USB scope enables high-res measurements of differential voltage waveforms

February 21, 2017 // By Graham Prophet
USB oscilloscope specialist Pico Technology's PicoScope 4444 high-resolution differential oscilloscope features four true differential input channels and a range of accessories for measurements from millivolt to 1000V CAT III applications.

This PicoScope addresses the problem of making accurate voltage waveform measurements on circuit elements that are not ground-referenced, without the risk of short circuits that could damage the device under test or the measuring instrument. “The 4444 gives electrical engineers the freedom to make differential voltage measurements in the presence of common-mode signals,” said Trevor Smith, Business Development Manager, Test & Measurement, at Pico Technology. “The instrument has 14-bit resolution on four channels and 256 megasample capture memory, for precise analysis of complex waveforms ranging "from biomedical sensors to current probes and 1000 V CAT III power distribution circuits.”

To make low-voltage measurements in the presence of high common-mode noise or varying offset voltages, ground-referenced oscilloscopes require use of two input channels and an A–B math function to observe the differential signal of interest. Pico comments that as well as requiring two input channels, most scopes don’t have a high enough common-mode rejection ratio (CMRR), nor the resolution to make measurements with sufficient precision.

Engineers working on polyphase power distribution systems need to measure phase-to-phase AC voltages, rather than phase to ground. A ground-referenced scope can’t be used as it will cause a short circuit. External differential probes are a solution but they are quite expensive and cumbersome, with each probe requiring its own power supply. The PicoScope 4444 has differential inputs and a choice of 1:1 low-voltage and 25:1 1000V CAT III probes to address both of these problems.

The scope has an intelligent probe interface that, in addition to delivering the differential signal to be measured, can also provide power for active probes such as Hall Effect AC/DC type active current probes. The scope detects when a compatible probe is connected and sets the corresponding units and vertical settings in the PicoScope 6 software. Compatible probes available at launch are:

- PicoConnect 441 passive differential 1:1 20 MHz voltage measurement probe

- PicoConnect 442 1000 V CAT III