unity gain Opamp problem

[Abusaleh]

I have a problem with LM358 (dual op amp). It is cofigured as a unity gain buffer for ADC input of dspic30f6014a microcontrollar. That means I connected the negative input to the output terminal and to the ADC input, and the positive input to the current sensor output. The input of this buffer is a dc voltage from CSLA2CD current sensor (https://datasheet.octopart.com/CSLA2CD-Honeywell-datasheet-9695429.pdf ). Therefore the measurement of this voltage should be accurate because the current is calculated inside the uC using the following formulla: I=(V_ADC-V_offset)/0.0327. where V_offset=Vcc/2=4.0V. That means the V_ADC should be accurate to get good results. The problem is the unity gain buffer amplifies the voltage. i.e. when the input is 3.92 V, its output is 3.98 which leads to wrong readings. I want the output of buffer exactly equals the input. How to solve this problem?

 

[captain251731]

you can calculate these things easily . use below link and put you values(for unity gain amplifier) and check where u r getting wrong.
http://www.bcae1.com/opamp.htm

 

[erikl]

... when the input is 3.92 V, its output is 3.98

60mV deviation is really a lot for an opAmp with an input offset voltage of typ. 2mV (max. 7mV). So I think its due to the input bias current (45 nA typ., 250nA max.) times the output resistance of your sensor (which isn't given in its data sheet, but perhaps is of the order of MΩ).

Try and compensate it by a feedBack resistor of this order between the opAmp output and its inverting input - instead of the direct connection.