Model34904

 

 

SectionIV

 

 

 

 

I N P U T

CONVERTER

 

 

ATTENUATOR

A M P

RECTIFIER

r N e u r {

? M

 

 

 

K ?

 

 

DC FEEDBACK AMP 25oK

OUTPUT

FILTER D C A M P

3 4 9 0 A - A - 2 8 2 4

50K

FEEDBACK

ATTENUATOR

Figure4-6. SimplifiedDiagram,AC Conyerter.

Table4-1. AC Amplifier Ranging.

Input Amplifier Total

Range Attenuator Gain Gain

1 V

1

1

1

1 0 v

1

o.1

0 . 1

1(X)v

0.o1

1

0.01

1000v

0.01

0 . 1

o.001

a gain of I or gain of 0.1 may be selectedDC. feedback stabilization is provided through an integating amplifier. Saturation of the Converter Amplifier is prevented by a diode protection circuit which limits the output to approximately1 7 V peak.

&58. Rectifierand Filter.

zt-59.The output of the Converter Amplifier is rectified by a half-wave rectiher, resulting in a positive output. A complementary diode in parallel with the output rectifier diode is used to provide a full-wave ac feedback to the amplifier input. Active filtering is used so that the necessaryfiltering can be obtained with capacitorsof a practical size. In ac measurements,the output of the converter is applied through a FET switch (E in Figure 44) Io the DC Amplifier during run-up.

4.60.OHMSCONVERTER.

461.The Ohms Convertersuppliesa referencecurrent through the resistancebeing measured.The resulting voltage drop, which is proportional to the resistance,is measuredin the same manner as a dc voltage input, except that the Q Referenceis used for run-down. A resistancemeasurement,then, is the ratio of the voltage developed across the unknown resistance,to the Q

Referencevoltage. A simplified diagam of the Ohms Converteris shownin Fizure4-7.

4-62.CurrentSource.

443. One input of the operational amplifier in the current sourceis referencedto ground.The ,fl Reference voltage (approximately - I V) is applied to the other input through a referenceresistance,Rr.1, whosevalue is selectedaccordingto the ohmmeterrange.The nature of an operational amplifier is suchthat it tends to maintain both inputs at the same potential. This requirement cannot be satisified by cunent drawn from the FET input; consequently,it must be met by the feedback current, which passes through the resistancebeing

-measured,R1. As the amplifier output goes negative becausedf the negativeinput, the transistorat its output

'"'

is forward biased. The resulting feedback current is automatically adjusted by the amplifier to causea I V drop across R.e;. The value of the current, then, is inverselyproportionalto the valueof R1s1.

464. 0hmmeterPowerSupply.

4-65. An output from the State Clock (see Logic Circuits) is applied to a divide by six counter. The counter output is then applied to both ends'of a center-tapped transformer primary. (The signal at one end is invertedand the other is not.) The output of the transformer secondary is rectified by a full-wave recti- fier, and this floating voltage is used as a supply for the Ohms Convertercircuits. This permits 4-terminal resistance measurements,since the A Sipal Low is not internally connected to circuit common during run-up. The Ohmmeter Power Supply is disabled during all measurementsexceptresistancemeasurements.

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