R
Richard Rasker
Hi all,
I'm currently working with a mass air flow sensor (a Honeywell AWM3100V, see
http://datasheet.octopart.com/AWM3100V-Honeywell-datasheet-57019.pdf), and
I would like to convert the rather non-linear response curve of this device
into a voltage which bears a linear relationship to the actual air flow.
Ideally, I would like to see the air flow converted in millivolts, so that
it can be fed into a 3.5 digit voltmeter directly.
These are the values (F=flow):
F (ccm) Vout (V)
0 1.00
25 1.90
50 2.67
75 3.27
100 3.75
125 4.17
150 4.50
175 4.80
200 5.00
The first problem was simple: finding a suitable mathematical function which
fits the curve; I looked at something along the lines of
Vout=c1*(1-e^(-F/c2))+1, and it turns out that c1=5 and c2=125 provides a
near-perfect fit. The second problem was to find an inverse function -- no
problem there either: F=-c2*ln(1-(Vout-1)/c1) -- leading to the third and
rather trickier problem, which of course is to implement that inverse
function in an actual circuit.
I've been doing some trial-and-error experimenting with a simple circuit,
based on a simple Si-diode with some bypass and series resistors in several
configurations, but that doesn't produce satisfactory results -- the best
curve I get is easily 10% off at the extremes, and that's even without
temperature instability. All this is of course no surprise, as the
exponential function of a forward-biased diode is something different than
a logarithmic function, and a simple PN junction has a temperature
coefficient of approximately 2 mV per degree Celsius.
Does anyone know of designs which provide a better fit for this type of
logarithmic function, and preferably a better temperature stability?
Thanks in advance, best regards,
Richard Rasker
I'm currently working with a mass air flow sensor (a Honeywell AWM3100V, see
http://datasheet.octopart.com/AWM3100V-Honeywell-datasheet-57019.pdf), and
I would like to convert the rather non-linear response curve of this device
into a voltage which bears a linear relationship to the actual air flow.
Ideally, I would like to see the air flow converted in millivolts, so that
it can be fed into a 3.5 digit voltmeter directly.
These are the values (F=flow):
F (ccm) Vout (V)
0 1.00
25 1.90
50 2.67
75 3.27
100 3.75
125 4.17
150 4.50
175 4.80
200 5.00
The first problem was simple: finding a suitable mathematical function which
fits the curve; I looked at something along the lines of
Vout=c1*(1-e^(-F/c2))+1, and it turns out that c1=5 and c2=125 provides a
near-perfect fit. The second problem was to find an inverse function -- no
problem there either: F=-c2*ln(1-(Vout-1)/c1) -- leading to the third and
rather trickier problem, which of course is to implement that inverse
function in an actual circuit.
I've been doing some trial-and-error experimenting with a simple circuit,
based on a simple Si-diode with some bypass and series resistors in several
configurations, but that doesn't produce satisfactory results -- the best
curve I get is easily 10% off at the extremes, and that's even without
temperature instability. All this is of course no surprise, as the
exponential function of a forward-biased diode is something different than
a logarithmic function, and a simple PN junction has a temperature
coefficient of approximately 2 mV per degree Celsius.
Does anyone know of designs which provide a better fit for this type of
logarithmic function, and preferably a better temperature stability?
Thanks in advance, best regards,
Richard Rasker