LM35 series resistor

[PerranOak]

I have seen circuits with a 1k resistor between the output of an LM35 and the pin (for ADC) of a microcontroller.

I get that with the input impedance of the uC the current is low and so the resistor will not materially affect the reading but for what purpose is the resistor? I mean, what good does it do?

Cheers.

[David Hess]

Sometimes a series resistor is used in combination with a capacitor to filter out noise and provide a low AC impedance to the ADC.  Just a series resistor might be used to limit fault current but that seems unlikely unless the LM35 and ADC are separately powered.  Or if the series resistor is located at the LM35, it might be to prevent instability when driving a long line.

[rstofer]

A real schematic might help.  Links to datasheets...
FWIW, Figure 12 of the LM35 datasheet shows a 2k series resistor for use when driving a capacitive load > 50 pf.  Most ADCs are capacitive due to the sample and hold circuit but I don't know that they have that much capacitance.
The LPC1768 ADC has an input capacitance of 15 pf.

[PerranOak]

Thanks both.

Sorry, I hadn't a particular circuit in mind it just that my circuit doesn't have a series resistor. I put one in and nothing seemed to change so I wondered what was up.

I though that it would confuse things as the voltage drop across the resistor surely reduces the voltage "seen" by the ADC but it didn't seem to!

[rstofer]

I though that it would confuse things as the voltage drop across the resistor surely reduces the voltage "seen" by the ADC but it didn't seem to!

Sure, it reduces the voltage, WHEN the current is flowing.  The idea behind a sample-and-hold ADC is to charge a capacitor through the resistor during the sample period and then analyze it during the hold period.  The sample circuit is designed to limit current flow such that a given driving impedance (check the datasheet) won't affect the reading.  Some ADCs want < 10k Ohms driving impedance, others want somewhat less.  It is important to check the datasheet.

[atmfjstc]

It could be there for safety. Some microcontrollers such as ATMEGAs allow you to switch the direction of pins such that an ADC input pin becomes a digital output. While all pins start off as inputs by default, it is possible to accidentally switch them to outputs if a crash or a bug occurs. Nothing good will happen if the LM35 and the ATMEGA try to drive the same wire to different voltage levels without any resistor there to limit the current.

[PerranOak]

Cheers, thank you both.