HOW-TO GUIDE
How to Use Oscilloscope Math Functions to Subtract Two Signals
By the OhmSavvy Research Team ·
Safety first: subtraction does not isolate anything
- On a mains-powered bench scope, every probe's ground clip is connected to mains earth through the power cord. For CH1 − CH2, clip both grounds to the same circuit ground. Clipping either one anywhere else shorts that point to earth, which can damage the circuit, probes or scope, or injure you.
- Channel subtraction is not a substitute for a differential probe on mains or high-voltage circuits. Each probe still sees its point's full voltage to earth, so each probe and input must be rated for it, and a non-isolated mains circuit cannot be ground-clipped at all. Use a rated differential probe, an isolation transformer on the device under test, or an isolated scope. Never float the scope by defeating its earth.
How channel subtraction works
A digital scope samples CH1 and CH2 at the same instants. The math function takes each pair of samples, subtracts one from the other, and plots the answer as a third trace. If CH1 is at 5 V and CH2 at 3 V at a given moment, the math trace shows 2 V at that moment, which is the voltage between the two probe tips.
Pseudo-differential, in one paragraph
1Connect both probes to the same ground
Put the CH1 probe tip on one end of the component (or the signal you want to subtract from) and the CH2 tip on the other end. Clip both ground leads to the same circuit ground, close to the area you are measuring. Don't clip a ground lead to either end of the component itself unless that end really is ground; that is the whole point of using two channels. Use the same probe model on both channels, set both to 10x, and compensate each one first using our guide to compensating an oscilloscope probe.
2Match V/div, probe ratio and coupling
The scope converts each channel to volts before subtracting, so the settings have to be right on both. These are the ones that matter:
| Setting | What to do | Why |
|---|---|---|
| Probe ratio | Set each channel to match its probe (10x and 10x) | One wrong ratio scales that channel by 10 and ruins the difference |
| V/div | Use the same V/div on both channels | Equal resolution on both; the difference isn't limited by the coarser channel |
| Keep on screen | Neither trace may run off the top or bottom | On many scopes off-screen data is clipped, so the math result is wrong |
| Coupling | DC on both | AC coupling removes each channel's DC part, so DC differences disappear |
| Bandwidth limit | Same setting on both | A filter on one channel only changes its timing and amplitude |
| Invert | Off on both | An inverted channel turns subtraction into addition |
Vertical position doesn't matter: on most scopes the math uses the measured voltages, not where the traces sit on screen, so you can stack CH1 and CH2 wherever they are easiest to see. Check your manual to confirm.
3Open the MATH menu and choose CH1 − CH2
Press the MATH button (some scopes put it inside a menu), choose subtraction as the operator (shown as A − B or a minus sign), set source A to CH1 and source B to CH2. The math trace appears in its own color. Swapping the sources flips the sign of the result, which is useful when you want a positive answer. Trigger on whichever channel carries the cleaner edge, as described in how to trigger an oscilloscope. Menu names and the operators on offer vary by model, so check your manual. If you only want to see how an output differs from its input at a glance, start with how to compare input and output signals on an oscilloscope.
4Scale and read the math trace
The math trace has its own vertical scale and position. Subtracting volts from volts gives volts, so its scale is in V/div, but it is set separately from CH1 and CH2. A small difference between two large signals often needs a finer scale than the channels themselves. Read it the same way you would read any channel, as in how to measure voltage with an oscilloscope.
Worked example: a resistor divider has R1 on top and R2 below it, with R2 twice R1. The input node swings 0 to 6 V and the junction swings 0 to 4 V. Neither end of R1 is at ground, so probe the input with CH1 and the junction with CH2.
The numbers check out: R1 carries one third of the input swing and R2 two thirds, so R1 sees 2 V of the 6 V. Many scopes can also run automatic measurements (peak-to-peak, RMS, mean) on the math trace; check your manual.
Check the setup before you trust it
A quick test catches most mistakes: touch both probe tips to the same point. The math trace should be a flat line at 0 V. If a scaled copy of the signal remains, the channels don't match: check probe ratios, compensation and V/div.
Common uses for CH1 − CH2
| Use | CH1 | CH2 | Math trace shows |
|---|---|---|---|
| Voltage across a component with both ends above ground | One end | Other end | Voltage across the component |
| Remove a shared offset | Signal riding on a reference (e.g. a mid-rail) | The reference | The signal relative to its reference |
| Compare input with output | Buffer input | Buffer output | The error between them (near 0 V when they match) |
| Low-voltage gate drive | MOSFET gate | MOSFET source | Gate-to-source voltage |
Comparing input with output only gives a near-zero result if the two have the same gain and timing; any delay through the circuit shows up as spikes at each edge. If what you want is the time or phase between two signals rather than their difference, use the phase difference method instead. For a steady DC voltage across a resistor, a multimeter is simpler: see how to measure voltage across a resistor.
Limits of subtraction
Subtraction struggles when the difference is tiny compared with the voltage both points share. Take a current-sense resistor at the top of a 12 V rail, with 0.1 V across it. Both channels sit near 12 V, so at 2 V/div each 0.1 V is only 0.05 of a division.
The 1% and 8-bit figures are illustrations: channel gain accuracy and ADC resolution are on your scope's datasheet, and many entry-level scopes use 8-bit converters. In cases like this, a differential probe or a current probe does a better job. Fast edges cause a similar problem: on switching nodes, tiny timing differences between channels leave spikes in the result, which is one reason switching power supply measurements often call for a differential probe. Noise on either channel also adds up in the math trace; our guide on why a waveform is noisy helps there.
Which scope you need for channel math
You need at least two channels, so a single-channel unit like the FNIRSI DSO152 (1 channel) can't do it. Math functions are common on two- and four-channel digital scopes, but the operators on offer vary, so check the manual of any model you are considering. Four channels let you watch CH1, CH2 and the result while keeping spare inputs for other signals. Compare models in our best beginner oscilloscopes roundup.
Two-channel-and-up scopes in our reviews
50MHz, 4 channels, 1 GSa/s. The default beginner oscilloscope recommendation for years.
100MHz, 4 channels, 1 GSa/s, with more bandwidth and a better UI than the Rigol.
100MHz, 2 channels, 1 GSa/s, plus a built-in waveform generator.
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Frequently asked questions
Is CH1 minus CH2 the same as using a differential probe?
No. The math result shows the same difference, but each probe still measures its point relative to the scope's earthed ground, so each channel must handle the full voltage at its point. A differential probe measures between its two tips directly and rejects the shared voltage far better. Subtraction is fine for low-voltage circuits; mains and high-voltage work need a rated differential probe.
Can you subtract channels on a two-channel oscilloscope?
Usually yes. CH1 minus CH2 only needs two inputs, and math subtraction is a common feature on two-channel digital scopes. The exact menu, the available operators and whether you can choose the source order vary by model, so check the manual. Single-channel pocket scopes cannot do it because there is no second channel to subtract.
Why is my oscilloscope math trace off the screen?
The math trace has its own vertical scale and position, separate from CH1 and CH2. After you turn it on, adjust its V/div and position until it fits. If the result looks far larger than expected, check that both channels have the correct probe ratio set, because one channel left on 1x with a 10x probe makes the difference wildly wrong.
Can an oscilloscope measure the voltage across a resistor?
Yes. If one end of the resistor is at circuit ground, probe the other end directly. If both ends are above ground, probe each end with its own channel, ground both clips to circuit ground, and display CH1 minus CH2. For a steady DC value, a multimeter across the resistor is simpler and usually more accurate.
What does A minus B mean on an oscilloscope?
It is the math operation that subtracts source B from source A, sample by sample. On most scopes you pick which channel is A and which is B in the math menu, so CH1 minus CH2 and CH2 minus CH1 give the same shape with opposite sign. The result is a voltage, shown as its own trace.
