Sect.11 Page 2 Model 405C
5) Press CALIBRATE switch and adjust CALI- Response time will then be about 50 msec. How-
BRATE control until readout agrees with internal ever, ac rejection will be much less than with the ..
secondary standard voltage. standard filter. For example, a 60-cps signal with
a peak value equal to full scale of the displayed
range will cause readout variations as much as
i200 counts.
2-4 INPUT FILTER
The 405C measures instantaneous dc voltages.
To avoid readout variations due to superimposed
ac signals, the instrument has a low-pass RC filter
at its input. The filter has a sharp cutoff and
attenuates 60-cps signals about 50 db. Thus a
60-cps signal with a peak value equal to full scale
of the displayed range typically causes a readout
variation of * 4 counts.
Since the filter is at the input, it becomes charged
to the full value of the input voltage. With the in-
put open-circuited (probe lifted off test point), the
filter capacitors must discharge through the input ,
attenuator, and after 2 seconds they still haveabout Voltmeter Indication Printed Record
37% of their original charge. (Youcansee the dis-
charge by watching the readout decrease after the + 3.48 348 2
probe is removed from a voltage source.) There- - 565. 565-0
fore, if you quickly change the probe from a high- + .283 283 3
voltage point to a low-voltage point, you will apply
nearly the full high voltage stored in the filter
capacitors to the low-voltage point through 500K
resistance. If the low-voltage point happens to be The 405C can drive the 561B directly; the standard
561B cable makes the necessary connections be-
a high-impedance point such as a vacuum tube grid tween the voltmeter and recorder. However, the
or, possibly, a transitor element, the voltage ap-
405A-95C Digital Recorder Adapter is required to
plied through the probe may upset circuit oper- connect the 405C to the 560A. The adapter mates
ation to the point that the circuit will damage itself.
A GOOD OPERATING PRACTICE IS TO TOUCH with the 100-pin output connector on the rear of the
405C and is held in place by two machine screws.
THE PROBE TO GROUND BRIEFLY OR ALLOW
THE FILTER SUFFICIENT TIME TO DISCHARGE The standard 560A cable connects to the adauter.
When connecting either recorder to the 405C, be
WHEN CHANGING THE PROBE FROM HIGH- sure both the recorder and 405A are turned off.
VOLTAGE POINTS TO LOW-VOLTAGE POINTS.
In any case be sure the readout is less than the CAUTION
expected voltage before connecting the probe into
a low-voltage circuit. The405C chassis is connected to power-line ground
through the adapterl550A system. Do not connect
the voltmeter common lead to an off-ground potential
The input filter makes instrument response to step
while the 560A is connected to the voltmeter. This
functions about 0.75 second compared to about 0.2 caution also applies when using 405C with Models
second between successive samplings at maximum 561, 562 with dual input couplers.
sampling rate. If you require a response time -----------
compatible with maximum sampling rate, remove
C1 and replace C2 with two 0.02-pf, ~O-V~CW, 2-6 ELECTRICAL OUTPUT INFORMATION
polystyrene-dielectric capacitors in series.
You can use the electrical information provided by
the 405C for other purposes than operating a re-
CAUTION
corder. For example, you might use the information
Since the two capacitors are in series, one will be to operate a remote readout identical to the one in
off ground potential. Be sure to insulate the off- the instrument or to record voltage information on
ground capacitor from the chassis. punched cards or tape. Table 2-1 gives full details
-------------- of the information available.
00518-2 |
ACEC
