Ok I breadboarded your original circuit with as close to your original values as I could get:
R1: 17.56 k
R2: 5 k
R3: 3.75 k (Lowered from 4.7 k to adjust Gain to fit the range 9V to 14V in to Vdivider)
R4: 4.64 k
R5: 8.44 k
Input Voltage VDo_OA_In AmplifierOutput
9.0 V 2.000 V 0.012 V
10.0 V 2.227 V 0.314 V
11.0 V 2.451 V 1.295 V
12.0 V 2.663 V 2.356 V
13.0 V 2.875 V 3.43 V
14 V 3.099 V 4.31 V (saturated at rail)
The circuit works as advertised (ignoring the fact that I don't have a rail-to-rail O/A.
(See attached PDF file for graph of gain.)
Truthfully, that's my first time using a voltage divider to bias the Virtual Grnd (-V pin) of an op amp. I must admit I was a bit skeptical but after seeing it work it really is pretty straight forward.
The amplifier has a gain of (R5+R3 || R4)/(R3 || R4) = (R5+2.35)/2.35.
[Edited]
that appears to mean 8.46 k + 2.35 (the parallel combination of two 4.7 k resistors)/
2.35 = 4.6 (8.46+2.35)/2.35 = 4.6
My resistor values:
R1: 17.56 k
R2: 5 k
R3: 3.75 k (Lowered from 4.7 k to adjust Gain to fit the range 9V to 14V in to Vdivider)
R4: 4.64 k
R5: 8.44 k
(R5+R3 || R4)/(R3 || R4) = (R5+2.35)/2.35
=8.44 k + (3.75 k || 4.64 k) = (8.44 k + 2.07)/2.07 = 6.15
The only question I have is why is the gain NOT consistent ?
The slope of the ramp (from the graph) is about 45 degrees starting with a gain of 0 and ending with a gain of approximately 6.15.
This is obviously the result of R3.
Conclusion: If you add a resistor between the -V op amp input pin and Vcc, the gain
becomes a function of the input voltage.
With Vin = 9V, the voltage on the -V pin = 2.200 V.
With Vin = 14V, the voltage on the -V pin = 3.049 V.
This may make perfect sense from an op amp standpoint but it nevertheless comes as a surprise because instead of a consistent non-inverting amplifier behavior , with a consistent gain across the board, you have look at the graph to predict what output you can expect for some given input. The criteria for the circuit was
Vout = Vin-9V
Thus ,
Vin Vin-9V
9 V 0 V
10 V 1 V
11 V 2 V
12 V 3 V
13 V 4 V
14 V 5 V
I would like to compute my motor tachometer's signal. This signal is a variable voltage going from 9v to 14v.
As Arduino can only accept signals from 0v to 5v on analog inputs, I have to decrease my signal's voltage.
I think the classic op amp subtractor (Differential amplifier) would deliver output values much closer to the above table. It would be nice if the OP would come back
and clarify WHY he needs this circuit and if my above assumptions are correct.
Op Amp Subtractor.pdf (168 KB)