# Negative Feedback Op-Amp as input to ADC

I have a target board(With Nuc240,Cortex M0) with ADC connected through an OP-Amp(NE5532APSR) as shown in the image.

where AINV1 is the input(0-30 V) and AINVI_ADC is the input to Microcontroller(0-3V) and AINV1_GND - Analog input voltage supply ground


I configured the inputs and I'm getting the Digital values in the range 0 to 4096. for the 12 bit ADC. The input to Op-Amp(negative feedback) ranges between 0 to 30 volt for which corresponding microcontroller input will be 0 - 3 volt.In the code,should I consider the input resistance R44 and R48 in calculation or just multiply the ADC reading by 7.324*10^-3 ie. -> (3/4096 * 10) to get the exact input voltage, if not what are the parameters I must consider?

• How is your AINV1_GND referenced to your system GND? And a 10k/1k5 voltage divider doesn't produce 0-3V from 0-30V. Jan 11 '17 at 13:57
• A 5532 can't swing rail to rail, if you're running it from 3v7 and gnd it's unlikely to make a 3v output. Jan 11 '17 at 14:06
• I agree, but can u answer the question please,what parameter I must consider in calculating the Vin? Jan 11 '17 at 14:09
• (1.5/11.5) * ADC COUNT * 7.324*10^-3, the adc count * 7... gives you the 0-3v level going into the adc, and the 1.5/11.5 is from the voltage divider before the buffer. With the problems mentioned though this won't quite work as it should. Jan 11 '17 at 14:17
• It's shown as 0 ohms, and the adc should be high impedance input so the drop would be negligible Jan 11 '17 at 14:35