When and Why to Use a High Impedance Voltmeter in Your Lab?
Introduction
In laboratory environments, a voltmeter’s precision and accuracy are critical—especially when monitoring sensitive circuits. A high impedance voltmeter is more than just a tool; it’s a must when working with high-resistance sources or sensitive devices. In this post, we’ll look at when and why a high impedance voltmeter should be your go-to instrument, using important insights from Electro-Tech Systems (ETS) and broader industry knowledge.
What is a High Impedance Voltmeter?
A high impedance voltmeter has a substantially higher input resistance (impedance) than a conventional meter. The ideal case is one with infinite input impedance, which means that the voltmeter pulls almost no current from the circuit being tested, minimizing circuit disturbance.
Modern digital multimeters (DMMs) offer input resistance ranging from 10 MΩ to over 10 GΩ, which significantly reduces circuit burden.
Why Use a High Impedance Voltmeter?
- Prevents Circuit Loading and Inaccurate Measurements.
If a voltmeter has a low input impedance, it pulls current and changes the circuit, serving as a parallel resistor and skewing voltage measurements. This effect is particularly noticeable in high-resistance circuits, such as voltage dividers or amplifier inputs.
In contrast, a high impedance voltmeter pulls negligible current, retaining circuit behavior and providing more precise results.
- Maintains the Charge in Reactive Components
When measuring stored charges in capacitors or timing circuits, a low impedance voltmeter may discharge the component, impairing the measurement. A high impedance voltmeter prevents this by reducing current draw.
- Essential for High-Voltage, High-Impedance Sources
Special measurement tools are required for high impedance sources, such as those found in radiation monitoring systems and high-voltage power supplies. The ETS Model 220B High Impedance Voltmeter has an ultrahigh 70 GΩ input resistance and can measure up to ±10 kV DC with better than 1% accuracy and 1 V resolution, while minimizing source load.
When should you use it?
- High Impedance Circuits
When working with logic circuits, sensor nodes, amplifier inputs, or any circuit with high resistance, use a high impedance voltmeter to verify you’re measuring genuine voltages—not artifacts created by the meter.
- Voltage Dividers and Precision Measurement
High impedance is required for measuring intermediate nodes in resistor networks or bridge circuits. Any meter-induced current can have a considerable effect on voltage levels.
- Electrostatic and High Voltage Applications
In ESD-sensitive and high-voltage situations, where circuits have very high source impedance or are readily disturbed, instruments like the ETS Model 220B are optimized for precision without interrupting the circuit or compromising safety.
- Calibration and Metrology Laboratories
When calibrating devices or doing precise validation, even minor loading might bias the results. High impedance instruments—such as null detectors and electrometers with near-infinite input resistance—are needed.
Real-World Example: The ETS Model 220B
- Input Resistance: ~70 GΩ—ideal for measuring high-voltage sources with minimal loading.
- Voltage Range: Up to ±10 kV DC; ±11 kV absolute max
- Accuracy: Better than ±1% full scale; resolution: 1 V
- Power Options: Battery (two 9 V) or AC adapter; compact and field-ready
- Ideal Use Cases: Measuring ESD sources, bias voltages in sensitive electronics, or high-voltage outputs without disturbing the circuit
When Is a Lower Impedance Voltmeter Preferable?
Yes, there are scenarios where low impedance could be advantageous:
- Intentional Circuit Loading: In voltage drop tests or load simulations, a low impedance meter can help stress-test the circuit.
- Ghost Voltage Mitigation: Some meters offer dual impedance (e.g., Fluke Auto-V/Lo Z) to filter out stray voltages—useful in mains troubleshooting.
- Solid, Resistant Applications: Instruments like solenoid voltmeters are durable but not ideal for high-impedance circuits due to the loading effect.
Conclusion
The Voltmeter remains a foundational instrument in any lab, but selecting the right type—especially when precision matters—makes all the difference. A high impedance voltmeter is the go-to choice for high-resistance circuits, high-voltage sources, and applications where accuracy is non-negotiable. Models like the ETS 220B exemplify how such tools enable precise, non-loading measurements even in challenging environments.
Still not sure which to pick for your setup? We are happy to help you compare models or recommend alternatives based on your specific needs!