Hello,

I have took a look at those 4 devices' datasheets, and I wasn't happy with
any of them, so to speak. I think that your best bet could be a discrete
solution (transistor based), exploiting the schmitt-trigger input of a
very small PIC. Put a RC filter before the transistor. Now the key is in
the PIC software: when you receive a pulse, count how many ticks it last,
and reject it if it was too short (interference). Given the nature of what
you're measuring, a minimum, VALID pulse width will be in the hundreds of
microseconds at least, i.e. you can virtually reject all the practical
noise out there. Just an RC filter before the transistor, to remove the
cream, and all the rest is done via software (input compare if available,
bit banging otherwise). I have played with this sort of things a lot, and
this was the best method among all those that I tried. And it's cheap.

On a side note, I need to ask a question now: when you have a reluctor
pickup, how can you maximize signal to noise ratio? i.e., what input load
should you show to the pickup? While I have found empirically the best
values for my application, I still have thoughts about it. From the signal
point of view, I know I want maximum possible voltage, and this suggests
me to have a high input impedance. But if I have an high input impedance I
will also pickup more EMI and capacitive coupled, low current interference,
which on a low-medium input impedance would be instead shunted to ground,
but I'd lose signal.

So what is the best input impedance to interface with a reluctor pickup,
in the end?

With kind regards,
Mario


At 23.49 2012.02.13, you wrote:
>Thanks to everyone for all the replies. I think I will prototype the
>Maxim part and see if it is superior. It looks like it uses a fraction
>of the output of a peak detector to set the comparator level. I like the
>idea, it seems like it should eliminate false crossings on noisy high
>amplitude signals.
>
>If it turns out to be really good, I may design it in and also put some
>pads for a header on the PC board to allow for an emergency piggyback
>board with a less integrated solution.
>
>I'll post info on the results of my tests when I get some samples from
>Maxim.
>
>Cheerful regards,
>
>Bob
>
>
>On Wed, Feb 8, 2012, at 11:29 AM, Bob Blick wrote:
>> I need to design a device that interfaces to the speed sensor of heavy
>> trucks. These are variable reluctance sensors and will remain connected
>> to the vehicle's ECU, my circuit will piggyback. These are two-wire,
>> differential signals full of common-mode noise and vary in amplitude
>> greatly with frequency and also position as the tailshaft bearing wears
>> in the vehicle.
>>=20
>> Right now I am designing the front end and would love to use something
>> like the MAX9924 since it makes things simple. But it's a Maxim part
>> which I am still hesitant to spec.
>>=20
>> Parts like the LM1815, LM2907, NCV1124 are all single-ended input. The
>> NCV7001 is end-of-life.
>>=20
>> Any pointers toward a differential VR sensor interface IC?
>>=20
>> This is a smallish volume product, so I don't expect to be able to get
>> any high-volume ABS interface parts. Which at this point are probably so
>> highly integrated with other functions as to be unusable to repurpose
>> for my use anyway.
>
>--=20
>http://www.fastmail.fm - Same, same, but different...
>
>--=20
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>http://mailman.mit.edu/mailman/listinfo/piclist

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