Teledyne LeCroy DA1855A - Manual (Page 46)

DA1855A Differential Amplifier
40 922258-00 Rev A
have a low frequency cut-off of 40 Hz or higher, eliminating the ability to measure power at line
frequencies
Effects of Probes on Saturation Voltage Measurements
To measure switching’s device saturation voltage while the device is operating in circuits requires
the combination of several capabilities in the measurement system.
First because the measurements are not ground referenced, thus differential voltage measurements
are needed. The amplifier must also be able to quickly recover from overdrive and the amplifier as
well as the probes must have very low high frequency aberrations.
Figure 3, Saturation Voltage
The most obvious problem with this measurement is the signal’s wide dynamic range. The voltage
across the device can be several hundred Volt when the device is off and then drop to less than a
Volt as the device turns on (Figure 3).
To measure the saturation voltage of a device to 100 mV accuracy when the OFF voltage is 400 Volt
requires 250 ppm measurement capability. To accurately view the device’s approximately 1 Volt ON-
voltage with an oscilloscope, the vertical sensitivity must be set to 200 to 500 mV/div. Almost all of
the signal will be off-screen. Also this voltage change occurs in a fraction of a microsecond. This
means that the oscilloscope must be able to accurately display sub-1 Volt signal less than a
microsecond after being overdriven by several hundred divisions. It is obvious that the oscilloscope
input or an input preamplifier such as the DA1855A needs to recover and therefore that a probe
with high frequency performance is required.
To overcome the overdrive problem is to use a specially designed fast clipping circuit to limit the
magnitude of the signal at the oscilloscope’s input to a value within the linear range and to turn ON
and OFF fast enough so not to compromise the measurement integrity. Another solution is to use a
differential pre-amplifier specifically designed to recover from being overdriven and to cleanly clip
the signal so the oscilloscope is not overdriven. Of equal importance to the amplifier’s overdrive