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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?
 
hi Mosiac,
Its possible to shift the 12.4V to 0Vref and amplify the 0.3V change.

Do you have a circuit to post.?
 
Not yet...Do u think the mc34071 is a suitable unit to use for millivolt accuracy?
 
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.
 
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'.?
 
Opamp Schematic

Here's the functioning sim:
 

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