AD Result with Decimal Point

[ericgibbs]

Hello eric

Can you tell me why i need a 12V supply to the op amp?cant i work with a 5V supply?

hi Suraj,
The opa requires about a 2volt overhead so that it can drive upto +5V [which is about 102Cdeg]
A +8V or +9Vdc would be OK for 100Cdeg.

If you only wish to measure upto about +60Cdeg max,[thats +3V out] then a +5V supply would be OK.

Most simple opa's are not rail to rail, that is they cannot swing from 0V thru +Vsupply, they need a 2V overhead.

The CA3140 can swing with about +10mV of 0V and to a high of [Vsupply -2V].

Do you follow OK.?

 

[Pommie]

The LM35 gives 10mV/°C and so at 100°C it will output 1V. A 2V reference and an opamp with an adjustable gain around 2 could be calibrated to give an ADC output of 1023 for a temperature of 102.3°C. The reference could be a zener/resistor combo and this would alleviate the need for a rail to rail opamp.

Mike.

 

[kchriste]

If you haven't bought the OpAmp yet, look at tip#12:
https://www.electro-tech-online.com/custompdfs/2008/08/41215a.pdf
I've never used the on-chip comparators as an OpAmp, so I'm not sure if this method would even give you the voltage swing you need. Might be worth experimenting with it if your PCB is tight for space/money.