WTD 80c51 PWM to Analogue Voltage conversion - Page 2

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Re: WTD 80c51 PWM to Analogue Voltage conversion

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If we are talking about a simple DtoA connected directly to the output of an
8051 then there are already several factors that mean we are not going to
be making the world's most accurate or consistent DtoA.  The additional
errors due to the diode are small. If you read my post you will notice
there is a load specified by the parallel R and any following circuit.
Indeed your simple low pass filter also load dependent.

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I agree.  Remember, an 8051 output is not the pulse source you will get in a
spice simulation that would lead you to think the simple low pass filter
method will work as expected. The 8051 has a relatively week pull up and
relatively strong pull down.  I have used the diode version with great
success in several very cost sensitive products.

Ian

Re: WTD 80c51 PWM to Analogue Voltage conversion

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    You didn't re-read the referenced posts as suggested.
 
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    Which is why I suggested the circuit you didn't read about.  Your
suggested circuit, two resistors, one diode, one capacitor, has twice the
number of components, is vunerable to the lack of 8051 output drive, and
has an additional error source in the diode.

    As you state, the 'quasi-bidirectional I/O port' cannot adequately charge
the capacitor, an inverter or buffer is required.  Your second resistor
and diode aren't needed.

    Both circuits are subject to loading unless an output buffer is used.

-- Regards, Albert
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