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[Moved]: op amp circuit selection

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janosandi

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hello Guys
i've connected an thermistor to ADC input of PIC MCU but it seems tht it needs an amplifier so wht you suggest ?
which type of op amp circuits works for this?

thx
John
 

Hi,

You give no information so how can we suggest a circuit?

Our answers can only be as good as your informations.

Klaus
 

At a minimum need:
Thermistor circuit with impedances.
Thermistor circuit signal level.
Signal level desired at ADC input.
Signal accuracy required.
Power available.
 

My Thermistor resistnace is about 3 k ohm at room temp

I want to use the volt between 0v - 3v i guess
im using PIC16f877 ADC so the range is 0-5v but i want to read the first 99 of the converter Not the all readings which its 255 at 5v
 

Hi,

Sure? You want the thermistor range from 0 to 3V?

I assume there is a 10 bit ADC. So the result is 0...1023.
For ratiometric measurements mind to use same low pass filters on VRef and signal.

If you use only 0..99 of an 10 bit ADC, then the input voltage range is only 0...0.5V
Shifting the result to the right enables you to use input voltages 0..1V, 0..2V, or 0...4V.(about)
Using a simple multiplication enables you to adjust the range very precisely to your needs.

Klaus
 

My Thermistor resistnace is about 3 k ohm at room temp

I want to use the volt between 0v - 3v i guess
im using PIC16f877 ADC so the range is 0-5v but i want to read the first 99 of the converter Not the all readings which its 255 at 5v
You guess?
Can I then guess at a circuit?

What is the temperature range you want to measure?
Is that 0-3V at the ADC input?
 

i Guess because no datasheet for the thermistor i've got

Dear Mr klaus

How i can make a simple multiplication ? you mean y software or hardware?
i didnt notice its more than 255 for 10 bit ADC
its my first time use the analog input of the PIC
im designing an thermometer for freezer so the range of temp i want to measure its about -20 to +20
& i've connected the thermistor via 2k2 resistor directly to ADC pin with 2k2 rsistor to ground
now i can read the room Temp at about 2.8v at ADC pin But one more problem im facng is the ADC result is variying quickly before it got stable readind so my units display is flckering till it have a number
Any idea to solve this or should i move this problem to MICROCONTROLLER section ?

thx for help
John
 

Hi,

Datasheet: urgent
Multiplication: software
Flickering: update display only once per second.
Varying: use filters. Analog or digital.

Read about LM73 digital temp sensor or similar. No flickering. No calibration. Maybe you don´t need multiplying.

Klaus
 

thx Klaus about flickering my display routine is ok
how can i multiply in software ?
i'll search for filter circuit & i need the circuit to work with thermistor i cant use any 3 wire sensoers
 

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