Op-Amp Question

[Spasm]

Hey all,

I'm quite new to this topic. I want to amplify a signal (around 360mV) to 20 times its origin. Now, there are lots of product out there that can do this, but there are many types(like audio amp). Therefore, I would like to know which op-amp circuit to choose if I want to amplify the voltage signal for ADC detection? Do I look for a 26dB chip?

Thanx

 

[motion]

There are many things to consider. First is how fast is the signal you are trying to capture? Does it all go down to DC? Or is it within the audio frequency range? How much noise can you tolerate? How about gain accuracy?

 

[Spasm]

OpAmp

Hi.

Well...actually the voltage input is from a temperature detection IC. Therefore, i think is should have low noise amplification and good gain precision. Any idea on how to build the circuit?


Thanx

 

[kinjalgp]

You can use low noise precision op-amp like OP07. See sample non-inverting DC amplifier with offset adjustment below.

 

[Spasm]

Thanx

Hey kinjalgp,

Thanx for the op-amp info. Will try it to see whether it works for the temperature sensor. But is there another op-amp with 5V Vs instead of 15V? Btw, how does anyone look for the correct op-amp for their circuit? Is there a page that can search for specific op-amps based on the characteristic input from us?( Other than searching thru search engines and manufacturer's website)

Thanx

 

[kinjalgp]

Most of the manufacturers have special search facility called "Parametric Search" using which you can find out the best suitable IC for your application. Check out www.national.com & www.maxim-ic.com
These have really good parametric search facility.

Regarding 5V supply, you can use OP07 right from +-3V to +-18V.
So just change the supply voltage to +-5V in the above given circuit. It will work.

 

[StupidDum]

For general use, i think 741 is ok. I usually use LM358.
For special application, such as high frequency, high speed, high voltage,current...etc. then you will have look for a suitable 1.

Don't feel panic if there are so many op amps that are suitable for your application, well, just pick any.

 

[motion]

Adding an amplification stage between the sensor and the A/D converter would introduce noise and gain errors. You might consider using 18-24 bits sigma-delta A/D converters from Maxim, Linear Tech and others. These allow you to connect directly to the temperature sensor without amplification. Power line noise filtering is also available.

MAX1400 from Maxim and LTC2410 from Linear Technology are some examples.

 

[Spasm]

OpAmp

Hi all,

Thanx for the replies. Well, the temperature sensor readings are quite low voltage, therefore i need to amplify the signal for reading in my microC. The resolution in the microC ADC is around 20mV. Therefore, I'm trying to boost up the voltage signal so that I can sense the temperature reading with 0.1 accuracy(I only need a range of 3 Celcius). Any suggestion on how to deal with the noise from the amplification? A filter maybe?

Thanx

 

[Roff]

I'm guessing that you have an 8 bit A/D with a 5 volt reference. It sounds like you need to amplify the signal and subtract an offset from it. What is the range of the voltage you need to amplify, i.e., what is the sensor voltage at the low temperature, and what is the sensor voltage at the high temperature?

 

[Spasm]

OpAmp

Hey ron

Yeah your're right...Im using 8 bit ADC with 5 volt voltage reference. According to the temp. sensor data sheet, the output voltage at 36Celcius is 360mV(10mV/C). Therefore I thought of first dropping the voltage before the opamp and then amplify it with 36C as the 0V(or any other reference point) and 38 or 39C as the max. Btw, I dont exactly understand the offset voltage. Is it the voltage for calibration when comparing to melting/freezing point?

Thanx

 

[Roff]

I'm posting a "hint" schematic below. What are you doing this for? Is it for a class?

 

[Spasm]

Op_Amp

Hey ron,

Hrmm..i guess that a simple differentor circuit right? Doing this for a simple project of mine. Well, thanx for the tips ron.

 

[Roff]

If you get stuck, let me know. I'll post a full solution.

 

[Agent 009]

What IC are you intending to use? The uA741?

 

[Agent 009]

I used the uA741 or the LM741 with a 12Vdc supply voltage, not 5V

 

[Spasm]

Op-Amp

Hey agent,

Not sure yet on the opamp IC, thats why the forum posting. Preferably its a 5V supply so the circuit can be easily built. Btw, if yr using 12V supply, usually where does the 12V come from? Batteries?

Thanx

 

[Optikon]

Re: OpAmp

Hi all,

Thanx for the replies. Well, the temperature sensor readings are quite low voltage, therefore i need to amplify the signal for reading in my microC. The resolution in the microC ADC is around 20mV. Therefore, I'm trying to boost up the voltage signal so that I can sense the temperature reading with 0.1 accuracy(I only need a range of 3 Celcius). Any suggestion on how to deal with the noise from the amplification? A filter maybe?

Thanx

Where did you get a temp IC that has accuracy better than 1 degree C?
Is this a PTAT / diode type output? I've been keeping my eye out for a IC temp sensor (with a buffered output) that can do better than 0.1 degree accuracy. Sounds like your sensor doesnt have the buffering / built-in gain though.

 

[Spasm]

Hey,

The temperature sensor is a normal temperature sensor from national. Yeah, I cant find one that gives 0.1 accuracy as well. But the characteristic of these sensors are mainly 10mV/Celcius. Based on the data sheet, the voltage/C is linear. Therefore theoratically, it is able to give 1mV/0.1Cel.

Thanx