# 358 op amp interfacing with 0804(a-d converter)

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#### iwineuro2008

##### New Member
hi friends,
i have a sensor whose output varies from 14 milli volt to 70 milli volt. i have to use a 0804 A to D convertor.so i want to use an op amp(preferably 358) with suitable gain so that my analog voltage can be detected by 0804 IC.

what should be the gain of opamp and how should i select the value of opamp resistors for noninverting mode??

#### ericgibbs

##### Well-Known Member
hi friends,
i have a sensor whose output varies from 14 milli volt to 70 milli volt. i have to use a 0804 A to D convertor.so i want to use an op amp(preferably 358) with suitable gain so that my analog voltage can be detected by 0804 IC.

what should be the gain of opamp and how should i select the value of opamp resistors for noninverting mode??

To amplify the 14mV thru 70mV, you need to offset the 14mV [zero point] and amplify 70mV-14mV = 56mV by 5V/0.056 = 89.28 times.

For the NON INV input the +Gain = 1 + [Rf/Rin]
Where Rf is the feedback resistor value and Rin is the Input resistor.

You must offset the 14mV low value.
or if you used a gain of 71.4 * 70mV to give +5V for the top limit, the low 14mV would be 71.4 * 14mV =0.999V

Do you follow.?

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#### iwineuro2008

##### New Member
ya , getting you pal. all i want to know is what should be Rf and Rin? as in gain of 70 can be obtained by 1+(69000/1000) os even 1+(6900/100) ???

#### ericgibbs

##### Well-Known Member
ya , getting you pal. all i want to know is what should be Rf and Rin? as in gain of 70 can be obtained by 1+(69000/1000) os even 1+(6900/100) ???

If Rin was say 1KΩ then for 70 [ on the NON INV] the Rf would be 69K.

This is the value I would choose.

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