OperatingInstructions, Controls and Indicators-- 1850A/1855A
recommendedthat the +INPUTcoupling first be changed
to OFF(precharge) whenmeasuring a newcircuit point.
This will safely recharge the ACcoupling capacitor in less
than 0.3 seconds. TheACcoupling capacitor is 0. I~F and
rated at 400VDC.
DCand low frequencies are attenuated by the ACcoupling
capacitor and the input resistance. With the ATTEN-
UATORset to +Ill, or set to +1 with the INPUT
RESISTANCEset to 1 megohm,the low frequency cut off
(-3dB point) is approximately 1.6Hz, lower than mostos-
cilloscopesby a factor of 5. Whenthe input attenuator is set
to +1, the INPUTRESISTANCEmay be set to I00 me-
gohms, and the -3dB point is 0.016Hz. This extremely low
frequency cut off is often handyin observing low fre-
quencynoise riding on large (up to 400 volts) DCvoltages.
In the DCmode. the +INPUTconnector is connected to
the amplifier either directly or throughthe input attenuator,
and the ACand DCattenuation are the same.
-INPUT COUPLING(AC OFF DC Vco~n,)
inverting input. The1850A/1855Aoutput is therefore zero
wheneverthese two voltages are equal. For this reason, the
voltageapplied to the inverting input is called a comparison
voltage,Vco.~n,.
Vco.~pis often used to makeprecise measurementsof large
signals by comparingthe accurately knownVco,~n,with the
unknownsignal. It can also be used to measurethe actual
voltage at any point of a waveform
PRECISION VOLTAGEGENERATOR(PVG) output
range is +15.500volts. ThePVGis never attenuated bv the
input attenuator. Attenuation of the +INPUTsignal bv the
+11)input attenuator will cause the PVGto null out an input
voltage up to +155.01)volts whichis ten times larger than
the actual PVGvoltage. Whenthe 1855Ais used with at-
tenuating probes that feature readout, the PVGdisplay is
changedto indicate the voltage at the +INPUTprobe tip
whichwill bring the amplifier output to zero.
The -INPUTconnector is not useable whenVco~n, is se-
lected.
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The -INPUThas the same coupling modes as the +INPUT
plus one additional option, Vco~,(comparisonvoltage).
The1855Agenerates a voltage controlled by the push but-
tons above and belowthe front panel numerical display.
This voltage is called the Precision Voltage Generator
(PVG). The 1850Adoes not have a built-in PVGand the
user must supply a dc voltage in the range of -15.5V to
+ 15.5Vto the rear panel OFFSETconnector.
In Vco~n,mode,the 1855A’sPVGis connected to the am-
plifier’s inverting input throughan internal filter designed
to eliminate radio and television signal interference. The
1850Adoes not have the PVG,but uses an externally sup-
plied voltage. See Page 2-6 for Vco~operation with the
1850A.
The1850A/1855A’samplifier subtracts the voltage applied
to its inverting input fromthe voltage applied to its non-
VDxw(differential offset voltage) is an instrument mode
rather than a type of input coupling. TheVmrrmode’allows
the PVG(or an external source in the case of the 1850A)
inject an offset signal into the 1850A/1855Awhile still
using both inputs for full differential operation. This mode
can be used as a position control to movethe trace on the
oscilloscopescreen in preferenceto using the oscilloscope’s
position or offset control. Theoscilloscope’s position and
offset controls should alwavs be set to zero so that the
1850A/1855A’sdvnamic range is properly centered. Op-
eration of the 1850A/1855Ausing the VDXWfunction is the
sameas VcoMrexcept for the following:
¯ The -INPUTremains active, allowing full use of the
1850A/1855Aas a differential amplifier.
¯ The maximumrange of the PVG(1855A)or the external
source (1850A)is _+10.000 volts in X1GAINand ±1.0000
volts in X10 GAIN. The effects of the +10 input
ATTENUATORand probe attenuation are the same as
whenusing Vcosn,,i.e., anyinput attenuation multiplies the
effective offset.
¯ TheVDtrVmodeoffset accuracyis slightly less than that
obtained using Vcosw.
The1855A’sPVGdisplay is changedto indicate the volt-
age that. if applied between the +INPUTand -INPUT,
wouldbring the amplifier output to zero. Whenthe 1855A
is usedwith attenuating probes whichfeature readout, the
PVGdisplay is scaled to include the effect of probe at-
tenuation.
2-2 03110199
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