Electrical – MAX9926 Filtering Issue


We are using a MAX9926 IC to filter 2 VR signals (from a HONDAS2000 Crank and Cam VR sensors) One being 24 tooth, and one being 3 tooth (per cycle of the Engine). At RPMs below approx 5000-6000 the signal comes through the 9926 fine, and triggers the MCU correctly such that it decodes it fine. At around this RPM we start to get extra/missing teeth on the IRQ lines to the MCU. Note we can 'push through' this noise and get to 9k RPM – just seems to be a 5-6K RPM issue…

We also used a scope to capture the signals at the entrance of the ECU to rule out electrical noise. The signals are smooth and clear at both idle, AND when hitting the point the ECU loses sync due to the 'phantom' teeth. The scope doesen't show these phantom teeth so I am assuming it is a 9926 issue, or the parts R1, R2, C1, R3, R4, C26. The only difference between perfect idle signal and higher speed 'issue' signal is the frequency of the wave, and its amplitude, as shown in the plots. We have 50v P-P at high speed and around 20v P-P at low speed.

1) Maybe I should and how could I attenuate both signals say by half? (Adding what, where)
2) Should I reduce the value of C1/C26 as they might be acting as a filter?

With regard to (2) if I keep the throttle down and 'push through' the noise the engine will regain sync and run all the way to 9k RPM without issue, so that is also odd!

Low speed, IRQs passed via 9926 perfectly
idle speed scope

6K RPM, phantom events passed out of the 9926: (Note the signal shows no issues at all, just different voltages and frequency.
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Schematic of the input system:

Best Answer

We found it to be a software issue!

We scoped the outputs from the sensor (which seemed perfect) so then we moved back, and scoped the inputs to the MCU from the MAX chip, they seemed perfect - we then added some debug lines on the MCU to show the IRQ handlers fire- low and behold, we had missing pulses even though they were being presented to the MCU! Software :(