Agilent 1958-03 free download

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HEWLETT-PACKARD JOURNAL T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - L A B O R A T O R I E S Vol. 9 No. 7 3UBLISHED BY THE HEWLETT-PACKARD COMPANY, 275 PAGE MILL ROAD, PALO ALTO, CALIFORNIA MARCH, 1958 An Increased -Sensitivity Micro Volt- Ammeter Using A Photoconductive Chopper TWO OF THE paramount considerations mechanical chopper consisting of long-life in designing a dc microvoltmeter are mini photoconductor elements. The modulator is mizing noise so that high sensitivity can be operated at other than line frequency so that obtained and then achieving freedom from extremely low effect from 60- and 120-cycle drift on the resulting sensitive ranges. In gen ripple in the measured dc is obtained. Neg eral, freedom from drift is usually sought by ligible zero drift is obtained by meticulous at using mechanical choppers to permit an ac- tention to overall design, while calibration is coupled amplifier to be used, while low noise stable and accurate because of feedback. The is sought by heavily restricting the bandwidth resulting instrument has a maximum full-scale of the amplifier. Using mechanical choppers is voltage sensitivity of ±10 microvolts and a expensive, however, and often introduces other full-scale current sensitivity of ±10 micro- sources of noise into the amplifier. In addition microamperes combined with a high input im and more important from a measurement pedance for voltage measurements and a drift standpoint, if line frequencies are used to drive of not more than 2 microvolts per hour. Under the chopper, as is almost always the case, the usual operating conditions, drift is less than 1 measurements are susceptible to large errors microvolt over a period of several hours. The when line-frequency hum is present in the dc instrument also has a floating