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Bandwidth of Analog Inputs 
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LQFP112 - Up with the play

Joined: Mon Jun 21, 2010 4:18 pm
Posts: 192
As I know very little about what it takes software-wise to run an engine, I am curious as to what impact the bandwidth of the analog inputs has on performance.

I assume that the bandwidth of a particular signal should be limited to as low as possible, without impacting performance, in order to filter out "noise".
For temperature inputs, in which values change relatively slowly, a low-pass filter of 1 - 100Hz would seem to be reasonable / sufficient.

But for potentially rapidly changing inputs such as TPS, MAP, Battery Voltage,..., how does one determine what bandwidth is needed?

I started to wonder about this because I ran across a Freescale app note in which they recommended a 5kHz low-pass filter for the MAP input and I saw that the Jaguar design uses a ~723Hz filter for internal MAP applications (and a ~1540Hz filter for external MAP applications).

Thanks,
TonyS


Fri Nov 29, 2013 3:36 am
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LQFP112 - Up with the play

Joined: Mon Jun 21, 2010 4:18 pm
Posts: 192
For what it's worth -
The Freescale guide KT33812ECUUG lists the following low-pass values -
On / Off or temperature inputs = 100Hz
O2 = 500Hz
TPS = 1.0kHz
MAP = 5.0kHz

Thanks,
TonyS


Mon Dec 02, 2013 4:01 am
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Sorry for the lag, been super busy, still am, but am currently waiting for someone, so clearing up some overdue things, such as replying to this thread! :-)

If you check out the RavAGE TODO list, you'll find "tune ADC inputs".

It's a little more complicated than just calculating a filter, though, if you want to do it properly. Each sensor type has an output impedance which forms part of the filter. For MAP it's pretty low, for TPS it's pretty high, so the TPS filter needs to be softer to drive or it will be laggy. TPS is important as a first sign of things changing, so lag isn't acceptable there, but neither is noise. Hence good quality cabling should be used.

There is a thread around asking users about their TPS overall resistance that is helpful for tuning the above in the general case. I imagine this thread was motivated by a specific case, though, which makes it easier, as you don't have to judge a good compromise for ALL possible engines as you must in a standalone setting such as FreeEMS.

The same is true for temp sensors, however most of those are pretty close, except old fords, which are very weak IIRC.

Hope that helps.

Fred.

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Wed Dec 18, 2013 10:04 pm
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