Teledyne LeCroy DA1855A - Manual (Page 47)

Operator’s Manual
922258-00 Rev A 41
recovery performance is the oscilloscope or differential amplifier’s probe performance. Probes play
an important role in device ON voltage measurements. They attenuate the voltage’s magnitude as
well as provide a convenient way of connecting to the device under test. in addition to attenuating
the input signal they also attenuate the rate of change of the oscilloscope or differential amplifier’s
input signal. A ÷100 passive probe will attenuate a 400 Volt signal with a dv/dt of 10 V/nsec to a 4
Volt signal with a dv/dt of 0.1 V/nsec. Less obvious is the effect of a probe’s low frequency
compensation adjustment on the measurement accuracy of device saturation voltage. Most
oscilloscope users are familiar with the requirement of adjusting passive probes for low frequency
compensation. Under normal usage, the entire waveform is on screen when a passive voltage
probe’s low frequency compensation is adjusted. A low frequency compensation made with the
entire waveform visible on screen is usually adequate for most measurements.
However when a signal’s amplitude is greatly magnified as can be the case when using a differential
amplifier, a small error in the low frequency compensation flatness can cause major error in voltage
measurements, especially when measuring saturation voltages.
Figure 4 through Figure 7 illustrate how this seemingly minor adjustment can make the saturation
voltage’s DC level appear to be incorrect.
A voltage probe appears to be properly compensated on a 400 V square wave when viewed at
100V/div.
Figure 4