Specifications

182011822 Operator’s Manual
VOLTAGE INPUT also indicates the waveform’s
differential voltage with respect to its -INPUT.
By using the 1822 in the differential offset mode and
the oscilloscope in a high sensitivity setting, high
resolution voltage measurements can be made. The
-INPUT is the reference for these measurements.
The Precision Voltage Generator can be used as a
position control which allows the 1822 to operate in
its most linear region.
Which Offset Mode Should be Used?
The operation of the Comparison (VcoMv) and
Differential Offset modes (VwvF) are quite similar. The
Comparison mode is easier to understand and has a wider
range, 15.5 volts versus 10.0 volts. The Differential
Offset mode provides offset operation while allowing the
1822 to function as a true differential amplifier.
For most applications, the Differential Offset (VDIFF)
mode has advantages over the Comparison (VcoMv)
mode. When using the Comparison mode, the Precision
Voltage Generator’s output is subtracted from the
+INPUT. Except for the PVG’s offset, operation is the
same as a standard single-ended oscilloscope - only one
1820/1822 input is available. In the Differential Offset
mode, the 1820/1822 functions as a differential amplifier
- both +INPUT and -INPUT function. This allows the
operator to choose a measurement reference point other
than ground. Even in ground referenced measurements,
signal degradation can be reduced by using the -INPUT
probe to select a ground reference point with the least
noise. This method is especially useful in eliminating
hum and noise from ground loops.
There is one instance in which the Differential Offset
(VmFv) mode might result in more noise. Magnetic pick-
up is proportional to the area between the probes. If
twisting the probe leads together is not sufficient to
reduce magnetic pick-up, the Comparison Offset (Vco~)
mode may be preferable.
Because the Comparison Offset mode uses the CMRR of
the 1822 while the Differential Offset mode uses an
internal amplifier, the Comparison Offset mode is
slightly more accurate.
The Differential Offset (VDIFF) mode is usually the mode
of choice if the wider range or higher accuracy of the
Comparison (VcoMP) mode is not needed.
User Traps to Avoid
There are a few situations the user of either the 1820 or 1822
should be aware of to avoid some potential measurement traps.
Exceeding the Common Mode Range
The 1820 and 1822 Differential Amplifiers have the largest
common mode range available for this type of amplifier and
are very good at measuring small differences between two
large signals. However, care still must be taken not to allow a
large common mode signal to exceed the available common
mode range.
The maximum common mode range is +15.5 volts when a
signal is applied directly (+1 ATTENUATOR and no probes)
to the 1820/1822’s + and - inputs. The yellow OVERDRIVE
light illuminates to warn the user of possible waveform
distortion caused by exceeding +15.5 volts.
Attenuating the input signal extends the common mode range
by the same factor as the attenuation. Pressing the +10
ATTENUATOR button increases the common mode range to
+155 volts, and using a probe with a +10 attenuation factor
will too. The effect of the internal +10 ATTENUATOR and
the attenuation factor of probes is multiplied just as the signal
is attenuated. As an example, using the amplifier’s +10
ATTENUATOR with a probe having a +100 attenuation
factor (total attenuation of+1000) results in a common mode
range of 15,500 volts. In this case, the probe’s maximum
voltage rating probably limits the maximum common mode
input voltage.
The gain setting of the amplifier has no effect on common
mode range; it is the same in XI000, XI00, XI0 or XI gains.
When making measurements on circuits that are line
referenced, be sure to use enough total attenuation to keep the
peak voltage at the amplifier input below 15.5 volts. The US
line can exceed 170 peak volts and therefore at least a total
attenuation of +100 should be used. Line voltages in some
other countries are larger but their peak voltages do not exceed
the 1550 volt common mode range that a +100 attenuation
factor provides.
Moving the oscilloscope position setting away from
center screen
When operating the 1820/1822 with an oscilloscope, it is very
important to set the oscilloscope position and/or offset control
to center screen. There are a couple of reasons for this:
3-4