A four-channel Arduino DC voltmeter is a four-way divider and ADC exercise. All channels share one ground and one analog-to-digital converter, so readings are taken one after another rather than at the same instant, and each channel needs its own scaling and calibration. Handled carefully it is a good low-voltage monitoring reference; confirm the range and scaling against your own board and components, and remember that displayed decimal places are not an accuracy statement.
What the ADC is actually doing
On the classic UNO R3, analogRead() uses a 10-bit ADC and A0 through A5 are usable analog inputs, per the official Arduino reference. The ATmega328P datasheet notes that the conversion is single-ended and referenced to the selected VREF, and its input discussion recommends a source impedance of roughly 10 kilohms or less while explaining sample-and-hold charging limitations. Those three facts define what a voltmeter channel can and cannot do: it digitises a pin voltage against a reference, and it expects the source to be stiff enough to charge the sample-and-hold quickly.
Four channels, one converter
Because there is a single ADC, the channels are multiplexed. The practical consequence is that readings are sequential, not simultaneous, so a value read on channel 1 and a value read on channel 2 belong to slightly different moments. For slow monitoring that is usually fine; for anything changing faster than the sampling loop it is a real limitation to state honestly. The channels also share a ground, which makes them convenient but not isolated inputs.
Diagnostic table: common symptoms
| Observation | Plausible cause | What to check |
|---|---|---|
| Channels disagree on a common source | Per-channel divider tolerance or missing per-channel calibration | Each divider’s measured ratio and its own constants |
| Drift on one channel only | High source impedance or a poor connection on that divider | That channel’s source impedance and contact quality |
| Crosstalk between adjacent channels | Insufficient settling between channel switches at the shared input | Whether settling time is respected between reads |
| Values change when leads move | The ground return is not truly common | Ground path and where the source ground connects |
| Reads correct at low voltage, wrong higher up | Nonlinear behaviour near an endpoint or an over-range input | Whether the divider keeps the pin inside its safe input window |
An ordered evaluation process
- Restrict the design to isolated low-voltage DC. These inputs share ground and are not independent, and mains or floating high-energy sources are out of scope.
- Measure each divider ratio individually instead of assuming identical scaling across channels.
- Verify the ADC reference arrangement, since a 10-bit result is only meaningful against a known VREF.
- Check each divider’s source impedance against the recommendation to avoid sample-and-hold lag.
- Calibrate every channel separately against a known reference voltage, deriving a scale and offset.
- Confirm the ground reference is genuinely common to all sources before trusting any comparison between channels.
- Document a per-channel uncertainty figure so the display does not imply more confidence than exists.
Accuracy caveats worth keeping
Displayed decimal places are not an accuracy statement. The dominant errors are divider tolerance, VREF stability and input behaviour near the ADC endpoints. Drive the display according to its own datasheet, and confirm any input protection against the voltages you actually expect. For a related treatment of divider and ADC calibration in a single-channel meter, see the OLED voltmeter guide, and for resistance-measurement limits see the milliohm meter article.
Reader questions
Are the four readings simultaneous?
No. A single ADC is multiplexed, so the channels are sampled one after another and belong to slightly different moments.
Can the channels measure four separate floating supplies?
No. They share a ground, so they are not isolated inputs. Treating them as independent floating inputs would be a safety and measurement error.
How do I make the channels agree better?
Calibrate each divider individually; shared scaling hides real differences between channels.
Will averaging fix the accuracy?
Averaging reduces random noise but not tolerance-based error. It cannot substitute for per-channel calibration.
Documentation: Arduino analogRead() reference (official reference-en source); Microchip ATmega328P datasheet, 7810D automotive edition.
Documentation only: the cited references cover ADC resolution, input mapping, conversion referencing and source-impedance guidance. No bench testing or calibration was performed, so treat per-channel readings as indicative until checked against a known reference, and keep the inputs within the isolated low-voltage DC scope above.
