Quick answer: a high-resolution differential ADC sets how finely you can digitise a small voltage, not how accurately you know a resistance. End-to-end milliohm accuracy is a budget, and the excitation current, its stability, the ADC reference, and the gain setting all sit inside it. Resolution is one line in that budget, not the answer.
Resolution is not accuracy
The Microchip MCP3422/3/4 datasheet describes the MCP3422 as having two differential input channels, an internal reference, and programmable gain. It pairs 18-bit mode with 3.75 samples per second, and offers higher data-rate options at lower resolutions. That is a resolution-versus-speed trade-off, and on its own it says nothing about how accurate a resistance reading will be. A differential converter can resolve very small voltage differences while the overall resistance result is still dominated by other terms in the chain.
The excitation source is not covered here
A milliohm measurement needs a defined excitation current; the voltage developed across the unknown component is what the ADC reads, so the current sets the scale. The current source’s own documentation is not among the verified sources for this guide, so its behaviour and limits cannot be stated here. Treat the excitation as a requirement you confirm from the exact regulator or current-source datasheet you choose: an unverified current means an unverified scale factor, no matter how many bits the converter offers.
Comparison: the accuracy-budget terms
| Term | Role in the result | Status here |
|---|---|---|
| Differential ADC resolution | Smallest resolvable step | Documented (18-bit mode, programmable gain) |
| ADC reference and gain error | Scale-factor error | Internal reference documented; end-to-end not verified |
| Excitation current value | Sets volts per milliohm | Not covered by the sources here |
| Excitation stability | Reading repeatability | Depends on the part you choose |
| Thermoelectric offsets | Small-voltage error | Not measured here |
| Range scaling | Gain chosen per range | Depends on the chosen excitation |
Ordered evaluation process
- Work on unpowered, detached components only: no live-circuit probing or injection into batteries or unknown electronics.
- Define the excitation current you intend, and treat it as an assumption until its documentation is checked.
- Choose an ADC gain and range so the expected voltage fits comfortably inside the converter input window.
- Decide the resolution-versus-speed trade-off (for example 18-bit at 3.75 samples per second) against how fast you need readings.
- List every scale-factor term: excitation value, ADC reference, ADC gain, and range setting.
- Account qualitatively for small-voltage error sources such as thermoelectric offsets.
- Calibrate against known resistors, and record the reference used and the conditions.
- Publish an accuracy budget rather than a single resolution figure.
Reader questions
Does 18-bit resolution mean 0.1 milliohm accuracy?
No. Resolution is the smallest step the converter can represent. Accuracy depends on the excitation current, the reference, the gain, and offset sources, none of which 18-bit mode alone establishes.
Why is the excitation source not covered here?
Its official documentation is not among the verified sources for this guide, so its current behaviour and limits cannot be stated. Treat the excitation as a documented requirement you must confirm separately against the exact part you use.
Is a differential ADC overkill for this job?
It suits small differential voltages well, which is why it appears in milliohm designs. But it only improves the digitisation term of the budget; a proper four-wire connection and a stable excitation still matter.
Can I measure resistance in a live circuit?
No. This reference covers unpowered, detached components only, with no precision current-sink wiring, live probing, or current injection.
Documentation
Documentation-based reference only: the single source is the Microchip MCP3422/3/4 datasheet (22088C). No implementation, calibration, or measurement was performed, so range figures, any resolution claim, current levels, thermoelectric offsets, and complete meter accuracy still need to be established for your design. A general auto-ranging ohmmeter reference covers higher resistance values, and the fluxgate current-probe reference addresses current sensing separately.
