TA044

Application Note Choosing the right Pico Technology active differential probe Pico Technology offers many active differential probes covering a wide range of voltages, category (CAT) ratings and bandwidths. As the name suggests, these probes have two major features: Active: Active probes achieve low input capacitance and high sensitivity by buffering and amplifying the signal close to the point of measurement. Differential: Differential probes present two non-grounded terminals for connection to the measurement and can thus measure accurately without directly connecting one test node to ground or drawing significant current from either test node. This ability to measure without a low-impedance connection improves safety when measuring high voltages or in the presence of high voltages. Before we look at choosing the correct probe for your application, lets go back a step and explain why you would want to use a differential probe.Application Note Choosing the right Pico Technology active differential probe 1. The problem: signal with unwanted common-mode noise Consider the circuit below. It consists of a signal source (purple) with AC and DC components producing a total output of V , which is the signal we wish to measure. In addition, the circuit contains an unwanted voltage SIG source (green) that also has AC and DC components adding up to V . V stands for common-mode voltage, CM CM which means a signal that is added equally to both measurement terminals. As the following diagram shows, viewing the output and ground terminals with a single-ended probe would result in a distorted waveform (V ) on the display. We cannot solve this by connecting the probe ground to the SUM negative terminal of V , as that would short-circuit V to ground through the oscilloscope and possibly cause a SIG CM damaging current to flow. We need a probe that can safely detect V and ignore V . SIG CM V SIG V V SUM CM V V SIG CM 0 V V SUM 0 V Figure 1 - Using a single-ended oscilloscope probe