Problem with scaling for ADC

[garg29]

scaling for adc

Dear freinds

i have drawn this circuit for amplifying the signal of temperature sensor LM35.i am using adc0831 .since LM35 gives 10 mV /C so for full scale reading i need to get 5volts for 1.5volt(i.e at 150degree celsius)
so that counts up for the gain of 5/1.5 =3.3333
so for this i m using 10k/3k resistors as shown in figure but the circuit isnt working. what must be the problem.

please help me out
thanks
with best regards
amit

 

[IanP]

Re: scaling for adc

Couple of issues here:
- If you supply this circuit of single +12V you will never have output in the range of 0V (LM324 is not linear al low voltages).
- LM35 requires a load, and the best results you will have if the load is connected between LM35 output and -Vs (see Typical applications: **broken link removed** ) .. and
therefore I think you should supply your circuit from +12/-5V and compensate for DC offset caused by unbalanced supply with A1 amplifier/buffer+10kpot so at 0V on (+) of A2 you will have 0V at output of A4 ..

Good luck ..

 

[garg29]

scaling for adc

Thanks for replying.

You wrote:
"therefore I think you should supply your circuit from +12/-5V and compensate for DC offset caused by unbalanced supply with A1 amplifier/buffer+10kpot so at 0V on (+) of A2 you will have 0V at output of A4 .. "

what changes should i do in circuit for DC offset, i mean on which pin what should i do????

sorry for asking too basics, but i'm poor in analog circuits.

With best regards,
Amit

 

[IanP]

Re: scaling for adc

Pin(11) of LM324 will go to (-5V), bottom of 10k Pot to (-5V) - multiturn pot will be good choice - and 100k resistor between LM35 output and (-5V) .. rest stays unchanged ..