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58
VII-1.6.
VII-2.
VII-2.1.
VII-2.2.
PM 2522
AC - to - DC converter (Fig. 14)
The supplied voltage which is always
2 Vr, m.s,
at end-of-range in all ranges is converted
IO2 M dsc.
To eliminate the non-linearity of the diode characteristic an operational amplifier with a
high open-loop gain is used (IC102).
The gain for positive periodes is determined by the ratio
ee
, for negative periodes
R139
R137 +R138
by the ratio "13718138
and should be 2.22 (twice the form factor).
As IC102 is controlled at the inverting input the input impedance is low (about 4.5 kQ).
However, the converter is preceding by a buffer consisting T$104 and TS105.
Offset compensation is made by potentiometer R141.
Potentiometer R137 is used to make end-of-range calibration.
DIGITAL SECTION
The digital section of the instrument serves to convert the supplied d.c. voltage from the
analog section into a digital information corresponding to the input signal.
The digital section contains the following circuits:
= FVREF and -VREF
- Analog to digital converter
(ADC)
- Display
+ and - VREF
(Fig.
15)
These voltages are required as reference voltages for the ADC.
The circuits of -- Vggp
and -УрЕр
are equal except the polarity of the zenerdiodes.
A constant current flows through the diodes so the voltage is also constant.
Resistance R194 is used to correct differences in switching times of the ADC switches.
Analog to digital converter
(ADC)
In this system the delta-pulse-modulation principle is used.
For the function of the ADC, refer to chapter IV-2.
Practical execution
In PM 2522 this system is obtained as shown т Fig. 16.
The comparator consists of TS112 followed Бу an amplifier (ІС103) fed back for ac.
The subtracting of pulses as described before is effected by an up down counter which is included
in IC104. This (P-channel-MOS) integrated circuit compromises also the next circuits:
- an input flip-flop. This flip-flop sets the analog switches, which select the reference voltage.
- an analog switch which controls the connection of the reference voltages to the chopper output.
- ап up/down
counter.
- a timing circuit for synchronisation of : reset pulse
transfer pulse
scan pulse
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