Olin,

The buffering has been the general feedback from everyone which has been=20
great.  And yes I swapped the inputs when I transfered the circuit from=20
paper to eagle.

One of my questions was" how to think" when constructing the PWM LPF. Is=20
it right to think that one just should remove the AC component. So if=20
you say 1/2 bit on the A/D input I should filter for one tenth of that=20
(having a gain of 10 ) in the difference stage?

Thanks for commenting. Appreciated.
/Ake

Olin Lathrop wrote:

> Ake Hedman wrote:
>
>> I need some advice comments on this circuit
>> http://www.vscp.org/images/barometer001.gif
>
>
> Hmm, where to begin.  I admit I didn't read all of your post (a bit=20
> long) so
> maybe some of this is explained or addresed there.  However, several=20
> things
> look seriously wrong if this is supposed to do what I think you intende=
d.
>
> 1  -  This circuit performs thresholding with hystersis, which is=20
> contrary
> to feeding the output into an A/D input.  You probably have the opamp=20
> inputs
> backwards.
>
> 2  -  If this is supposed to be a linear amplifier and the opamp=20
> inputs are
> flipped, then why attenuate the input?  R2 and R4 divide the input=20
> voltage
> by 2, which doesn't make sense if you ultimately want to amplify it.
>
> 3  -  The R5 and C1 PWM filter has a rolloff frequency of about=20
> 900Hz.  The
> PWM frequency needs to be well above that to filter out the individual=20
> PWM
> pulses.  You need the residual PWM noise to be about 1/2 LSB on the=20
> A/D or
> less, and I doubt this filter will do that for reasonable PWM=20
> frequencies.
> You probably want a two pole filter at least.
>
> 4  -  If this is supposed to be a diff amp with the PWM setting the "0"
> level, then R1 is definitely wrong.  Since the PWM filter output=20
> impedence
> will vary with frequency and will be impossible to adjust for, it=20
> should be
> buffered.
>
>
> *****************************************************************
> Embed Inc, embedded system specialists in Littleton Massachusetts
> (978) 742-9014, http://www.embedinc.com



--=20
 ---
Ake Hedman (YAP - Yet Another Programmer)
eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden
Phone: (46) 657 413430 Cellular: (46) 73 84 84 102
Company home: http://www.eurosource.se     =20
Kryddor/Te/Kaffe: http://www.brattberg.com
Personal homepage: http://www.eurosource.se/akhe
Automated home: http://www.vscp.org


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