Temperature sensor LM35 and long leads

I sure hope you (they) meant 161uA ....

Thanks, Gary.

Unfortunately the LM50 only comes in a surface-mount package which isn't suitable for my application. I see from the datasheet that it has a 2K output impedance, so I guess they solved the LM35's capacitance load problem by increasing the output impedance.

I'll keep the supply voltage low and use a decoupling capacitor across the power supply at the LM35 end.

The LM35 datasheet has two solutions for the capacitance load problem. A 2K series resistor or a R-C (75 ohm, 1uF) damper circuit. Having done a bit more searching on the web it seems that (anecdotally) the damper circuit has proved effective, so I wonder if I should go for that?

Unfortunately, I haven't got the resources to test which method is best.

Oops...thanks John....sorry for misleading. I guess I better look at the data sheet more carefully. Yes,the LM35 looks much better than the LM335 in regard to self-heating.

Steve

Sounds like they put the resistor inside!

John

What's the performance difference between the two solutions? Anybody know?

Any idea which Nat Semi circuit is best to mitigate this capacitance? A

Thanks for your help.

Any idea which Nat Semi circuit is best to mitigate this capacitance? A

Thanks for your help.

Perhaps the damper circuit is used if the designer wants to keep his output impedance low. Not sure if this is more robust than the 2K resistor though!

Both try to keep the load "real" (ie resistive) at high frequencies, as opposed to reactive. The 2K sure looks smaller and cheaper.

John

I've just been looking for more information about this exact question, so this is a timely thread: I'm trying to use LM35 sensors with an AD626 instrumentation amp. The circuit is working fine when I just hook the the sensors directly to the amp, but if I add the 2K resistor to allow me to use longer leads, things get weird. Upon more detailed reading of the data sheet for the amp

Ah, that makes things much easier to deal with!

An additional question: Is there any need to use precision parts for this damper? Will temperature drift of the RC network affect the signal?

Thanks very much for the help.

mike

Nope, it's not at all critical.

John

John, are you sure? Figure 4 of the Nat Semi data sheet shows the damper circuit at the sensor end.

You may well be right. May I ask if your answer based on theory or practical experience?

Yup.

Instinct. Why would the end matter?

John

Because the resistor is to isolate the sensor output from the line capacitance?

Thanks, Rich

The network in question here is the R-C shunt damper, not the series

If the line is any sensible length, the entire length is equipotential, and the same capacitance, and the same damping impedance, appears anywhere along the cable that you choose to observe it.

Dang, you guys are forcing me to think about this, which is entirely unnecessary for a situation this simple.

John

OK, think that I understand now...

Thanks for your time and trouble, John