Bipolar to Unipolar Voltage Conversion Help Needed

[GFS666]

Greetings All,
I need help in getting/designing a practical cicuit to convert a Bipolar (+/-) signal to a unipolar (+) signal. The particulars of the cicuit are:

I am measuring an Amperage from a wire using an LA100-P current sensor from LEM. The sensor runs off of a +-15VDC power supply and outputs a current to a measuring resister. The voltage potential across this resister is +-1.5 VDC. I need to convert this to a 0-3VDC signal that will be measured by a Texas Instruments Microcontroller. TI has a aplication note that show some op-amps doing it, but the circuit has no practical information or real values showing how this can be done. It appears that I'll need to use some op-amps to do this, but does anyone have any practical cicuit (I.E. one that has values) that they can recommend/share?

Thank You,

Gordon

 

[ScottyUK]

I think the first thing to establish is whether you are just interested in the magnitude of the signal, or whether you are trying to shift the whole range into the positive region.

If the former then a precision rectifier would fit the bill, i.e. an absolute value circuit, with a gain of 2 to match the +1.5V F.S output of the precision rectifier to the 3V input span.

If the latter then a level shifter would let you scale, say, 0V = negative F.S, 1.5V = zero current, +3V = positve F.S.

So... which are you trying to do?


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If we learn from our mistakes I'm getting a great education!

 

[GFS666]

Hi Scotty,
I am trying to do a level shifter and your scale is exactly what I need (i.e. 0V = negative F.S, 1.5V = zero current, +3V = positve F.S.).

Gordon

 

[ScottyUK]

Easy way is likely to be a two-input summing amplifier to provide the offset correction, using one input for the signal and one for a reference to provide the offset, followed by an unity-gain inverting stage to correct polarity. If you're feeling clever you could do the latter in software. The summing amplifier is one of the commonest building block circuits there is.

You might want to invest in, or borrow, a copy of 'the Art of Electronics' by Horowitz & Hill. It's getting a bit long in the tooth in trms of the specific devices it uses but is very readable and full of good circuit ideas.


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If we learn from our mistakes I'm getting a great education!

 

[roydm]

Another possibility
Assuming the LEM(s) has a dedicated +/- 15 volt regulated supply simply use a voltage divider to give you -1.5V which can be connected to the micro-controller 0 reference.

I wonder what effect using an offset supply would have e.g. +15/-13.5.

The LA 100-P is an interesting transducer. It seems obvious to me that with the 3 wire connection shown, the supplies need to be regulated or at least go up and down together otherwise the output will suffer from zero offset.

If the micro-controller input impedance is high. A crude temporary fix would be to simply add a 1.5V cell into the circuit.
Regards
Roy