FW: Frequency to Voltage Conversion: how?

[Bohdan.L.Bodnar at att.com]

Comments and useful information...

>  Someone writes:
>
>MAP and BP sensors apparently do not return voltage as output.  Instead
>the oscillate at a frequency which corresponds to the absolute pressure.
>What electronics are involved with frequency to voltage conversion?
>
>>This brings up another point: what output frequency corresponds to what
>>pressure level?  I'm sure we could test one to find out, but isn't there
>>already some book or manual that gives in depth data on sensor readings?
>>If we have to test every sensor, it is going to take a LONG time.  Surely
>>Ford, Chevy, and/or Bosch, publishes a manual for servicing EFI that
>>describes how each particular type of sensor responds.
>>
>>  Most single chip computers can measure frequency much better than voltage,
>>which is why MAF sensors output frequency.  Since I don't know the frequency
>>range for this sensor, I can't comment on the best circut to do a freq. to
>>voltage conversion.
>>  My GM map sensor outputs a voltage proportional to the pressure *and
>>reference voltage*.  I did a pressure calibration and found it to be extreemly
>>linear and very dependent on ref. voltage.  Three terminals: ground, ref in,
>>and output voltage.  I gave mine 5.0 volts.  I don't have the data handy, but
>>can post if you would wish.  I plan on calibrating all that I own to find out
>>repeatability info.  The MAP sensor I use is used in my '89 Berettas (4 and 6
>>cyl), my sister's '89 Corsica, and my friend's Jeep.
>>  - Steven ciciora





Steven, et al,

With the exception of Ford's MAP/BP sensors, I know of no other major
manufacturer which uses a digital output MAP/BP sensor;  all others use an
analog output.  The pressure/frequency relationship is given in Ford's
customer training literature;  if there's enough interest, I can post the table.
Typically, about 155 Hz is atmospheric pressure (157 Hz, +/- 1 Hz, yesterday
afternoon in Chicago -- I was working on an emission problem on my Mustang)
with the frequency being around 80 Hz at about 23" Hg vacuum.  The comment
about frequency measurement and MAF output is not quite true.  I know of NO
technical reason why frequency measurements are easier than voltage
measurements -- both are easy to do in real-time (keep in mind that
"real-time" in a automotive engine control environment is about 10
samples/second).  In one case, averaged period measurements are used to
indirectly obtain frequency whereas in the other analog to digital conversion
is used;  I suspect the latter doesn't even use sample-and-hold amplifiers
since the input signals change so slowly w.r.t. the conversion rate (around 20
microseconds for a typical 8-bit successive approximation converter using a 1
MHz clock).

Any MAF system based on currently deployed Bosch technology will have an analog
output from the MAF sensor with approximately reference voltage (5.0v nominal)
corresponding to atmospheric pressure.  This covers most Chrysler, most 1980s
GM stuff, and virtually all Japanese vehicles.  The "virtually" excludes stuff
using the Hitachi MAF sensor (a high-frequency digital output) and the Chrysler
import which uses the Karman-vortex sensor (Dodge Colt?).

Cordially,

Bohdan Bodnar
bohdan.l.bodnar at att.com