Electronic – How to do two points scaling in electronics? 0.6 V – 3.2 V to 0.0 V – 3.3 V

measurementoperational-amplifierscalingvoltage measurement

I'm seeking a solution how to do a two point scaling in electronics. I have tried an Op-amp, but it did not work very well. So I wonder if you have some ideas how to do two point scaling from e.g 0.6 V – 3.2 V to 0.0 V – 3.3 V?

When I apply 4 mA, I what the output to be 0.0 V or very close to 0.0 V.
When I apply 20 mA, I want the output to be 3.3 V or very close to 3.3 V.

The Zener below is a 3.6 V Zener.

Is that possible?

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Best Answer

I'm using a STM32 but the ADC reference is 3.3V and when I apply 4mA, then I'm using a lot of the ADC. I don't what to do that. Or is it possible to change the ADC reference for the minimum voltage too?

You're only losing 20% of the range and you have the advantage of a "live zero" which can be used to detect a break in the 4 - 20 mA loop. This is the standard solution taken by industrial PLCs and many of those do that on a 0 - 10 V input by addition of a 250 Ω shunt resistor to give only 5 V at 20 mA. Usually its simplicity wins out over the loss of resolution.

A further advantage in the industrial sensor applications is that sensor faults can be indicated by sending a 3 mA signal, for example.

From the comments:

But isn't 20% a little bit to much? I could detect current loop breaks with 5% waste.

That's for you to decide. You're using a 12-bit / 4096 step ADC. That's a step resolution of 0.02% of full-scale per step. Losing 20% gives you 0.03% of full-scale per step. You're going to have difficulty getting better than 0.1% shunt resistors - see Vishay for example and noise will introduce further problems. Introducing an op-amp to do the offset properly only makes it more difficult.

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Figure 1. It appears that the top and bottom of the STM32 ADC are accessible.

The other option is to offset the ADC VREF- to 20% of VREF+. See page 68 of the datasheet.

Added by OP:

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