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

--- Quote from: sokoloff on July 04, 2019, 11:24:10 am ---In an automotive application, at idle and partial throttle low cruise (which is pretty common), you will see intake manifold pressures under atmospheric (what the automotive world calls "vacuum")
--- End quote ---
Vacuum relative to the absolute pressure of 1 bar... the limitation of > 0.5bar is therefore not that bad, because it´s purpose is to check and limit the turbo function and check for intake manifold leakage (air drawn into a cracked hose/gasket). In other words it is more relevant on the upper end.

Anyway, just as a reminder: the more modern ones have a built in SENT bus...
james_s:
In many cases the MAP sensor is not directly related to the turbo at all, it is used to measure air flow which is used to control the amount of fuel injected. When you know the absolute pressure in the intake manifold, the temperature of the air and the characteristics of the engine you can calculate the amount of air being ingested.
HwAoRrDk:

--- 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.

--- End quote ---

Wow! Thank you very much. :clap: I would not have been able to come up with something like this on my own!

I'm no good with LTSpice, so I recreated it in the Falstad simulator, and it works great! To test it, I made the addition of a voltage-controlled-voltage-source node to simulate the voltage output of the GM sensor by taking the node's voltage input as pressure instead of volts, and setting the output to be the transfer function of the sensor.

A few questions about this circuit:

- I see that the parallel 470k/130k pairs give a combined resistance of 101.8k. I suppose one could use instead a series pair of 100k and 1K, but was the parallel arrangement made to get exactly 101.8k, or is it something to do with tolerance error? Because, as I make it, if one were to use 1% grade resistors, the series pair has lower overall tolerance error than the parallel pair - assuming you don't need exactly 101.8K.

- I looked up the LT1782, and it is described as an 'over-the-top' op-amp, where inputs can exceed the supply. I presume this has no bearing on the circuit, seeing as both inputs are ultimately derived from the same 5V reference supply as the op-amp has, so can never exceed it. Is this the case, and almost any rail-to-rail-input-output op-amp could be used? For example, MCP6001.

- Extra protection of the op-amp inputs is not needed, right? Because the high resistance values used will limit current greatly? So, if for example the sensor input was accidentally shorted to +12V, the protection diodes of the op-amp should be able to handle it?

Finally, could you explain a bit about how it works? Also how you derived the appropriate resistance values?
mikerj:
The resistor combination matched almost perfectly (to something like 5 decimal places) the ratio required (or at least the ratio I derived by squinting at your chart!) and are E24 series values.  As mentioned this doesn't mean much if you use normal 1% resistors as the error will stack up.

The LT1782 is simply a really nice Linear Tech RRIO op-amp that is included in LTSpice, so anyone can download the model and run it without any third party component models.  AN MCP6001 will probably work ok, but has a limited supply voltage of not much over 5v, so you'll need to provide some protection to ensure it's not destroyed by small spikes.  The input offset voltage range is also not the best IIRC.

The reason for the high resistance values is so that the potential divider supplying the offset voltage isn't loaded much, you'll see the potential divider resistors are lower by a couple of orders of magnitude.  If the offset voltage was buffered I would use lower value resistors around the diff amp, but the offset voltage is small enough that even a RRIO op-amp is going to be almost saturated.

Supply decoupling/filtering, overvoltage protection and input and output protection would really ideally be needed to make this reliable for automotive use.
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