Meter Loading Effect
A voltmeter is not a passive observer. When you connect it to a circuit, it becomes part of that circuit — drawing a small but real current through its input terminals. In a robust, low-impedance circuit this extra current is negligible. But in a high-impedance circuit — such as a valve grid, a JFET gate, or a resistive bias network with megohm values — the meter's own impedance can drag the voltage down significantly below the true value. This phenomenon is called meter loading, and understanding it is the difference between a measurement you can trust and one that misleads you.
What Is Meter Loading
Think of it with a water pressure analogy. You have a high-pressure water system fed through a narrow pipe (high source impedance). The pressure at the tap with no flow is fine. But connect a wide-open tap (low input impedance load) and the pressure drops because water is now rushing through the narrow feed pipe. The pressure you now measure is lower than the true no-load pressure — the load has changed the thing you are measuring.
A voltmeter connected across a circuit draws a small current through its input resistance (typically 10 MΩ for a modern DMM). That current flows through whatever resistance exists between the measurement point and the supply or ground reference — and it produces a voltage drop across that source resistance. The meter reads the voltage after this drop, not the true open-circuit voltage.
The voltmeter's 10 MΩ input resistance (Rin) forms a voltage divider with the source resistance (Rsource). The meter reads the divided voltage, not the true open-circuit voltage.
View LargerThe Loading Formula
The meter and the source resistance form a simple voltage divider. The voltage the meter actually reads is:
The loading error as a percentage of the true voltage is:
Or equivalently: Error % = Rsource / (Rin + Rsource) × 100
True voltage = 10 V, Source resistance = 100 Ω, Meter Rin = 10 MΩ
Vmeasured = 10 × 10,000,000 / (10,000,000 + 100) = 10 × 0.99999 = 9.9999 V
Error = 0.001% — completely negligible.
True voltage = 10 V, Source resistance = 1 MΩ, Meter Rin = 10 MΩ
Vmeasured = 10 × 10 / (10 + 1) = 10 × 0.909 = 9.09 V
Error = 9.1% — this reading is seriously wrong.
Voltmeter Loading Error Calculator
Enter the source voltage, source (Thévenin) resistance, and meter input resistance to find the actual reading and the error percentage.
When Loading Matters
As a rule of thumb, loading error is negligible when the source impedance is less than 1% of the meter's input impedance. For a 10 MΩ DMM, that means any source impedance below about 100 kΩ will give less than 1% loading error — entirely acceptable for most measurements.
| Source Impedance | Loading Error (with 10 MΩ meter) | Verdict |
|---|---|---|
| 1 Ω – 1 kΩ | <0.01% | Negligible — measure freely |
| 10 kΩ | 0.1% | Negligible for practical work |
| 100 kΩ | 1% | Borderline — acceptable for most purposes |
| 1 MΩ | 9% | Significant error — results are misleading |
| 10 MΩ | 50% | Severe — reading is roughly half the true value |
Analog vs Digital Input Impedance
Older analog multimeters specified input impedance as "ohms per volt" — the resistance divided by the full-scale voltage. A typical VOM (Volt-Ohm-Milliammeter) rated at 20,000 Ω/V on the 10 V range has an input impedance of 200 kΩ. Switch to the 100 V range and it becomes 2 MΩ. This means analog meters become better (higher impedance) on higher ranges, but can be very poor on low voltage ranges where the impedance drops substantially.
A modern DMM presents a fixed 10 MΩ across all voltage ranges, which is far better than any analog meter except on their highest ranges. For working on high-impedance valve circuits or MOSFET bias networks, even 10 MΩ may be insufficient, and an electrometer or FET-input voltmeter with 1 GΩ or higher input impedance is required.
Ham Radio Examples
- 50 Ω RF circuits: Loading is entirely negligible. A 10 MΩ meter in parallel with 50 Ω gives less than 0.001% loading. However, note that a DMM cannot measure RF voltages — only DC or low-frequency AC.
- Bias networks for BJT amplifiers: Typical voltage divider bias resistors are 10–100 kΩ. Loading error with a 10 MΩ DMM is 0.1–1% — generally acceptable.
- JFET and MOSFET gate bias: Gate resistors are often 1–10 MΩ. A 10 MΩ DMM in parallel with a 10 MΩ gate resistor cuts the measured voltage by 50%. Measure at a lower-impedance point in the circuit whenever possible.
- Valve (vacuum tube) circuits: Grid leak resistors of 100 kΩ–10 MΩ are common. Loading is significant — a 10 MΩ meter on a 1 MΩ grid leak reads only 91% of the true voltage. Where possible, power down and use a voltage divider probe to reduce source impedance to the meter.
Frequently Asked Questions
Does meter loading affect current and resistance measurements as well as voltage?
In current measurement the meter introduces a small series resistance (shunt), which increases the total circuit resistance slightly and reduces the current — this is sometimes called burden voltage. In resistance measurement the meter supplies its own current source so loading in the traditional sense does not apply, but parallel paths in the circuit can corrupt the reading (covered in the resistance lesson). The loading effect discussed in this lesson applies specifically to voltage measurement.
Why do all modern DMMs have 10 MΩ input impedance?
10 MΩ became the de facto standard because it is high enough to produce negligible loading in almost all practical low-frequency circuits, yet low enough to be reliably and stably implemented with standard semiconductor components. Some high-end instruments use switchable 10 MΩ/10 GΩ input impedance for working on ultra-high-impedance circuits such as piezo sensors and electrometer applications.
If I use two identical 10 MΩ meters to measure the same high-impedance node, is the loading doubled?
Yes. Two 10 MΩ meters in parallel present 5 MΩ to the circuit, approximately doubling the loading error compared to one meter. Never connect multiple meters to the same high-impedance node simultaneously. Use one meter and move it, or use a single dedicated buffer amplifier probe.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.