How to convert voltage -5V to +5V to a range of 0-5v

[garfy]

I have an inclinometer that outputs a range of -5 to +5v. The output of the inclinometer will be fed to my PIC that can only takes in positive voltage in the range of 0-5V.

I have the following circuit as shown in the attachment. The gain of the op amp is --0.5 in this case.

When I put Vin to Ground, Vout has a voltage of 3.75V. When I put Vin to 5v, Vout measures 1.24V from the voltmeter. Why isn't the voltage 2.50V and 0V respectively. The op amp i'm using is LM358. I'm prototyping on breadboard.

I am new to op amp, would appreciate any help. Thanks!

 

[santi001]

When vin = 0.
The gain of this amp is =(1+(R/2R)) = 1.5

 

[FvM]

The gain is ΔVout/ΔVin, and it's -R/2R or -0.5, as said. But the bias voltage at +In has to be 1.67 rather than 2.5 V for intended operation, so the voltage divider must be 2R:R instead of R:R.

 

[garfy]

U are right, the voltage divder should be 2R:R, based on the calculation.

So, i use the following resistor values,
Rg = 10K, Rp = 20K, Rq = 20K, Rf = 10K, Vp = 5V

Vcc = 5V (LM358)
Vcc- = 0V

With the above calculation, i should get the following Vr based on the Vq value.
Vq = 0, Vr = 2.5v, Vr (voltmeter) = 2.5v (OK)
Vq = 5v, Vr = 0v, Vr (voltmeter) = 0.62v (Wrong!)

When Vq = 5v, my voltmeter gives me 0.62v, which is incorrect. I have checked my connection, even did 2 prototype separately. Both gave the same voltmeter readings.

When I replace Rp by 10K (in the above circuit), i got the right Vr value.
Vq = 0, Vr = 3.75v, Vr (voltmeter) = 3.75v (OK)
Vq = 5v, Vr = 1.25v, Vr ( voltmeter) = 1.24v (OK)

Can somebody enlighten me on this? why is there a discrepancy?

Am i missing out something that i do not know, i checked the LM358 datasheet... everything seems to be OK...

I'm using resistors of 1% tolerance, 0.25W.


Help....

 

[FvM]

LM358 doesn't work for this design because it's not rail-to-rail output. It almost reaches V- but, if I remember right, about 3.5 V Vout,max. There's a rich choice of RtoR output OP's, e.g. TLV271 or many Microchip MCPxx types.

 

[garfy]

LM358 doesn't work for this design because it's not rail-to-rail output. It almost reaches V- but, if I remember right, about 3.5 V Vout,max. There's a rich choice of RtoR output OP's, e.g. TLV271 or many Microchip MCPxx types.

Thanks, just as i have suspected. I will look for another op amp.