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РМ 2522
The basic circuit of the ADC is shown in Fig. 7, раде 44. It operates as follows:
The input voltage V, is compared with voltage У- by means of a comparator.
The output of the comparator controls switch S via flip-flop in such a way that the positive
reference voltage Ver) is connected to R1 if Ve < Vx or the negative reference
(-Vpgp?) if Ve > Ух.
Suppose Vy is positive with respect to Vc and switch S to +VREF:
This results in charging the capacitor so that Ve increases until it becomes larger than Vx .
At the first clock pulse after V. has become larger than Vx, flip-flop FF is set; switch S is set
RE
Then switch S is set again to the starting position (S to +V
to position -V
F' Capacitor C1 will be discharged until Ус becomes smaller than Ух.
at the next clock pulse.
REF)
Due to this compensation method the average value of Ус will be equal to V.
If V has а
а constant value (d.c. level) the average charge of capacitor C1 must be also constant.
This means,
that the charge supplied (Q <) is equal to the charge drained off (Qy) when inte-
grating over a longer period (measuring time tm).
TM
=V
Charge supplied:
@ =
REF
E
te
|
te ta
(Че
a
Charge drained:
Qd = Vx = ЖЕТУШ
VREF or Vx = fo Hq
k
VREE:
in which t< = sum of the charging times, tq = sum of the draining times during time tm
(+V
and -V
have the same absolute value).
REF
REF
By counting the number of clock pulses during the charging time (tc) and subtracting the clock
pulses during the draining time (tq) this results in a number of clock pulses which corresponds
to the input voltage.
Assume:
N
Il
total number of clock pulses during tm
T
period time of clock pulse
I
then tm =
measuring time
=
NT
=
te tata
If during tc, n clock pulses are used, then during tq N-n pulses are used, so that Vx
_
nT - (NT - nT)
_
2n- М
becomes
Vx
NT
VREF
=
N
VREF -
I
N
If 2n -
N =n'
= the number of pulses to be counted, then:
n'
= V,
VREF
М
is a fixed number of pulses determined by a timer and amounts to 2046. When using a reference
voltage of 2.046 V the formula becomes
in volts)
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х
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