{"id":3464,"date":"2026-10-09T04:17:00","date_gmt":"2026-10-08T20:17:00","guid":{"rendered":"http:\/\/www.arasdamco.com\/blog\/?p=3464"},"modified":"2026-10-09T04:17:00","modified_gmt":"2026-10-08T20:17:00","slug":"what-is-the-input-impedance-of-honeywell-temperature-transmitter-4697-cef41c","status":"publish","type":"post","link":"http:\/\/www.arasdamco.com\/blog\/2026\/10\/09\/what-is-the-input-impedance-of-honeywell-temperature-transmitter-4697-cef41c\/","title":{"rendered":"What is the input impedance of Honeywell Temperature Transmitter?"},"content":{"rendered":"

Hey there, if you\u2019re hanging around industrial automation long enough, you\u2019ve probably stared at a Honeywell temperature transmitter\u2019s spec sheet and scratched your head at a random line item: \u201cinput impedance.\u201d I know I did, back when I was a new account rep for our firm\u2014we\u2019ve been a trusted Honeywell temperature transmitter supplier for over a decade, and every time I\u2019d hop on a call with a plant engineer or a systems integrator, this question would pop up faster than a quick coffee break. \u201cWhat\u2019s the input impedance on that STT250? Or the STT850? I don\u2019t want my RTD signal getting fried mid-process.\u201d Let\u2019s break this down like we\u2019re geeking out over the tiny details that actually keep your refineries, food processing lines, and power plants running. Honeywell Temperature Transmitter<\/a><\/p>\n

<\/p>\n

First off, let\u2019s get the basics straight: input impedance, for any temperature transmitter, is basically how much resistance the device\u2019s input stage puts up when your sensor (whether it\u2019s an RTD, thermocouple, or even a resistance temperature device) sends it a signal. Think of it like a gatekeeper at a concert\u2014if the gate is super wide (high impedance), the signal flows through without fighting; if it\u2019s narrow (low impedance), the signal gets bottlenecked, distorted, or even lost entirely. That\u2019s not just a fun analogy\u2014this is a make-or-break detail for accuracy, especially when you\u2019re dealing with millivolt-level thermocouple signals or low-resistance RTDs.<\/p>\n

Now, let\u2019s talk Honeywell-specifics because that\u2019s what you\u2019re here for. Honeywell doesn\u2019t just slap a one-size-fits-all input impedance on all their temperature transmitters\u2014they tailor it to the type of sensor they\u2019re pairing with, and that\u2019s where most engineers mess up when cross-referencing specs. Let\u2019s start with the big two: thermocouples and RTDs, since those are the most common sensors we ship every day.<\/p>\n

Thermocouples are the finicky ones, right? They produce tiny, low-voltage signals\u2014like, millivolts, not full volts. For their line of thermocouple temperature transmitters, including the popular STT250 and STT850 series, Honeywell specs input impedance at a minimum of 10 M\u03a9 (megaohms) for standard applications, and up to 100 M\u03a9 for high-noise or long-lead setups. Wait, why so high? If a thermocouple\u2019s signal is only 5 mV at 100\u00b0C, a low input impedance would act like a sponge siphoning that tiny voltage away, leading to wild, inaccurate readings. I once had a customer at a petrochemical plant who didn\u2019t check this\u2014they used a no-name transmitter with 1 M\u03a9 input impedance, and their furnace temp was off by 150\u00b0F. By the time they figured it out, they\u2019d scraped three batches of catalyst. Ouch. Honeywell designed their thermocouple transmitters specifically to avoid that headache, so that 10+ M\u03a9 number is non-negotiable for reliable signal capture.<\/p>\n

Next up, RTDs\u2014resistance temperature detectors, the ones that are sturdier and give more stable signals than thermocouples, especially in moderate temp ranges. RTDs come in different resistance values: 100 \u03a9, 500 \u03a9, 1000 \u03a9, even 2000 \u03a9, and you can\u2019t just pair any input impedance with them. For their 2-wire, 3-wire, and 4-wire RTD transmitters (like the STT250 RTD variants), Honeywell\u2019s input impedance is tailored to be low enough to avoid noise pickup, but not so low that it loads the RTD\u2019s signal. Wait, hold on\u2014this is the opposite of thermocouples! For RTDs, Honeywell typically specs input impedance between 1 k\u03a9 (kiloohms) and 10 k\u03a9. Why? Because RTDs work by passing a small excitation current through the sensor and measuring the voltage drop. If the transmitter\u2019s input impedance was too high, you\u2019d get stray capacitance and inductance from long cable runs picking up electrical noise from motors or VFDs. That 1-10 k\u03a9 sweet spot balances signal stability and noise rejection\u2014again, Honeywell\u2019s engineers tested this over decades, so it\u2019s not a random number. I\u2019ve seen engineers try to swap in a transmitter with higher impedance for an RTD project, and they ended up with random temp spikes that made their quality control team pull all-nighters. Never again, right?<\/p>\n

Wait, what about other sensor types? There\u2019s also thermistors, which are common in HVAC applications, and even current-output sensors for some custom setups. For thermistor inputs, Honeywell uses input impedance in the 10 k\u03a9 to 100 k\u03a9 range, just because thermistors have much higher resistance changes with temp, so they can handle a bit more impedance without loading. And for our newer STT850 smart transmitters, they have configurable input impedance for some third-party sensors, which is a game-changer for integrators who are mixing and matching hardware.<\/p>\n

Now, let\u2019s get into the weeds of why this actually matters beyond the spec sheet. Input impedance ties directly to something called \u201cloading error\u201d\u2014that\u2019s the inaccuracy you get when the transmitter\u2019s input takes too much (or too little) of the sensor\u2019s signal. For thermocouples, loading error is the big one because their signal is so small. Honeywell\u2019s 10 M\u03a9 minimum means loading error is less than 0.1\u00b0C for most standard thermocouple types, which is well within industrial accuracy requirements. For RTDs, loading error is minimal because their signals are larger, but that 1-10 k\u03a9 impedance keeps cable length effects (another common pain point) from throwing things off. If you have a 100-foot cable run to an RTD, a transmitter with too high input impedance would let the cable\u2019s resistance add tiny offsets, but Honeywell\u2019s spec eliminates that.<\/p>\n

Another thing: our firm (as a Honeywell temperature transmitter supplier) gets a ton of questions about how this interacts with other parts of the control loop. Like, if you\u2019re pairing a Honeywell transmitter with a PLC or DCS, do you have to match the input impedance of the receiver too? Short answer: not usually, because most modern DCS inputs have impedance way higher than 10 M\u03a9 for thermocouples or way lower than 10 k\u03a9 for RTDs, so there\u2019s no conflict. But it\u2019s worth double-checking if you\u2019re doing a custom retrofit, not just a straight replacement. Pro tip: if you\u2019re swapping out an old transmitter for a Honeywell one, never skip checking the sensor type and matching the input impedance spec\u2014we\u2019ve had customers come back after a bad install, and 9 times out of 10, it\u2019s a mismatch here.<\/p>\n

Wait, let\u2019s clear up a common myth I hear all the time: \u201cHigher input impedance is always better.\u201d No, that\u2019s only true for thermocouples. For RTDs, too high an input impedance is a bad thing. I had a customer in the food and beverage industry last year who was upgrading their pasteurization lines. They went with a premium competitor\u2019s transmitter that had 100 M\u03a9 input impedance for their RTDs, and they were getting random temp fluctuations that made their cheese batches fail safety tests. We swapped in a Honeywell STT250 with its 5 k\u03a9 input impedance (perfect for 1000 \u03a9 RTDs), and the fluctuations disappeared. Turned out the high impedance was picking up stray signal from their conveyor motors 50 feet away. That\u2019s the kind of real-world stuff we deal with every day as a Honeywell temp transmitter supplier, not just reading specs off a page.<\/p>\n

Now, let\u2019s talk about how Honeywell tests this, because it\u2019s not just a marketing number. Their temperature transmitters go through rigorous testing for input impedance over temperature ranges from -40\u00b0C to 85\u00b0C (and even higher for extreme industrial models, up to 150\u00b0C for hazardous locations). That means the input impedance doesn\u2019t drift as the transmitter heats up or cools down, which is crucial for applications like boiler monitoring where temps swing wildly. A lot of budget transmitters will have a static input impedance number, but it drops or rises with temperature, leading to accuracy errors after a few months of use. Honeywell\u2019s testing ensures that input impedance stays within spec even in the harshest environments\u2014think refineries with high humidity, chemical plants with corrosive fumes, or outdoor oil rigs.<\/p>\n

What about for the smart transmitter models, like the STT850 that supports HART, FOUNDATION fieldbus, or Modbus protocols? Their input impedance is still tailored to the sensor, but with the added benefit of digital signal processing that compensates for minor impedance-related drift. We\u2019ve had a pharmaceutical client use these for their vaccine cold storage monitoring, and they\u2019ve never had a temp reading error that\u2019s attributed to input impedance in 3 years of use. That\u2019s the kind of reliability you pay for, and it\u2019s why Honeywell transmitters are our top-selling line.<\/p>\n

Now, if you\u2019re someone who\u2019s actually reading this because you\u2019re in the market for a new temperature transmitter, here\u2019s your quick cheat sheet for input impedance and Honeywell:<\/p>\n