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56
РМ 2522
УП-1.4. Resistance measuring circuit
The principle of the resistance measuring is shown in Fig. 12.
A constant current passes through the unknown resistor Rx.
The voltage across
Rx, Vx, is supplied to the + input of ІСТОТА (1 x/ 10 x amplifier).
Depending on the range, Vx will be amplified 1 x or 10 x.
The operational amplifier IC101B generates a output voltage of 1.2 V.
As Vx is supplied to the input of IC101B the output will be 1.2 V + Vy volt.
Resistor Rs causes a voltage drop of 1.2 V. This voltage is independent of Ух and thus of Вх.
Rs determines also the current through Ry.
The next table gives a survey of the various measuring currents and voltages for the selected
ranges.
R
R
та
Composition
Meas. Volt.
1х/10х
ADC
pi
È
of Rs
at end-of-range
amp.
input
200
Q
12220
1 тА | R105 + R107 + R206
{ 200 mV
10х
20V
2 КО
1229
1 mA
+ R fuse
20M
1х
2 W
20 kQ
WO
Kẹ
10 pA
R105 + R104 + R103
{ 200 mV
10x
2۷
200 kQ
120
КО
10 pA
| + R106 + К fuse
22V.
1х
22V.
2000 kQ
12
М
100nA
R109 + R110 + R104
{ 200 mV
10x
2V
20 MQ
12
MQ
| 100 nA | , + R103 + R102 + R101
2
V
1х
ZEN.
+ R119 +R fuse
All resistance ranges can withstand 250 V ac or dc. In the case of incorrect operation in the
ranges .2 and 2 КО fuse VLI will blow. The voltage is limited by two zenerdiodes GR201 and
GR202. The zener current is limited by R206.
The other resistance ranges are protected because the current will be low due to the very high
value of Rs (120 kQ and 12 MQ).
VII-1.5.
1x/ 10x amplifier (Fig. 13)
The amplifier consists of operational amplifier 1С101А preceded by dual Fet TS103.
This Fet has a very high input- impedance at a low offset current. The offset voltage is com-
pensated for by R120 and potentiometer "0" (R1) at the frontpanel.
The gain of the amplifier is determined by ratio.
R127 + R128 + R129 // R130
=
10х
R128 + R129 // R130
With potentiometer R130 the gain can be adjusted to 10 times exactly.
For the measurements capacitors C109 and C110 are connected to circuit zero via switch
V == 2, thus forming together with R118 a RC-filter by which
a.c. voltages are suppressed.
For a.c. measurements an improvement of the frequency response is achieved by C109 and
C110 // R118.
Protection
The protection of the amplifier is made by transistors TS101 and TS102. By connecting bases and
collectors two zener diodes are obtained, the zener voltage is approximately 8 V with a low
leakage current. If the voltage at the gate of TS103 exceeds 8 V the transitors become con-
ductive thus the voltage at the gate is limited. The input current is limited by R118.
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