Puma board MAP vs AAP circuit

Marcos' unmaintained, but still in-use, Puma for FreeEMS circuit board/hardware design!
slacker.cam
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Puma board MAP vs AAP circuit

Post by slacker.cam »

I see on the schematics for the Puma board that the AAP circuit has been designed as per the MPX4100A datasheet (ie. 10nF and 100nF filter caps plus 470pF on the output) but the MAP circuit however has a 1.5nF cap followed by a 68k resistor and then a 33nF cap on the output. Can someone tell me why they're different? I checked both datasheets and they both suggest using the AAP style circuit.

Secondly, what is the 680k pullup for on a lot of the analog inputs? The schematics suggest something about diagnosis...?

I didn't get in on the group buy so I'm having to build up some hardware from scratch to use with the TechArts board. I want it to be as similar to the Puma board as possible to make life simple...
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jharvey
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Re: Puma board MAP vs AAP circuit

Post by jharvey »

I seem to recall those were per a BOSH device datasheet. I seem to recall it was very similar to this one, and that they recommended it for diagnostics purposes. You don't need them for a TA design using the MPX.
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Re: Puma board MAP vs AAP circuit

Post by slacker.cam »

Link to said datasheet? The schematics do say 'Bosch's signal evaluation recommendation' which is fine. But we aren't using a Bosch part... It seems inconsistent to use two different filter techniques for the pressure sensors given that both transducers come from the same family of parts.

I know I don't need the 680k 'pull ups'. I just want to know why they're on the board for interests sake? They are on a lot of the analog inputs (the maf input for example) so it doesn't have anything to do with the pressure sensor.

Perhaps this could be simplified on spin 2. This is not so much a point that needs to be made in this thread but it's just popped into my head. Although saving a couple of 0805 parts may only save $0.02 on the BOM, you actually save much more in placement costs. Our SMD assembler costs out at about $0.20 per part in volume so that'd save $0.42. Assume a 100% markup and we've just saved nearly a dollar on the sell price...
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Fred
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Re: Puma board MAP vs AAP circuit

Post by Fred »

Those pull resistors should not be in the design. They will skew the reading. It should already be in the list of shit to rip out for spin2.

The filtering stuff on map vs aap could be different due to the possibility of external map, it's on the footprint AND header. Otherwise, it's not a big deal which exact values are being used, it's just another analogue feed, really.

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slacker.cam
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Re: Puma board MAP vs AAP circuit

Post by slacker.cam »

Yes, obviously they shouldn't be in the design. As i mentioned I'm just curious as to why they were put on the schematic in the first place.

Depending on what the time constant of the filter circuit is it could actually have an effect on the performance of the design. Anyway, I'm not too fussed but it would be nice if these things could be clarified.

I have plenty more questions too but wanted to start somewhere simple :)
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jharvey
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Re: Puma board MAP vs AAP circuit

Post by jharvey »

Circuit resistors and caps are per page 35 (pg 4 of 13) of this data sheet

http://www.bosch.com.au/content/languag ... cation.pdf

It notes the 680K, and the caps. Marco didn't do the Temperature feed back, but did add optional items that follow the Bosh recommendations. I would wager a guess that unless someone can point out that Bosh is wrong, that those PCB footprints will be included. Marco seems less interested in making decisions based on gut feel, and he tend to follows more concrete things like datasheet recommendations or calculations.

I believe the long term solution for this kind of confusion is that the board will come pre-populated win a 0 ohm resistor, not the 680k, making it follow the MPX map datasheet recommendations. Then there might be (or at least could have as an optional rework) instructions that allows you to change some parts and use the Bosch sensor.
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Fred
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Re: Puma board MAP vs AAP circuit

Post by Fred »

0 ohm is a piece of wire! Do you mean left unpopulated? I hope so. Or are we talking about different things? Possibly.
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jharvey
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Re: Puma board MAP vs AAP circuit

Post by jharvey »

R134 the 68k ohm would use a 0R, and R133 the 680k ohm would be open / left off. Caps would also be left off or changed to match the MPX datasheet.
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Re: Puma board MAP vs AAP circuit

Post by Fred »

OK! :-)
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Re: Puma board MAP vs AAP circuit

Post by Fred »

You know what is really weird? Slacker Cam seems to be interested/doing stuff with this. I'm not sure that I believe it. He'll have to post videos of it in action, I reckon. :-p
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