LT DIN Rail Analog Transmitters with Serial Data Communication and Analog Outputs for Process and Ratiometric Signal Applications P/N LT20P

Process & Ratiometric DIN Rail Transmitter Frequently Asked Questions

Why does ratiometric mode use a different excitation voltage for potentiometers (5V) than for bridges (5V or 10V)?

Documented specification specifically lists 5V or 10V as available for bridge circuits, but only 5V for potentiometers — this reflects that bridge circuits and potentiometers are documented as genuinely different sensor types with different excitation requirements; the specific voltage options available for each are tied to what that particular sensor type is designed to accept, rather than being a single universal setting for all ratiometric applications.

Does ratiometric mode eliminate the need for the excitation supply to be stable, since the transmitter is documented as unaffected by excitation changes?

Not entirely — while documented ratiometric operation specifically compensates for changes in the applied excitation level (since both the sensor's output and the reference are affected proportionally), this compensates for excitation level drift specifically, not for other independent noise or instability the excitation supply might introduce into the signal path; a genuinely noisy excitation source can still degrade the measurement even in ratiometric mode.

Why does custom curve linearization use multiple non-linear spline-fit segments rather than a single formula covering the whole curve?

Documented explanation specifically states this approach provides much better accuracy than linear segments — real-world nonlinear relationships (such as volume versus level in an irregularly shaped tank) rarely follow one simple mathematical formula across their full range, so breaking the curve into multiple segments, each independently fit to the actual data points, lets the linearization follow the true nonlinear shape far more closely than a single equation could.

Can the same physical transmitter be switched between absolute and ratiometric measurement modes, or is this a fixed hardware choice made at purchase?

Documented note specifically states the same DC signal conditioner can be user-configured for DC, process, bridge, and potentiometer signals — this points toward configuration-level flexibility on the same hardware rather than requiring a separate physical transmitter purchased and fixed for each mode, though the specific field-configuration process itself isn't detailed beyond this documented capability statement.

Does the documented 0.01% of reading accuracy in ratiometric mode mean a small-signal reading near the bottom of a range is just as accurate in absolute terms as a full-scale reading?

No — "of reading" accuracy is documented as scaling with the actual measured value, meaning the absolute error in real units shrinks proportionally as the reading itself gets smaller; this differs from "of full scale" accuracy (used for absolute mode), where the absolute error stays roughly constant regardless of where in the range the actual reading falls, making ratiometric mode's accuracy figure relatively more favorable at low readings within its range.

Does the P1/SG1 custom scaling option change the transmitter's underlying accuracy compared to the standard P/SG default scaling?

No — documented distinction between these options is specifically about how the input-to-output relationship is configured (default fixed scaling versus user-specified custom minimum/maximum input and reading pairs), not a change to the underlying signal conditioner's accuracy, CMR/NMR, or conversion technique, which are documented as shared specifications regardless of which specific scaling option is selected.

Why are altimeters and thermistors specifically mentioned as custom curve linearization applications alongside tank volume readout?

All three documented examples share the same underlying need: a genuinely nonlinear relationship between the raw sensor signal and the desired displayed value — tank volume versus level in an irregular shape, altitude versus pressure (which follows a nonlinear atmospheric model), and temperature versus resistance in a thermistor (which follows a nonlinear resistance curve) are all documented as benefiting from the same spline-fit linearization approach rather than a simple linear scale.

Does the 1 GΩ input impedance on the 200.00 mV and 2.0000V ranges apply the same way when the transmitter is used in ratiometric mode with a potentiometer?

Documented specification lists 1 GΩ specifically for these two voltage ranges as a general input characteristic, without carving out a separate figure for ratiometric versus absolute use of those same ranges — since ratiometric mode changes how the reading is interpreted and referenced to excitation, not the physical input circuitry's loading characteristic, the same 1 GΩ impedance applies regardless of which mode those ranges are operating in.

Can the Extended board's custom curve linearization and ratiometric potentiometer mode be used together on the same transmitter?

Documented capability describes these as addressing different stages of the measurement — ratiometric mode governs how the raw potentiometer signal is measured relative to excitation, while custom curve linearization applies a nonlinear correction to whatever signal is being read; nothing in the documented feature descriptions restricts them to mutually exclusive use, so a nonlinear potentiometer application could genuinely benefit from combining both.

Why does the 600V voltage range carry a flat ±0.4V accuracy figure instead of the 0.01% FS ±2 counts pattern used by the other ranges?

This is documented as a genuine, stated exception specific to that top range — extending accurate measurement to 600V involves practical tradeoffs at the high end of the voltage scale, and the flat ±0.4V figure is the transmitter's documented accuracy specification for that specific range rather than a typo or an application of the same percentage-based formula used elsewhere.

Precision Potentiometer Wiper & Position-Sensing Questions From the Field

Why is reading a potentiometer ratiometrically specifically recommended for precision position feedback applications?

Documented guidance specifically explains that ratiometric reading references the potentiometer's output to the same supply driving it, so that supply drift affects both the signal and the reference proportionally and cancels out — this is documented as the reason ratiometric reading keeps the position measurement accurate even when the excitation supply itself isn't perfectly stable.

What is "contact-resistance-variation" (CRV) noise in a potentiometer, and why does it matter for precision position sensing?

Documented technical explanation specifically describes CRV as noise arising from inconsistent contact between the wiper and the resistive element as it moves — since this noise appears as small, spurious variations superimposed on the genuine position signal, it directly degrades the precision with which small position changes can be reliably detected, independent of the potentiometer's nominal accuracy rating.

Does wiper contact design (such as a multi-finger contact versus a single-point contact) genuinely affect measurement noise?

Yes — documented design comparison specifically notes multi-finger contact designs exhibit lower contact noise than single-contact designs, since individual contact fingers track the resistive element's surface somewhat independently, reducing the interruption and "make-and-break" noise associated with a single contact point losing and regaining contact.

Does keeping wiper current low actually help protect long-term measurement accuracy, or is it mainly about component lifespan?

Both, according to documented guidance — limiting wiper current is specifically recommended both to protect the resistive track from excess current-driven wear and, separately, because lower wiper current is documented as contributing to more stable, repeatable contact behavior, which supports maintaining accurate readings over the potentiometer's service life rather than only extending its mechanical lifespan.

Should potentiometer linearity be evaluated only against the manufacturer's nameplate specification, or verified independently for a specific application?

Documented best practice specifically recommends measuring linearity across the actual working range being used in the application, not relying solely on the nameplate specification — since a potentiometer's real installed stroke or rotation range may not match its full rated travel, verifying linearity specifically within the actual operating range is documented as necessary for genuinely trustworthy position accuracy.

Can mechanical wear on a potentiometer's resistive element cause its noise characteristics to change progressively over time, even without any change in wiring or excitation?

Yes — documented analysis specifically describes how abrasive wear on the resistive element's surface during repeated wiper travel can cause the contact area to enlarge over time while spring-applied contact pressure stays constant, and this documented mechanism is specifically identified as producing a systematic drift in noise characteristics, and potentially calibration, purely from accumulated mechanical wear.

Does adding a small filter capacitor from the wiper to ground genuinely help without introducing its own measurement error?

Documented guidance specifically recommends a small capacitor (10-100 nF) from the wiper to ground specifically to filter noise without disturbing DC accuracy — the small capacitance value is specifically chosen to filter high-frequency noise while remaining small enough not to meaningfully affect the DC signal level the position measurement actually depends on.

Is periodic continuity or resistance testing a genuinely useful maintenance practice for potentiometers used in long-term position sensing?

Yes — documented maintenance guidance specifically recommends periodic continuity testing between the wiper and end terminals to detect breaks or inconsistencies, along with periodic resistance and linearity measurement using precision test equipment, as a way of catching developing wear-related problems (such as the documented CRV drift mechanism) before they meaningfully affect the accuracy of position readings in service.