Service Manuals, User Guides, Schematic Diagrams or docs for : Agilent journals 1958-03
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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 input, a factor
voltage or current being measured. that has special advantages in the microvolt
The new dc micro volt-ammeter shown in region, as described later.
Fig. 1 achieves higher sensitivity and lower The basic power sensitivity of the instru
drift than established designs by using a non- ment is better than 10 18 watt, which, for an
Fig. 2. Typical stability of instrument is indicated by above
tu-enty-ftve minute zero drift record made on unifs most sensitive
range. One semi-major vertical division is equal to 1 microvolt
(one-tenth of full scale). Drift is typically well below I Mv per hour.
Fine structure in record is described in Figs. 4 and 5.
Fig. 1 (left). New -hp- Model 425A Micro Volt-Ammeter has
maximum full-scale sensitivity of ±10 microvolts and ±10-micro-
microamperes or a basic power sensitivity of better than 10-'s watt.
At the ripple time it has very low sensitivity to line-frequency ripple
in dc being measured.
P R I N T E D I N U . S . A .
C O P Y R I G H T 1 9 5 8 H E W L E T T - P A C K A R D C O .
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