Input Resistance
When the input ATTENUATORis set to -1 and no
attenuating probeis connected,the input resistance can be
increased from 1 Mr2 to 100 Mr2 by pressing the
100M/IMbutton. This is advantageous whenmeasuring
high impedancecircuits or whenACcoupling is needed
with a very low frequency cut off. Whenthe input
ATTENUATORis set to -10 or an attenuating probe
with readout capability is attached, 1 Mr2(IM) input
resistance is automaticallyselected.
Unbalanced source impedances can have an adverse
effect on commonmode rejection. For example, a
differential source with impedancesof 1000and2000[2,
each loaded with 1 M£2will have a commonmode
rejection ratio (CMRR)of 1000 to 1. The commonmode
rejection ration can be improvedto 100,000to 1 by using
100 M~input resistance.
This limitation is also apparent whentrying to make
accurate measurements using VCOMP.A 10.000 volt
reference with a 1000 f2 output impedance will be
reduced to 9.9900 volts by the 1820/1822 1 M~input
resistance, introducing a 10 mVerror in the measurement.
Increasing the input resistance to 100 M~decreases this
error to 100~tV.
Oscilloscope inputs have a small input current whichcan
cause an offset whenmeasuringhigh impedancecircuits.
The offset can be observed by opening and shorting the
input to ground. The 1820/1822 has a temperature-
compensatedinput current pull away(cancellation) which
works in both the 1 M~ and 100 Mr2 INPUT
RESISTANCEmodes. Its input offset current is
considerablyless than that of mostoscilloscopes.
Effective Gain (1822 Only)
Sevenlights (LEDs)across the top of the 1822front panel
indicate the total gain fromthe instrumentinput to output.
Whenthe X1light is lighted, the overall amplifier voltage
gain (amplification) is unity. Similarly, XI0indicates
overall amplification of ten times. +10 indicates the
voltageamplificationis 0.1, andso forth.
WhenPreamble Instruments or other encodedprobes are
properly used, the effective gain includes the probe’s
attenuationfactor.
Upper and Lower -3 dB Points
The1820/22allows the user to select both the upper and
lower frequency -3 dBpoints. Selections for the high
1820/1822Operator’sManual
frequency -3 dB points are 3MHz, I MHz, 300kHz,
100kHz, 30kHz, 10kHz, 3kHz, IkHz, 300Hzand 100Hz.
Selections for the lower -3 dBpoints are 0.1Hz, IHz,
10Hz, 100Hz, lkHz and 10kHz. These filters makeit
possible to improvethe signal-to-noise ratio whenmaking
measurementson microvolt magnitudesignals.
Precision Voltage Generator (1822 Only)
The PVGgenerates the voltage which is used in the
VCOMPand VDWFmodes and appears at the rear panel
PRECISION VOLTAGE GENERATOR OUTPUT
connector.
Aboveeach digit is a push button whichincrements the
corresponding digit by one whenpushed. Whenheld, the
digit continues to increment, eventually incrementingthe
nexthigherdigit.
Similarly, below each digit is a push button which
decrementsthe correspondingdigit.
The4- button above the left-most digit changes the PVG
output polarity. The ZERObutton below the left-most
digit sets the output to zero and invokes the Autozero
function.
Autozeroresets the PVGoutput to zero to eliminate any
drift which mayhave occurred in the PVGdue to low
frequency noise, or long term drift. Autozerois invoked
each time the ZERObutton is pressed and re-invoked
approximately every minute thereafter. This is useful
whenthe instrument has been unpluggedand a cold start
is required.
Overdrive (yellow)
Whena signal is applied to either input of the 1820/1822
that exceeds its ±15.5 volt input range the yellow
OVERDRIVEindicator is lighted. The light remains on
as long as the input remainslarger than the linear range.
The linear range is multiplied by the ATTENUATOR
factor and by the use of anattenuating probe.
The yellow OVERDRIVElight is intended to warn the
user of potentially distorted waveforms.
Overload (red)
Whena signal whichcould damageto the 1820/1822has
been applied to either input connector, the 1820/1822
protects itself by disconnecting the signal. Theinput
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