Teledyne LeCroy DA1855A - Manual (Page 45)

Operator’s Manual
922258-00 Rev A 39
Applications
Introduction
Some parts of a power supply operation, such as saturation voltage, upper gate drive, loop response
etc., have been difficult to perform without the proper accessories to expand the measurement
capabilities of oscilloscopes. To obtain accurate voltage and current waveforms is a necessity before
any waveform analysis can occur.
Voltage Measurements
A significant limitation in power supply characterization is that many of the signals of interest are
referenced to voltages other than ground. Several techniques have been tried to overcome this
measurement limitation.
The most frequently used — and probably the worst one – is floating the oscilloscope by
disconnecting the ground wire in the power-line cable. This allows the chassis of the oscilloscope to
float to the potential to which the probe ground lead is connected. The most obvious danger is
electrical shock. When an oscilloscope is floated to hazardous voltages, accidental contact with any
metal component of the oscilloscope chassis can seriously injure or even kill the operator.
Another problem when floating a scope is the inability to externally trigger the oscilloscope, or the
waveform distortion that may occur when high slew rates appear on the ground lead. Another
technique used for measuring voltages not referenced to ground is quasi-differential, or channel A
minus Channel B. Even though this technique is safe, the oscilloscope is still grounded, it is still
limited to measurements where the differential mode (signal of interest) is approximately the same
amplitude or larger than the common mode signal (signal being rejected). A major problem however
is the limited CMRR (Common Mode Rejection Ratio) caused by gain mismatches between the two
input channels.
The best solution for measuring voltages that are not referenced to ground is to use a differential
amplifier. The DA1855 is ideally suited for these measurements.
Measuring Current
Current can be measured either by using a shunt resistor or by using a current probe. Adding a shunt
resistor requires cutting the current carrying conductor. Shunt resistors will add a resistance to the
circuit that can affect the operation. It is difficult to obtain accurate resistors with low resistance and
low inductance values necessary to measure large dynamic currents.
Current probes overcome these problems. Some models have a jaw that can be opened to install
around conductors without the need to cut them. They come in two different types, AC and DC. The
DC types can measure from DC to higher frequencies with relative flat frequency response. AC
current probes have both a low frequency and a high frequency response. Many AC current probes