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Lm2907 circuit design

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ullisees

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Can any one help me with a circuit that will take the squarewave produced by a Memsic 2125 Dual-Axis Accelerometer into a lm2907 to give a 0 to 5v analog output.

I've attached datasheet for the accelerometer

Thank
 

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Hi,

Yes.
But "help" means: you do the main job, show it and we will help you to rectify mistakes.

Don't expect us to do your job.

Klaus
 

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If you LPF a PWM output you will get an analog V proportional to its
duty cycle.

See attached help designing the filter.


Regards, Dana.
 

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  • Cancel PWM DAC ripple with analog subtraction _ EDN.pdf
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  • slyt076[1].pdf
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  • spraa88a.pdf
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The LM2907 isn't the best device to use for this although it might work. It is really a frequency to voltage converter rather than a pulse width to voltage converter but it does use an internal charge pump so the output will change with PWM ratio. The output wouldn't follow the manufacturers data sheet which expects a constant pulse width and variable frequency so predicting performance would be difficult.

Brian.
 

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This is a copy of the original circuit that I would like to replace AC1 and AC2 with the Memsic 2125. The LATACC and FACC are a analog 0 to 5v signal which are fed directly to a 68HC11 micro.

Sorry that there are no part numbers or values on this drawing, I have not had time yet to identify every component on the original PCB.
ACC Original.jpg


I have never had to convert a PWM signal into a analog signal before so all this is new to me.

Thank you
 

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Hi,

We need technical informations.
What frequency is the PWM?
What source impedance is the signal? (Type, datasheet....)

In simplest case an RC filter should work.
Maybe two RCs, maybe an active circuit with Opamps.

Klaus
 

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The PWM is either 100 or 400 Hz.

This shows a recommended single pole filter. Note use of PWM as a DAC has an
undesired (generally speaking) latency. So if you are trying to use this in a real
time control loop it can be a significant effect on control loop response. The
alternative is to use the processor to do a direct pulse width measurement, although
that too has a significant latency due to PWM pulse width fairly long, eg 2.5 or 10 mS.

1620812596679.png



Regards, Dana.
 

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