Probes And Differential Measurements - HBM Genesis GEN5i User Manual

GEN5i
IMPORTANT
For a correct compensation it is necessary that both impedances have
the same value, i.e. R1C1 = R2C2. In practice, this condition may not be
met exactly but can be approximated. The probe's compensation
capacitor is usually made adjustable somewhere between 10 pF and
50 pF to compensate for the instrument's input capacitance. Since the
Universal Amplifier has a 100 pF capacitance there is no way to
compensate correctly with standard probes. Therefore the probe
capacitance must be adapted to this situation. Various probe
manufacturers offer the possibility to purchase probes with other
compensation ranges on request.
D.5.1

Probes and differential measurements

Connecting the differential amplifier or probe to the signal source is generally
a great source of error. To maintain the input match, both paths should be as
identical as possible. Any cabling should be of the same length for both inputs.
If individual probes are used for each signal line, they should be the same model
and cable length. When measuring low-frequency signals with large common-
mode voltages, avoid the use of attenuating probes. At high gains, they simply
cannot be used as it's impossible to precisely balance their attenuation. When
attenuation is needed for high-voltage or high-frequency applications, special
passive probes designed specifically for differential applications should be
used. These probes have provisions for precisely trimming DC attenuation and
AC compensation. To get the best performance, a set of probes should be
dedicated to each specific amplifier and calibrated with that amplifier using the
procedure included with the probes.
532 I2679-4.0 en