard self-contained current or voltage
ranges are available from Weston as shown in Table I.
The instrument utilizes a dual slope integrating technique designed
with sampling intervals related to the power line frequency.
Dual
slope is a method of A/D conversion which,
grants immunity to
random and power frequency noise accompanying the input signal
or as power supply ripple.
Integrating time in the Model 1292 is
200 milliseconds,
providing excellent rejection of 50 Hz, 60 Hz to
400 Hz of whose periods are sub-multiples of 200 milliseconds.
Dual slope converts the input signal into a ratio of two time inter-
vals, both proportional to clock period making the accuracy of
conversion independent of clock accuracy.
The accuracy of con-
version is dependent upon the stability of the internal reference
circuits.
Waveforms
are shown in Figure 6, Page 15.
The integrating time
T is constant at 200 milliseconds,
corresponding to 8000 pulses
from the 40K Hz clock oscillator.
The cycle, is divided into two intervals; ramp up Ub and ramp
down tg.
During t1 the integrator charges to a voltage ein t1.
Я
ВС
During tg, the capacitor discharges an equal amount under the
influence of ein and Тре.
IRef such that
e Ul er
Ч ein) ty
ЕС
R
G
and Since ti +
to =
T,
Ein
т
2 =
Я
R
Ref
10
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