# Signal Conditioning using Opamp -Need Help

I need help in designing signal conditioning circuit using op amp.

I have my sensor out put as 0.6V for 0 deg Celsius and 1.6V at 100 deg Celsius.

So I need to level convert it to 0-5V range for an adc.

So I identified gain as 5 AND OFFSET Voltage as -3V.

• What you have been done? Oct 21, 2013 at 14:13
• What resolution and accuracy do you need? How accurate is the sensor in the first place? You may very well be able to use the 1 V range directly. Or, use a lower reference voltage for the A/D. Many built-in A/Ds in microcontrollers allow for a separate reference voltage that is lower than the micro's power voltage. For example, if the A/D can be set to a 2.5 V reference, then you've effectively doubled the usable range. You may be able to use a non-zero negative reference too. Using a opamp is a last resort that you haven't shown is necessary. Oct 21, 2013 at 18:41

If you just fed the ADC with the range 0.6V to 1.6V corresponding to 0ºC to 100ºC, you'd have a range of 1V going into a 5V ADC. If the ADC is 10 bit then you'd have approximately 200 least significant bits change for a 100ºC change in temperature.

That, is a resolution of 0.5ºC and, if you could average 10 measurements this would give you 0.05ºC resolution (I'm assuming here that noise would likely give you 1 or 2 bits of variation in the measurement i.e. you'd be implementing a form of dithering).

The question then remains, how much resolution do you require to justify adding an op-amp circuit?

• Thank you very much Andy! Before getting into an adc thing,I want to level convert my signal in the range of 0-5V.Please help me Oct 21, 2013 at 15:03
• @thilak I think you need to justify why you need to amplify it based on what I've said above. Oct 21, 2013 at 15:04
• @Andyaka +1 for the resolution question. But IMHO, here we are talking for a homework. Oct 21, 2013 at 15:31
• Andy,I want to measure the temperature with a resolution of 0.1 deg c.that is 1mv/c.so I need to go with a 10 bit adc Oct 21, 2013 at 20:18
• @thilak - currently, without amplification a single ADC reading gives you 0.5ºC resolution. This number, on several consecutive readings will move a round +/-1 bit due to noise and, if you accumulate (say) 8 consecutive readings this will give you a 13 bit number. This 13 bit number will have a resolution that is greater than 0.1ºC. This is not a con - using out-of-band noise to achieve this is called dithering: en.wikipedia.org/wiki/Dither Oct 21, 2013 at 21:00

I think you would benefit greatly from looking at Op Amps for Everyone Design Guide by Ron Mancini from TI

This is an easy to read, very practical guide on how to design with Op Amps. It should answer your question on designing the right network for the Op Amp so your input and output relationships are what you expect.