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I want to convert my 0 to (-40 )v signal to be 0-5 volt so that i can measure it using arduino,
i want to attenuate the signal first by the factor of 8 and then shift it up by the arduino 5 volt. lets say for example if the signal is -20 volt
-20/8 = -2.5 ,,, -2.5+5 =2.5 v
is that possible ?
but then this is how i emplemented it and the voltage is always fixated at 5 V
bellow is my circuit, tell me where did I go wrong
You can use an opamp to perform the translation if you like.
But there's an easier way : using 2 resistors you can both attenuate and level shift at the same time. I have chosen values that give a 10:1 attenuation and an 0.5V unused range at each end of the scale - scaling to the correct values is simple in software.
simulate this circuit – Schematic created using CircuitLab
You have the two voltages fighting each other.
Look at the 5 V supply: it's connected directly to the output.
Meanwhile the voltage at R1 and R2 is trying to pull it away from 5 V but can't because the lower resistance of the directly connected 5 V circuit will always win.
V2 seems to be drawn as a positive supply. Your question says it's a negative signal.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Inverting amplifier.
The inverting amplifier op-amp circuit does exactly what you need. In this configuration the relationship between output and input is given by the formula
$$ V_{OUT} = - \frac {R_F}{R_I} $$
Setting these as shown in Figure 1 gives a gain of -8 and your -40 V signal will be "amplified" to +5 V.
You need to select an op-amp capable of "rail to rail" operation. i.e., the output can swing to negative and positive rails.
