down converting ±10v to 3.3v using op-amp

[confusion]

down converting ±16v to 3.3v using op-amp

hi there, m working on a project where i need to convert ±16v signal to +3.3V (which will go to ADC with ref voltage 3.3v ) . can anyone suggest a good op-amp IC . I came across TLV237x but it down converts the i/p voltage range to 2.7 V. I need 3.3V .. plz help .

 

[barry]

Huh? I guess your name says it all. An opamp does not "down convert". And a TLV237x does not "down convert" to 2.7 V. I think you first need to understand how an opamp works. For one thing, the TLV237x can operate with a supply voltage up to 16 V.

What you REALLY want is to convert a 32 volt span to a 3.3V span, so you need to divide your signal by 32/3.3. And then you need to add an offset. There are many ways to do this, depending on your requirements. One way is to use a voltage divider and apply that output to the input of an opamp with an offset.

 

[confusion]

hmm :-o u guessed it right ..i have least knowledge of analog ckts... the tlv237x datasheet said "The TLV237x
takes the minimum operating supply voltage down to 2.7 V over the extended industrial temperature range" so i thought opamp can down convert :roll:
sorry my bad!! i did not go through the datasheet well..!!
coming to the point i understood the voltage dividing part ..but once 32v span is converted to 3.3v using voltage divider ckt.. (okay now this might sound very lame but i really don't know) why do we need to add an offset and op-amp ??

 

[barry]

Ok, here's a simple (not necessarily optimal) solution. Use one voltage divider to to divide down your signal by 33/3.2=9.7 (approx). Use a second voltage divider to generate 1.65V. Apply these two voltages to a unity-gain summing amplifier. You need to pay attention to accuracy requirements, etc.

 

[confusion]

thanx barry, your suggestions lead me on the right track ..exactly what i wanted to know.. just one more question..i totally understood the ckt you explained before .. but i have a ready circuit (which obviously is not working and made me post this thread) .
In this ckt the i/p signal is been given to the TLV2372 ckt (the one who made it expected i/p to be in the range ±8V though) and then o/p is given to the voltage divider circuit for offseting it to 1.65V and from there to the ADC.
And i still coudn't figure what is the gain of TLV2372. i went over the datasheet but its all Latin to me. proceeding with your circuit is the smartest thing for me to do now. but was just curious to know what was wrong with this ckt.

 

[tgootee]

If all you need is a 3.3V reference voltage, you might want to look at something like a TL431, a widely-used voltage reference IC. You can set the output voltage by adding two resistors.

 

[barry]

thanx barry, your suggestions lead me on the right track ..exactly what i wanted to know.. just one more question..i totally understood the ckt you explained before .. but i have a ready circuit (which obviously is not working and made me post this thread) .
In this ckt the i/p signal is been given to the TLV2372 ckt (the one who made it expected i/p to be in the range ±8V though) and then o/p is given to the voltage divider circuit for offseting it to 1.65V and from there to the ADC.
And i still coudn't figure what is the gain of TLV2372. i went over the datasheet but its all Latin to me. proceeding with your circuit is the smartest thing for me to do now. but was just curious to know what was wrong with this ckt.

You better study up on opamps if you don't know "what is the gain". The gain of the TLV2372 is probably about 100000. That's NOT what you want; you need to set the gain with feedback resistors. You should learn SOMETHING about opamps before you just start throwing them into circuits and hoping they'll work.