piecewise matching of two sensors?

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I want to match the outputs of two light sensitive sensors when they see the
same light level.

When I measure the outputs from the two sensors they donít follow each
other. So I have an idea that I should run one of the signals through an
variable gain amplifier to piecewise match their gain.

The amplifier should look up the gain value from a table. The correction
gain should be a piecewise linearization algorithm.

What is the best way to perform this piecewise linearization?

Any good links about this type of problem?

Re: piecewise matching of two sensors?
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You need to provide a bit more information:  What type of light sensors
are you talking about, and over what range do you want them to match.
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If you are using photodiodes and a proper current-to-voltage converter,
the outputs should be linear over several orders of magnitude of input
energy.   You should not need to linearize anything---just provide
a single gain matching circuit and offset adjustment.
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You'll need to specify the detector and amplifier characteristics
before I can provide any pertinent links.

Mark Borgerson

Re: piecewise matching of two sensors?
On Saturday, in article

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What sensor and HOW are you ensuring they are seeing EXACTLY the same light?
Too many setups have had their 'calibrations' setup by what was thought to be
the same light, for such obscure reasons as -

  one sensor was hotter than the other
  one light source to two sensors at same time
     was NOT the same level to multiple sensors
  internal reflections/shadows giving a difference
  different temp co-efficients
  different colour spectrum response between similar devices
  Tolerance differences between sensors

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Check that they are actually seeing the same light, and better still
first check each one INDIVIDUALLY against the SAME light source DIRECT
with no other obstructions or reflections or optics.

How far out are they and is that within the sensor and its front end
component tolerances?

You are of course checking the light levels with some form of light meter
for reference?

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First be sure you are looking at the correct problem.

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Depends on being sure you are looking at the correct problem.

Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: piecewise matching of two sensors?
Although not for light sensors, I have done this exersize about 10
years ago
to match the channell resistance of a pair of JFETS and to linearize
measurements - a gamma spectrum - to a 12 bit linearity, and to
thermocompensate the whole mess.
 The technique is viable for relatively low volume products.
 I used to select pairs of FETs (no good enough duals on the market)
for a start,
then a table with temperature dependent gain correction coefficients
built and stored in the products MCU EEPROM (it was a HC11, but it only
stored the values, the bigger host made use of them).
 For linearization, it turned out that the non-linearity of FETs was
well described by only 3 coefficients (second order), which were again
measured on a per unit basis and stored in the EEPROM. The host
used the values to create a correction curve, which was used to do
the  linearisation transformation of the spectrum.
 I hope this is more or less what you are looking for, although your
will probably be much much lower.
 Nowadays, we have unbeliavably fast and precise ADCs and we use
DSPs etc., life is much easier....


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