How to Use Differential Amplifier to Measure 0-50°C Human Body Temp Difference (5cm)

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How can I measure the temperature difference between two skin-mounted sensors about 5 cm apart over a 0–50°C range using a differential amplifier?

Use matched Pt100/Pt1000 sensors in a Wheatstone bridge and amplify the bridge output with an instrumentation or differential amplifier; an LM335 is not a good choice for very small skin-temperature differences because of its thermal mass [#21673186][#21673193] A Pt1000 bridge gives about 1 mV/K sensitivity, while two Pt100s give about 0.1 mV/K, and the amplifier gain can be set with A = (R2 + R1) / R1 [#21673186] Keep the sensor cables exactly the same length to avoid resistance errors, and use the trimmers R4/R5 to set the zero point [#21673186][#21673188] If you power the op-amp from a single supply, the second input must be biased to V+/2 (4.5 V on a 9 V supply), but that makes small differences hard to detect because the output sits around 4.5 V at zero difference [#21673191] Also avoid driving the Pt100s with too much voltage/current, because the resulting self-heating can be around 10 K in the posted schematic, so use series resistors or a current source instead [#21673191][#21673188] If you want automatic calibration rather than manual trimming, you will need a microcontroller [#21673188]

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Suggesting a differential amplifier to boost sensor sensitivity
#1 21673183 15 Jan 2015 10:28

zeyad asaad zeyad asaad

Anonymous

Suggesting a differential amplifier to boost sensor sensitivity
Post #1 21673183 15 Jan 2015 10:28

i am looking for a circuit to measure difference in temperature (+ or -) on the human body.
the distance is between two thermal sensors is about 5 cm, and the reading range is between 0 -50 Celsius.
i have a circuit but i need to make it more sensible. i was thinking of adding differential amplifier.
any one has got an idea of how to improve that by adding amplifier
i am struggle with the circuit
many thank in advanced
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#2 21673184 16 Jan 2015 09:11

Enrico Koeck Enrico Koeck

Anonymous

Post #2
21673184 16 Jan 2015 09:11

In the attached circuit the differential amplifier should be attached where the METER is drawn.
Clarifying sensor pin connections and body placement
#3 21673185 16 Jan 2015 16:41

zeyad asaad zeyad asaad

Anonymous

Clarifying sensor pin connections and body placement
Post #3 21673185 16 Jan 2015 16:41

i am stuck with the circuit because i don't know how to connect the temperature sensor because it has 3 pins, and i don't know which one is going to the human body
i will be very grateful if anyone can redraw the circuit for me and i don't know whether the circuit will work or not.
this what i want to acheive but i don't know whether the circuit will work or not
http://www.autospeed.com/cms/article.html?&title=Differential-Temp-Meter-Part-2&A=111357
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Pt1000 bridge recommended over LM335 for skin temperature
#4 21673186 18 Jan 2015 09:10

Enrico Koeck Enrico Koeck

Anonymous

Pt1000 bridge recommended over LM335 for skin temperature
Post #4 21673186 18 Jan 2015 09:10

In the datasheet of the LM335 is stated which pin has which function depending on your package (TO-92, SO-8, ...). Neither pin goes to the body but the Sensor itself needs to be attached where you want to measure temperature (the third pin is for adjusting the sensor output). Regarding its large size (big thermal mass or heat capacitance) the LM335 is IMHO not very suitable for measuring small temperature differences of an object near room temperature. Maybe it would be wiser to use a Pt100/Pt1000 with a very small footprint, put it directly onto the skin and isolate it against the ambient temperature. If you wire them in a Wheatstone bridge you would get a sensitivity of approximately 1mV/K (Pt1000 resp. 0.1mV/K using two Pt100). This voltage can be easily amplified if you are paying attention to noise and clibrate the bridge.

If you wanted to use the attached circuit:
- the wires to the sensors (dashed boxes) should be same length
- the gain of the amplifier is calculated by A = (R2 + R1) / R1 thus the sensitivity of the circuit is approximately (R2 + R1) / R2 * 1mv/K
- The lowpass filter at the output of the amplifier can be used to suppress noise, and its corner frequency is calculated by f = 1 / (2 * pi * R3 * C1). Its useful value depends on the expected speed of temperature changes you want to monitor. If they are very slow it would be necessary to use an active filter, otherwise the value for the capacitor would become very large. But the circuit works without this filter as well.
- the potentiometers R4 and R5 are used to set the zero point
- your voltage output has a positive value if the temperature at the second sensor (Pt1000-2) is higher than at the first one
- you need positive and negative supply voltage for the opamp as well
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#5 21673187 18 Jan 2015 12:43

zeyad asaad zeyad asaad

Anonymous

Post #5
21673187 18 Jan 2015 12:43

hi ENRICO KOECK
i am very grateful of what you have done
in order to get the difference between two sensors i have to calibrate one of them to zero then connect the other to have the difference
is there any method to get the temperature difference without calibrating one of them?
i have done some calculation and would like to know your opinion of their values
please see the attached circuit
Pt1000 self-heating and cable resistance concerns
#6 21673188 19 Jan 2015 05:10

Enrico Koeck Enrico Koeck

Anonymous

Pt1000 self-heating and cable resistance concerns
Post #6 21673188 19 Jan 2015 05:10

I'm sorry, I can't open your attached file because I don't have MultiSim installed. Maybe you can post a screenshot. Well if you need the sensors only over a small temperature range (as you stated) it is maybe sufficient to adjust the potentiometers only once or once in a while to level out the resistance difference caused by production tolerance. The prequesit would be that the cables for the sensors are exactly the same length otherwise you get errors caused by the resitance of the cables and the temperature dependence of it. If you want the system to calibrate itself you need some kind of microcontroller in my opinion. And you have to watch out for the self-heating of the Pt1000 which is about 0.5K/mW (P = U^2/R). So you should consider putting resistors in series with the Pt1000s (witch lowers the sensitivity) or use a current source.
#7 21673189 19 Jan 2015 05:51

zeyad asaad zeyad asaad

Anonymous

Post #7
21673189 19 Jan 2015 05:51

this is the circuit
#8 21673190 19 Jan 2015 05:54

zeyad asaad zeyad asaad

Anonymous

Post #8
21673190 19 Jan 2015 05:54

this what i want
PT100 self-heating and 4.5 V offset issue
#9 21673191 19 Jan 2015 15:22

Enrico Koeck Enrico Koeck

Anonymous

PT100 self-heating and 4.5 V offset issue
Post #9 21673191 19 Jan 2015 15:22

To your schematic: If you only use a positive power supply you have to change the amplifier to a differential amplifier with the second voltage set to V+/2 (in this particular case 9V/2 = 4.5V). As well if you have a difference of zero between the two sensors your output would be 4.5V. This makes it very hard to detect small voltage differences because thoose are added to the virtual zero of 4.5V. Second the voltage over the PT100's is way too high. It would mean that a current of approximately 4.5mA flows through them wich results in a power dissipation of roughly 20mW. This would result in a temperature increase of about 10K in the Pt100. So it isn't a sensor any longer but a heater.
But I don't understand your second picture. Do you mean the temperatures should be added? Or if you just connect one probe it measures the difference to what? Or displays absolute values?
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#10 21673192 19 Jan 2015 15:36

zeyad asaad zeyad asaad

Anonymous

Post #10
21673192 19 Jan 2015 15:36

hi
regarding second picture i would like to connect both sensors at the same time to measure the difference in temperature between those two point which mean subtract them from each other.
when i have 0 that mean both sides of the vertebrae have the same temperature.
when i have difference in the temperature that mean i have trapped nerve on that point.
Need for instrumentation amplifier and high-quality components
#11 21673193 19 Jan 2015 16:07

Enrico Koeck Enrico Koeck

Anonymous

Need for instrumentation amplifier and high-quality components
Post #11 21673193 19 Jan 2015 16:07

So you are talking about temperature differences in the range of 1/100s of K? Those changes are pretty hard to detect with this kind of setup. You would need pretty high quality components and an instrumentation amplifier circuit which is way more complex. So either take something already on the market or let it be designed by someone with experience in the field. Otherwise there are many books on designing amplifier circuits and sophisticated measuring equipment.
I belive you want to rebuild something like this http://www.theinstrumentcompany.com/?
#12 21673194 19 Jan 2015 16:12

zeyad asaad zeyad asaad

Anonymous

Post #12
21673194 19 Jan 2015 16:12

i appreciate your advice
which is totally right, it is better to buy ready made one as you said
thanks for your help
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Topic summary

The discussion focuses on designing a circuit to measure small temperature differences (0-50°C range) on the human body over a 5 cm distance using two thermal sensors and a differential amplifier. The original circuit uses LM335 temperature sensors, but concerns were raised about their suitability due to large thermal mass and slow response. Alternatives like Pt100 or Pt1000 sensors with small footprints and Wheatstone bridge configurations were recommended for higher sensitivity (~1mV/K) and better accuracy. Proper wiring with equal cable lengths is critical to minimize resistance and temperature-induced errors. The differential amplifier should be connected at the meter output point, with gain calculated based on resistor values. Using a single positive power supply requires biasing the amplifier input to half the supply voltage to handle zero-difference signals. High current through sensors can cause self-heating, distorting measurements, so current limiting or current sources are advised. Calibration is necessary to offset sensor tolerances, but automatic calibration would require a microcontroller. Measuring very small temperature differences (hundredths of a Kelvin) demands high-quality instrumentation amplifiers and noise management, often beyond simple DIY circuits. Ultimately, purchasing a ready-made precision temperature difference meter was suggested as a practical solution.
AI summary based on the discussion. May contain errors.

FAQ

TL;DR: For 0–50°C skin-difference sensing over 5 cm, a Pt1000 Wheatstone bridge gives ~1 mV/K; “use a Pt100/Pt1000 with small footprint and insulate.” [Elektroda, Enrico Koeck, post #21673186]

Why it matters: Clinicians and makers need stable, low-noise differential readings to spot small left–right skin temperature differences.

Quick-Facts

Target use-case: two points ~5 cm apart on human skin, 0–50 °C difference range. [Elektroda, zeyad asaad, post #21673183]
Bridge sensitivity: Approx. 1 mV/K with two Pt1000s in a Wheatstone bridge. [Elektroda, Enrico Koeck, post #21673186]
Self‑heating: Typical ~0.5 K/mW for Pt1000; limit excitation or use a current source. [Elektroda, Enrico Koeck, post #21673188]
Single-supply op-amp needs a mid-rail (V+/2) reference; zero reads ~V+/2, reducing small-signal headroom. [Elektroda, Enrico Koeck, post #21673191]
Hundredths‑of‑kelvin resolution needs an instrumentation amplifier and high‑quality components. [Elektroda, Enrico Koeck, post #21673193]

Quick Facts

Target use-case: two points ~5 cm apart on human skin, 0–50 °C difference range. [Elektroda, zeyad asaad, post #21673183]
Bridge sensitivity: Approx. 1 mV/K with two Pt1000s in a Wheatstone bridge. [Elektroda, Enrico Koeck, post #21673186]
Self‑heating: Typical ~0.5 K/mW for Pt1000; limit excitation or use a current source. [Elektroda, Enrico Koeck, post #21673188]
Single-supply op-amp needs a mid-rail (V+/2) reference; zero reads ~V+/2, reducing small-signal headroom. [Elektroda, Enrico Koeck, post #21673191]
Hundredths‑of‑kelvin resolution needs an instrumentation amplifier and high‑quality components. [Elektroda, Enrico Koeck, post #21673193]

How do I wire a 3‑pin LM335 for body measurements?

Use the LM335 per its datasheet pinout; the third pin trims output. The package, not a specific pin, contacts skin. However, for tiny skin temperature differences, LM335’s larger thermal mass is not ideal. Prefer small‑footprint RTDs (Pt100/Pt1000) placed directly on the skin and insulated from ambient. [Elektroda, Enrico Koeck, post #21673186]

What sensors work best for small left–right skin temperature differences?

Use two matched Pt100 or Pt1000 RTDs with very small footprints. Mount them directly on skin and thermally isolate them from air. This setup improves response time and reduces ambient drift when measuring subtle differential changes near room temperature. [Elektroda, Enrico Koeck, post #21673186]

How do I get a true differential output from two probes?

Wire the two RTDs into a Wheatstone bridge and read the bridge imbalance with a differential amplifier. Keep sensor cable lengths equal. Set gain with A = (R2 + R1)/R1, and polarity so output is positive when sensor 2 is warmer. Add an output low‑pass filter if needed. [Elektroda, Enrico Koeck, post #21673186]

Do I need to zero one probe every time?

For a narrow operating range, set the bridge zero with the trimmers once, or occasionally to counter part tolerances. Use equal‑length cables to minimize offset from wire resistance and its temperature coefficient. For automatic zeroing, add a microcontroller to supervise calibration. [Elektroda, Enrico Koeck, post #21673188]

How do I avoid cable resistance errors over 5 cm spacing?

Use identical cable types and keep lengths the same from the bridge to each sensor. Mismatched wire resistance and its temperature drift will create false imbalances that look like temperature differences. Good wiring symmetry reduces this error source. [Elektroda, Enrico Koeck, post #21673186]

What gain and filter values should I start with?

Set amplifier gain with A = (R2 + R1)/R1. Start modest, then increase while watching noise. Add a low‑pass at the output: fC = 1/(2πR3C1). For very slow changes, consider an active filter to avoid huge capacitors. The circuit also works without the filter. [Elektroda, Enrico Koeck, post #21673186]

Can I power the op‑amp from a single 9 V battery?

Yes, but create a mid‑rail at V+/2 as the reference for a differential stage. With zero temperature difference, output sits near 4.5 V, which makes tiny signals harder to resolve. Dual supplies avoid this offset issue but increase complexity. [Elektroda, Enrico Koeck, post #21673191]

Why are my PT100s reading hot?

Excess excitation current causes self‑heating. In one example, ~4.5 mA produced ~20 mW, warming the RTD by about 10 K, so it acted as a heater. Reduce sensor current and series resistance to keep power low, or readings will skew high. [Elektroda, Enrico Koeck, post #21673191]

How small a temperature difference can I realistically measure?

Hundredths of a kelvin are challenging with simple op‑amp stages. “You would need pretty high quality components and an instrumentation amplifier circuit.” Use precision parts, good PCB layout, and low‑noise amplification to reach that level. [Elektroda, Enrico Koeck, post #21673193]

What is a Wheatstone bridge, and why use it here?

A Wheatstone bridge is a four‑resistor network that converts small resistance differences into voltage. Using two RTDs in opposite arms yields a near‑zero common‑mode output and a signal proportional to temperature difference, about 1 mV/K for Pt1000. [Elektroda, Enrico Koeck, post #21673186]

What is an instrumentation amplifier in simple terms?

It’s a precision differential amplifier with high input impedance, tight gain accuracy, and strong common‑mode rejection. It resolves tiny differential signals—like hundredths of a kelvin—from noisy, drifting environments better than a basic op‑amp stage. [Elektroda, Enrico Koeck, post #21673193]

How do I minimize ambient air effects on the probes?

Use small sensors directly on the skin and insulate them from ambient air. Foam or tape insulators reduce airflow errors and improve stability when comparing adjacent body points. [Elektroda, Enrico Koeck, post #21673186]

How do I build a simple differential skin thermometer? (3 steps)

Mount two small Pt1000s 5 cm apart on skin and insulate from air.
Wire them into a Wheatstone bridge feeding a differential op‑amp; match cable lengths.
Set zero with the pots, choose gain A = (R2+R1)/R1, and add a low‑pass filter. [Elektroda, Enrico Koeck, post #21673186]

Can I just buy a ready‑made differential skin thermometer?

Yes. Given the precision needed for hundredths‑of‑kelvin differences, consider commercial instruments instead of DIY. They integrate low‑noise amplification, filtering, and calibration workflows out of the box. [Elektroda, Enrico Koeck, post #21673193]

Is the LM335 suitable for tiny skin temperature differences?

Not ideal. Although usable, LM335’s larger thermal mass slows response for subtle skin gradients. A matched Pt100/Pt1000 pair with careful mounting and insulation performs better for small differential measurements near room temperature. [Elektroda, Enrico Koeck, post #21673186]