Termination Impedance: 50 Ohm Versus 1 Megohm
A measurement is only as good as the load the instrument sees. Signal generators, spectrum analysers and network analysers are calibrated to drive and to sense a defined impedance, and when the load at the end of the cable does not match the assumption, the reading on the screen stops describing the device under test. The waveform changes shape, the amplitude becomes unreliable, and in some settings the instrument itself is at risk.
Two impedance values account for almost every measurement in a modern lab: fifty ohms, which is the standard for radio frequency work, and one megohm, which is what a high impedance probe or a low frequency amplifier expects to see. This article explains what goes wrong when the wrong one is selected and why the cable in between is part of the explanation.
The discussion is worth having because the failure mode is not subtle and it is not rare. It is one of the few measurement errors that can damage both the instrument and the device.
Two Standard Loads And Why They Exist
The 50 ohm standard became the reference for radio frequency equipment because it is a practical compromise for a coaxial line, balancing loss against power handling, and because an entire ecosystem of instruments and cables was designed around it. A source designed for a 50 ohm load delivers a defined power into that load and is calibrated on that basis, which is why the load has to be present before the number means anything. The same assumption underlies the way a controlled impedance line is designed on a board, where the trace and the load are specified together.
One megohm is the opposite choice. It presents so little loading to the circuit that the measurement barely disturbs it, which is what an oscilloscope input or a low frequency amplifier needs when it is observing a voltage. The two are not interchangeable, and the instrument manual normally states which one applies to each mode. Confusing them is a mistake that survives right up until the signal quality is actually examined.
Needing Fifty Ohms But Selecting One Megohm
This is the more common mistake in high frequency work, and its signature is unmistakable. The source is designed to drive a 50 ohm load, but it is terminated into a high impedance, so the lead and the cable ahead of it are no longer terminated. The termination impedance is effectively removed, and the cable capacitance, which would normally be absorbed by the load, is now magnified by the high resistance at the end of the line.
The result is a capacitive load on the source. A clean sine wave develops peaks, overshoot or an irregular shape, the fundamental amplitude wanders, and the harmonic content becomes unpredictable. Because the load is not absorbing the incident energy, a standing wave forms along the lead, and the measurement now depends on the length of the cable rather than on the device under test. In the worst case the source becomes unstable and oscillates, which is a real risk of damage.
Needing One Megohm But Selecting Fifty Ohms
The opposite error is less dramatic and more quietly destructive to the data. A source intended to drive a high impedance load is a voltage source with limited current capability. Asking it to drive a 50 ohm load is a heavy load by comparison, and the current it can supply is not enough to hold the voltage across that resistance.
The visible symptoms are a divided output, amplitude that falls away and a waveform that clips at the top or the bottom because the source has run out of headroom. The amplitude also varies with frequency in a way that has nothing to do with the device, since the load current rises as the frequency rises for a given voltage. The data looks plausible enough to be believed, which is what makes this mistake dangerous.
Why The Cable Matters
The cable is not a passive detail. A length of coaxial cable or even a test lead has a characteristic impedance of its own, and it also has capacitance distributed along its length. When the far end is terminated in the value the cable expects, the cable behaves as a matched line and the source sees its designed load. When the far end is not matched, the cable’s impedance is transformed into something else entirely at the source.
The effect scales with frequency. At low frequency the cable capacitance dominates and the load looks capacitive; at high frequency the reflection behaviour dominates and the length of the lead becomes a design parameter. This is why measurement leads are specified, why a proper 50 ohm cable is used for radio frequency work, and why an improvised lead on a bench can produce results that no amount of careful reading will explain. Where the signal quality of a fast link is being judged, the same reasoning that governs signal quality on a board applies to the cable on the bench.
Choosing The Setting Deliberately
The correct approach is to decide the load before connecting anything. If the instrument is a radio frequency source, a spectrum analyser or a network analyser port, a 50 ohm load is the expected termination and a feed-through terminator or the instrument’s internal termination should be used. If the instrument is an oscilloscope measuring a voltage, or a low frequency amplifier, the input is one megohm and the source expects to see that.
It also helps to know what the device under test expects. A module designed to drive a transmission line wants a matched load at the far end; a sensor whose output is a voltage wants to see a high impedance and no more. The setting on the instrument and the requirement of the device have to agree, and when they do not, the answer is usually a matching pad or an active probe rather than a change to the instrument’s input.
Avoiding The Mistake In Practice
Three habits remove most of the risk. The first is to check the instrument setting before the probe touches the circuit, and to note it in the test record, because a controlled impedance path that is measured with the wrong load produces data that cannot be interpreted later. The second is to use the right accessory: a passive high impedance probe for high impedance circuits, a 50 ohm cable and terminator for radio frequency circuits, and an adapter when the two have to meet.
The third is to sanity check the result. An amplitude that changes when the cable is moved, a waveform that clips, or an output that collapses relative to the datasheet are all signs that the load is wrong rather than that the device is faulty. Recognising that signature early is worth more than any amount of post processing.
FAQ
Can a fifty ohm instrument be used with a one megohm load? It can, but the reading will not represent the calibrated output. Where a high impedance load is genuinely required, an appropriate buffer or attenuator pad should be used.
Do I need a terminator if the instrument has an internal one? Sometimes, since not every input is terminated internally. Where the internal termination can be switched, it must be switched to the correct position rather than left at its default.
Why does the reading change when I move the cable? Because the load is not matched, so the line is resonating and the measurement depends on the lead. Terminating the far end properly removes the dependence.
Is a wrong termination setting dangerous? It can be. Driving a low impedance from a source that expects a high impedance load draws more current than the output stage was designed for, and an unterminated high frequency source can oscillate.
