# Resistor capacitor combination

The signal from LM35DZ temperature sensor was impacted by the signal from a light dependent resistor, when the light intensity changed. A resistor capacitor pair was connected between the LM35DZ signal and ground which resolved the problem. (see National Semiconductors datasheet Figure 4 on www.jameco.com/Jameco/Products/ProdDS/1276463.pdf).
The product of the resistor (R) and capacitor (C) values (tau = RC) determines the filtering threshold frequency and time for the capacitor to change/discharge.
So for a given RC value, there are many combinations of R and C. For example, a 100ohm resistor and a 1microF capacitor pair have the same RC value as a 10kohm resistor and a 10nF capacitor pair, so the filtering threshold frequency and time for the capacitor to change/discharge is the same for both pairs.

My question is what is criteria for choosing one RC combination over another ?

The impedance of the RC network. For instance a 10k resistor or less is fine for an analog input, higher
might lead to some inaccuracy (if doing analogRead on several pins, especially).

Too low an impedance might overload the sensor output (but only at high frequencies which you
aren’t interested in).

The range 1k to 10k is typically used for low frequency signals as a good compromise in most
circumstances, although higher values can be good for lower power circuitry.

Also you might look at the cost of a high value of resistor and a low value of capacitor, versus a low value of resistor and a high value of capacitor.

The cost difference is not very much for a single unit but might add up to quite a bit for 100,000 units.

How on earth could a LDR affect a temperature sensor reading ?? and how would a RC net fix it?

puzzled

Allan

It seems OP is confused with Arduino muxer crosstalk.
Leo…

Only significant with very high impedances - not reccommended.

Allan

OP did not post LDR pull up resistor value.
100n from LDR pin to ground could have fixed the cross talk.

OP did not post LM35 cable length or capacitance.
Seems like a guessing game.
Leo..

OP's got a solution - so what the hell..

Allan

To Wawa : LDR pull-up resistor was 4.7K ohm and the LM35 cable length was less than 10 cm

To Mark T : what is the difference between using a 1K or a 10K ohm resistor and how would you decide on what value of capacitor to use ?

How does - in terms of resistance, current etc - an RC combination (suggested by Mark T) differ from a capacitor only (suggested by Wawa) or from a resistor only for reducing interferance ?

what is the difference between using a 1K or a 10K ohm resistor

The impedance is higher with the 10K than the 1K.

and how would you decide on what value of capacitor to use ?

To keep the same time constant use 1K or 10K in the formula:-

Time constant = R * C

that is with R in ohms and C in Farads. You know the time constant you were using by multiplying the origional values of R & C.

and how would you decide on what value of capacitor to use ?

See reply #6 and action it for more specific information.