"2 



-? vAAA- 



/j 5 000JT- 



CR I 



-W- 



R 3 CR2 



AVV W- 



I 2 5000-n- 



Fig. 10. Simplified AC-DC converter. 



o — vw 



2 MEG 



LA/NAT 



500 -TL 



Fig. 11. Actual AC-DC converter. 



Fig. 12. Low pass filter. 



UNFILTERED, E . 



shows why the waveform distortion must be kept 

 low in this instrument . With total harmonic 

 distortion "below 0.5$, a conversion accuracy of 

 better than 0.1$ can be obtained related to the 

 rms value of the input voltage . 



A diagram of a simplified AC-DC converter is 

 given in Fig. 10. The open loop gain, A Q , of the 

 amplifier is approximately 66 db insuring over 

 60 db of net feedback around the loop including 

 the silicon diodes . The diodes are connected such 

 that on the positive half of the output wave, 

 E 0C; diode CR1 conducts and diode CR2 is biased 

 off. The reverse occurs on the negative portion 

 of the wave. The sum of currents I 2 and II4. are 

 equal to current I]_ since sensing current lr> is 

 reduced to below 0.1$ of 1^ as a result of the 

 60 db of negative feedback. 



Operation with diodes in the feedback is as 

 follows . If there is no input signal to the 

 amplifier and the amplifier output noise is below 

 1 volt peak-to-peak, the rectifier diodes are 

 for all practical purposes open circuits allowing 

 the amplifier to have its full raw gain of 66 db. 

 The waveform appearing at the output of the ampli- 

 fier will be that of white noise. If a positive 

 voltage is applied to the input of the amplifier, 

 a current I]_ will flow into the amplifier as 

 sensing current I? since the diodes are still 

 open. Sensing current I3 instantaneously drives 

 the output voltage, E oc , negative causing CR2 

 to conduct. Input current will now flow through 

 Ro as Ig. I3, the amplifier sensing current, 

 will drop to a negligible value. At this time 

 the forward gain of the amplifier has dropped to 

 slightly greater than unity. Therefore, the vol- 

 tage developed across Ro represents one-half of 

 1^ and the other half is developed across R 2 . 

 Further, the rectified voltage, E dc , developed 

 across R 2 is proportional to the input voltage, 



to better than 0. 



In Fig. 11 is shown 



Fig . 13 ■ Converter output . 



the actual AC-DC converter diagram used to obtain 

 the 2 megohms input impedance required for the 

 amplifier. This requirement is necessary since 

 the amplifier input impedance loads the staff. 



Low Pass Filter 



The output voltage, E dc , from the AC-DC con- 

 verter, though being a precise conversion of the 

 AC voltage across the staff, is still not in a 

 form usable for most applications due to its pul- 

 sating nature. It is necessary to smooth this 

 voltage to keep the ripple to less than 5 milli- 

 volts. The 2-stage LC low pass shown in Fig. 12 

 is employed for this purpose. The two inductors 

 have a series resistance of approximately 1,000 

 ohms . This resistance limits the load resistance 

 that may be connected to the instrument without 

 appreciable loss of filtered output voltage, Ep^. 

 At a carrier frequency of 1 Kcps the filter has 

 an attenuation of over 70 db. The filter has less 

 than 0.2$ attenuation at 5 cps giving additional 

 useful data up to 20 cps. Fig. 13 shows both the 

 unfiltered converter output, E D( -,, and the filtered 

 output, EpQ. The final output voltage is in a 



98 



