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Opamp Advice Needed

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Mosaic

Well-Known Member
Hi,

I am trying to build an opamp circuit (mc34071) to offset an auto battery's voltage by 12.4V downward so that the output of the opamp reads 0 when it sees 12.4V from the battery. Therefore 12.7V should translate to 0.3V output on the opamp!

If rail to rail is a prob...I can work with perhaps everything offset up by up to .2 V.

So perhaps 12.7V could translate to 0.5 V from the op amp, and 13.2V could be 1V.

I am using a 4.096 precision source divided toa 1.024V precision source on a 16F886 Vref+. I'll use an adc with the full 10 bit 1.024V range to precisely measure the voltage.

Any advice?
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Mosiac,
Its possible to shift the 12.4V to 0Vref and amplify the 0.3V change.

Do you have a circuit to post.?
 

Mosaic

Well-Known Member
Not yet...Do u think the mc34071 is a suitable unit to use for millivolt accuracy?
 

Mosaic

Well-Known Member
Ok I think I figured it out.

Using a precision 4.096 Vref to feed the adc & the dual opamp, MC34072.

Opamp A does a noninvering x3 gain to produce 12.288 VDC ref. which feeds the subtractin input of a difference amp, below.

OPamp B does a difference amp gain of x4 to produce a 4.096V output if the batt voltage is 1.024V higher than the 12.288. So the full scale adc at 4.096 Vref sees 4.096 V if the battery is at 13.312VDC.
This give best precision at 1mV = bit in a 10Bit adc.
 

ericgibbs

Well-Known Member
Most Helpful Member
Ok I think I figured it out.

Using a precision 4.096 Vref to feed the adc & the dual opamp, MC34072.

Opamp A does a noninvering x3 gain to produce 12.288 VDC ref. which feeds the subtractin input of a difference amp, below.

OPamp B does a difference amp gain of x4 to produce a 4.096V output if the batt voltage is 1.024V higher than the 12.288. So the full scale adc at 4.096 Vref sees 4.096 V if the battery is at 13.312VDC.
This give best precision at 1mV = bit in a 10Bit adc.

hi,
What is the voltage supply source to the 'project'.?
 

Mosaic

Well-Known Member
Opamp Schematic

Here's the functioning sim:
 

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