LT DIN Rail Analog Transmitters with Serial Data Communication and Analog Outputs for True RMS AC Voltage and Current Applications

True RMS AC Voltage & Current DIN Rail Transmitter Frequently Asked Questions

Why does the documented allowable crest factor increase from 3.0 at full scale to 300 at 1% of full scale, rather than staying constant?

Documented specification specifically ties the crest factor figure to signal amplitude — since crest factor is the ratio of peak to RMS value, a signal at a small fraction of full scale can have a much higher peak-to-RMS ratio while its peak voltage still stays safely within the transmitter's absolute input limits; the documented 3.0-at-full-scale to 300-at-1%-of-full-scale relationship reflects that the true constraint is on peak voltage, not on the crest factor ratio itself.

Does choosing the 10 Megohm input resistance factory special option reduce accuracy, or only maximum frequency?

Documented description specifically states the 10 Megohm version decreases maximum frequency from 5 kHz to 1 kHz for three of the voltage ranges — it doesn't document a stated change to the 0.03% of full scale accuracy figure itself, meaning the documented tradeoff for this factory special is specifically frequency bandwidth, not the base accuracy specification.

Why does the capture behavior change so much between signals above 50/60 Hz, signals from 3 Hz to 50/60 Hz, and signals below 3 Hz or at DC?

Documented capture strategy specifically adapts to how many complete cycles fit within a practical measurement window at each frequency range — above the power line frequency, multiple integral cycles are documented as averaged together; between 3 Hz and 50/60 Hz, a single complete cycle is captured; below 3 Hz and at DC, where a full cycle would take too long to wait for, the documented capture rate switches to a fixed 333 ms interval instead.

Does the documented "0-16.7 ms after completion of one input signal cycle" response time apply equally at every frequency the transmitter supports?

Not necessarily at every frequency — this documented fast-response figure is specifically tied to completing one input signal cycle, and since one full cycle takes different amounts of time at different frequencies (16.7 ms itself corresponds to one 60 Hz cycle), the practical response time documented for very low-frequency signals is governed instead by the separately documented 333 ms capture rate below 3 Hz and at DC.

Why does the accuracy calculation for the 600V range treat it as 2000V (20,000 counts) rather than using the actual 600V figure directly?

This is documented specifically as a footnoted accuracy-calculation convention rather than a change to the actual voltage being measured — the transmitter genuinely reads up to 600V, but the specific 2000V/20,000-count basis is documented as the reference used when calculating the applicable accuracy figure for that particular range, similarly to how the 5A range is documented as using a 20A/20,000-count basis for the same calculation purpose.

Does the 5A current range's built-in 0.01 ohm shunt mean an external current transformer still needs its own separate burden resistor?

No — documented description specifically frames the built-in shunt as what lets the output of a 5A current transformer be applied directly to the transmitter, with no need for a step-down transformer; the shunt itself is documented as serving the burden function for a 5A-output CT, rather than requiring the user to add a separate external burden component before connecting to the transmitter.

Does AC coupling change the transmitter's accuracy specification compared to DC coupling?

The page documents the 0.03% of full scale accuracy figure specifically for signals from DC to 5 kHz without stating a separate accuracy figure specifically for the AC-coupled configuration — AC versus DC coupling is documented as a jumper-selectable choice about which portion of the signal is measured (blocking or passing the DC component), rather than as a change to the underlying stated accuracy specification.

Does high common mode rejection specifically enable placing a current shunt on the high side of the line, or is it a general noise-immunity feature?

Documented description specifically connects these two: "High common mode rejection allows for stable readings with current shunts located on the high side of the line" — this is stated as a direct, specific enabling capability rather than only a general noise-immunity benefit, since a high-side shunt sits at a different common-mode voltage than a low-side (ground-referenced) shunt, and the transmitter's documented common mode rejection is what keeps that elevated common-mode voltage from degrading the measurement.

Does the transmitter's digital filtering option affect its documented fast response time?

The page documents digital filtering as separately selectable "for noisy signals," alongside the documented fast true RMS response of 0-16.7 ms after one signal cycle — since filtering by its nature trades some responsiveness for stability against noise, enabling a filter option would be expected to affect how quickly a given reading settles, though the specific quantitative relationship between a selected filter setting and the fast-response figure isn't detailed on this page.

Is the ±0.8V accuracy figure on the 600V and 300V ranges comparable to the ±0.4V figure documented on the DC voltage/current transmitter's 600V range?

They're documented as separate, range-specific figures on two different transmitter variants rather than directly interchangeable — this True RMS transmitter's 600V and 300V ranges are each documented with a flat ±0.8V accuracy figure, distinct from the DC voltage/current transmitter's own separately documented ±0.4V figure for its 600V range; the different underlying measurement technique (true RMS versus DC) is consistent with each variant carrying its own specific accuracy figure for its highest voltage range.

Crest Factor & Non-Sinusoidal True RMS Measurement Questions From the Field

Why do average-responding meters give incorrect readings on non-sinusoidal waveforms, while true RMS meters don't?

Documented explanation specifically describes average-responding meters as applying a sine-wave correction factor to whatever waveform they measure — since this correction factor is only valid for a genuine sine wave, feeding it a distorted or non-sinusoidal waveform is documented as producing readings that can be wrong by as much as 50 percent, whereas true RMS meters calculate the genuine heating-equivalent value directly from the actual waveform shape rather than relying on that sine-wave assumption.

Does a true RMS meter's sine-wave accuracy specification automatically apply to every waveform shape it measures?

No — documented guidance specifically flags this as a common misconception; the shape of the input signal can dramatically affect measurement accuracy even on a genuine true RMS instrument, since every meter has documented limits tied to crest factor and bandwidth, and a waveform that exceeds those specific limits can still produce a significantly inaccurate reading despite the instrument being true RMS.

What specifically causes nonlinear loads like switch-mode power supplies and VFDs to produce high-crest-factor current waveforms?

Documented explanation specifically describes these loads as drawing current in short, narrow pulses near the peaks of the voltage waveform rather than smoothly across the full cycle — a switch-mode power supply charging a capacitor near the voltage peaks is documented as a classic example, and this pulsed, peaky current draw is what produces a current waveform with a peak value large relative to its RMS value.

Why is a documented crest factor rating of 3.0 or higher commonly recommended for meters used on modern industrial power systems?

Documented guidance specifically ties this recommendation to the presence of harmonics in modern loads — while a pure sine wave has a crest factor of only 1.414, documented analysis notes that harmonics from nonlinear loads raise the peaks of both voltage and current waveforms relative to their RMS values, so a meter rated for only the sine-wave crest factor would be inadequate for accurately measuring the more sharply peaked waveforms actually present in many real industrial systems.

Can the peak-capture feature of a true RMS instrument be used to actually detect whether a waveform is distorted?

Yes — documented field practice specifically describes measuring the true RMS value, multiplying it by 1.414 to get the theoretical peak value for an undistorted sine wave, then comparing that theoretical figure to the actual measured peak captured by the instrument; a significant difference between the two documented values indicates the waveform is distorted and likely contains harmonics.

Do voltage harmonics and current harmonics typically push crest factor in the same direction?

No — documented field guidance specifically distinguishes these: for voltage harmonics, the typical crest factor is documented as below the sine-wave value of 1.414, producing "flat-top" waveforms, while for current harmonics, the typical crest factor is documented as considerably above 1.414, reflecting the sharply peaked current pulses nonlinear loads tend to draw.

Is instrument bandwidth a separate limitation from crest factor when measuring distorted waveforms, or do they describe the same constraint?

They're documented as genuinely separate constraints — crest factor describes how large a peak-to-RMS ratio the instrument's conversion circuitry can handle, while bandwidth describes the frequency range across which the instrument can accurately respond; documented guidance specifically notes all meters have limited bandwidth and cannot detect harmonic content above that bandwidth limit, regardless of how generous their crest factor rating is.

Can measurement error from a high crest factor be severe enough to matter for practical power system troubleshooting?

Yes — documented technical analysis specifically cites conventional RMS converters exhibiting error on the order of 4% at a crest factor of 4, with PWM signals used in motor controllers and switching power supplies documented as reaching crest factors greater than 30 — at that level, documented guidance specifically states commonly used RMS meters give unacceptable results, underscoring that crest factor limitations are a genuine practical accuracy concern, not just a specification-sheet technicality.