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How to make an analog circuit to proportionally scale a sensor output voltage?
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mikerj:

--- Quote from: Zero999 on July 05, 2019, 09:58:56 am ---
--- Quote from: mikerj on July 05, 2019, 08:57:15 am ---That quite a nice part, but I don't think it provides any over-voltage protection and it's voltage operating range is lower than the MCP6001.  I'd think I'd choose an op-amp with a wider supply range and then use a TVS and maybe a polyfuse for protection.  You'll want to add some resistance on the op-amp output to limit current if that is connected to something bad, it can go before the feedback resistor so any voltage drop is compensated.

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If 12V is available anyway, then why not use the cheap and cheerful LM358?

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12v typically isn't available on the sensor wiring, they run from a 5v reference.  He could run another wire but a module that just sits inline with the sensor would be much neater.
vk6zgo:

--- Quote from: HwAoRrDk on July 04, 2019, 03:15:21 am ---I have this device that takes an input from an air pressure sensor. The sensor gets a 5V reference, and the device's MCU reads the sensor's output voltage using its ADC. The sensor reads up to 2.5 Bar.

However, this particular sensor is expensive and hard to get, so I have been thinking about the possibility of substituting it with something more commonly available. My mind turned to automotive MAP sensors, which are ubiquitous and cheap. In particular, the common GM 3 Bar MAP sensor would be a great candidate. But, of course, it works over a different pressure range (2.5 versus 3), so therefore the output voltage will not match for any given pressure.

I found spec sheets for both my existing sensor and a Delphi-brand GM sensor, and made a graph of pressure vs. voltage using the provided transfer functions:



Existing sensor is in green, GM 3 Bar in dark red. As you can see, the voltage difference is proportional to pressure; for example, at 2.5 Bar the GM one is approx. 82% of the other, but 87% at 1 Bar.

The obvious solution might be to say "modify the MCU firmware with different sensor calibration data", but I can't because I didn't make the device and I can't re-program it. :)

So, my question: is there some way, using purely analog means (i.e. no MCU/ADC/DAC), that I can scale the voltage from the new sensor to match the output of the old one? Ideally, I would be able to add the circuitry in-line in between the sensor and the device.

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The OP isn't using it in an automotive application, see part of post I emphasised in red.
GerryR:

--- Quote from: mikerj on July 04, 2019, 06:23:04 pm ---That makes things much easier, a simple diff amp will do the job.  This shows the general idea, but is not a complete design.

Getting the gain and offset reasonably accurate will need low tolerance resistors, but I don't know how much error is acceptable.  Deriving the offset from the 5v rail should be ok as this is a reference supply for the sensors on an engine management system, so should be well regulated.

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Could you explain your circuit a little.  I see how you offset the voltage and match the gain of the original sensor, but don't see how you modified the output (slope) to make the GM sensor match the original sensor.  Please explain; sometimes I can be a little slow.  Thanks.
Rerouter:
Here is the breakdown, it is a normal differential amplifier summed with a fixed offset voltage,
mikerj:

--- Quote from: GerryR on July 05, 2019, 11:07:50 am ---Could you explain your circuit a little.  I see how you offset the voltage and match the gain of the original sensor, but don't see how you modified the output (slope) to make the GM sensor match the original sensor.  Please explain; sometimes I can be a little slow.  Thanks.

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Gain and slope are effectively the same thing, the higher the gain of the amplifier, the steeper the slope at the output.  The gain we need is = Slope Original / Slope GM ~ 1.277
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