AC/DC Current Probes and Probe Amplifiers

Current probes utilize different physical mechanisms depending on whether they measure alternating current (AC) only or combined AC and direct current (DC).

Passive AC-only probes, such as the Tektronix A621, rely entirely on transformer induction. The current-carrying conductor acts as a single-turn primary winding, inducing a magnetic field in the probe's core, which then generates a current in the secondary winding. Because transformer action requires a changing magnetic field, these probes cannot measure DC. The Tektronix A621 operates over a frequency range of 5 Hz to 50 kHz, making it unsuitable for DC or high-frequency power electronics testing.

For measurements requiring DC response, probes like the Tektronix TCP312 and Tektronix TCP404XL use a hybrid split-core design. This approach combines a transformer winding for high-frequency AC with a semiconductor-based Hall-effect sensor. The Hall-effect sensor detects static (DC) and low-frequency magnetic fields by generating a voltage proportional to the magnetic flux density. By combining both methods, hybrid probes deliver a flat frequency response from DC up to megahertz levels.

Active AC/DC probes cannot plug directly into standard passive oscilloscope inputs. They require dedicated external amplifiers, such as the Tektronix TCPA300, for several technical reasons:

• Power Supply and Bias: Hall-effect sensors are active components. They require high-precision bias currents and power, which must be supplied from an external source rather than the high-impedance input of a standard oscilloscope.
• Offset and Degaussing: Magnetic cores retain residual magnetism (remanence) when subjected to high currents or DC offsets. This residual magnetism introduces DC measurement errors. Dedicated amplifiers feature manual offset adjustments and integrated demagnetization (degauss) circuits to clear the core's residual field.
• Impedance Matching and Scaling: The amplifier converts the probe's current-derived signal into a calibrated voltage output (typically scaled to 50 Ohm nominal output impedance) that a standard BNC oscilloscope input can read accurately.

Selecting an AC/DC current measurement system requires balancing current-handling capacity against signal speed. In hybrid probe designs, larger core structures accommodate higher currents but increase insertion impedance and limit high-frequency response.

• High-Bandwidth, Low-Current: The Tektronix TCP312 is optimized for speed, offering a bandwidth of DC to 100 MHz and a fast rise time of 3.5 ns or less. It has a maximum continuous DC limit of 30 A.
• High-Current, Low-Bandwidth: The Tektronix TCP404XL handles large electrical loads, with a continuous DC and RMS rating of 500 A and a peak pulse capability of 750 A. However, its larger sensor structure restricts its bandwidth to DC to 2 MHz with a rise time of 175 ns or less.