FMR documents a 12-bit analog-input resolution and allows low-resistance sensors to be configured. Matrix also allows low-resistance sensor configuration. The available evidence does not establish whether either device switches excitation current when the configured sensor range changes.
Separate ADC Resolution from Measurement Range
A 12-bit converter provides 212 = 4096 discrete codes. That value describes conversion granularity; it does not identify the resistance span, excitation current, input gain, or sensor-transfer calculation. Therefore, the 12-bit rating alone cannot prove that one fixed excitation current is insufficient or that the hardware switches current automatically.
Confirmed Behavior and Open Questions
| Item | Evidence status | Engineering consequence |
|---|---|---|
| FMR analog-input resolution | 12 bit | 4096 converter codes are available before any documented scaling or processing. |
| Low-resistance sensor selection | Available in FMR and Matrix | The configuration supports this sensor category, but the evidence does not reveal the input circuit. |
| Excitation-current switching | Not confirmed | Do not assume that changing the configured sensor type changes excitation current. |
| Current values and switching thresholds | Not supplied | No numeric current, range boundary, or accuracy calculation can be stated. |
Determine the Actual Input Method
Possible implementations include switched excitation current, selectable gain, switched resistance networks, or software conversion over a fixed electrical range. These are diagnostic hypotheses, not confirmed FMR or Matrix behaviors.
- Review page 34 of the referenced FMR manual for the input circuit, supported sensor ranges, excitation-current data, or range-dependent settings. Do not infer details that the manual does not state.
- Configure each supported low-resistance sensor range separately and apply known resistance values that remain valid for the selected range.
- Measure the sensor-terminal voltage with a high-impedance meter. If the circuit behaves as a current-excited resistive input, calculate the apparent excitation current as
I = V / R. - Repeat at the same resistance after changing only the configured range. A repeatable change in calculated current supports switched excitation; unchanged current indicates that another mechanism may provide range adaptation.
Perform this test only if the device documentation permits external resistance substitution and terminal measurement. Record the configured range, applied resistance, terminal voltage, and resulting reading so that configuration effects remain separate from sensor and wiring errors.
FAQ
Does the FMR 12-bit input automatically switch excitation current?
The available evidence does not confirm that behavior. Verify page 34 of the FMR manual or compare calculated current at the same resistance under different configured ranges.
How many codes does a 12-bit FMR analog input provide?
A 12-bit conversion provides 4096 discrete codes because 212 = 4096. This does not, by itself, define the supported resistance span or excitation current.
How can I test whether excitation current changes with sensor range?
Apply the same known resistance in each valid configuration, measure terminal voltage, and calculate I = V / R. Compare the results without changing the resistance or wiring.
