Floating Oscilloscope Measurements ... And Operator Protection

Differential Measurement System

Pseudo-Differential Techniques. The most popular solution to the need for a "floating" measurement is the "A minus B" pseudo-differential technique. Most general-purpose dual-trace oscilloscopes have an ADD Mode where the two channels can be electrically subtracted (invert CH 2), giving a display of the difference signal. Higher voltage probes such as the P5100 (2,500 V, 100X - see Figure 4) are used, but they limit minimum sensitivities. This can be a problem when attempting to examine low-level control signals in the presence of high common-mode voltages. Also, the common-mode dynamic range is severely limited (+1 division beyond screen height) and common-mode rejection ratio (CMRR) is low - approximately 20 to 1.

Figure 4. - P5100 High Voltage Passive Probe.

Figure 5. - P6135A Differential Passive Voltage Probe pair.

Figure 6. - P5200 High Voltage Active Differential Probe.

Figure 7. - P5205 High Voltage Active Differential Probe.

Figure 8. - P5210 High Voltage Active Differential Probe.

True Differential Techniques.True differential amplifiers feature complementary inputs (+ and -) and offer high CMRR - as high as 10,000 to 1 or higher for many instruments. Since the amplifier's ability to reject the common-mode component depends on the degree to which the two channels remain balanced, its common-mode rejection ratio will decrease with frequency (imbalance due to effects of stray C, etc.) and with the magnitude of the common-mode signal (imbalance due to effects of amplifier overdrive).

Use of a differential probe pair such as the P6135A (see Figure 5) is essential to maintain maximum CMRR.

High-Voltage Active Differential Probes. The P5200 Series High Voltage Active Differential Probes are economical, heavy-duty solutions for making safe, accurate floating measurements with any oscilloscope. With true differential amplifiers in the probe heads, the compact P5200 Series are rated for differential voltage measurements up to 4,400 V_RMS (5,600 V (DC + pk AC)) depending on the probe chosen.

Circuit connections are made by leads terminated by standard 4 mm shrouded banana plugs. The included crocodile clips and insulated plunger style clamps enable safe, easy connections to a wide range of test points from bus bars to IC legs, even when "hot."

The P5200 (see Figure 6) connects directly to the BNC input of the measurement instrument and is powered by a 9 V wall adapter. The P5200 has a bandwidth of 25 MHz with CMRR of 50 dB at 1 MHz.

The P5205 (Figure 7) and P5210 (Figure 8) use the Tektronix, Inc. TekProbe^®interface system found on the TDS3000, TDS400, TDS500, TDS600, and TDS 700 oscilloscope systems. The TekProbe interface provides probe power, readout, and many other features not readily available on common probes.

The P5205 has a bandwidth of 100 MHz and a voltage level of 1,300 V_RMS. At 4,400 V_RMS, the P5210 can measure frequencies up to 50 MHz. Both probes provide a CMRR of 50 dB at 1 MHz.

The P5200 Series High Voltage Active Differential Probes satisfy today's third party safety standards