Saurabh,

 If I remember correctly, the ADC0804 is an 8 bit Successive
 Approximation A to D converter. In this case, whatever the
 maximum conversion time divided into one would be the fastest
 frequency you could sample at.   It seem to me that the worst
 case conversion time is about 16 to 20 uS (Not sure of this time),
 but lets assume it's 20 uS.  The the fastest the ADC can be operated
 at is 1/20 uS which is 50000.  So, the most you could sample is 50K
 times per second.  The practical limit would be somewhat less
 though.  If the Maximum conversion time is less than 20 uS, then the
 maximum sampling frequency would be higher, and vice versa.

 As far as there being a hardware setup for sampling frequency, the
 ADC itself does not have any.  What controls the sampling frequency
 is the processor it's connected to.   Whenever the host processor
 gives the ADC a convert command, the ADC goes to work converting.
 And it takes 8 clock cycles to complete the conversion.

 So, you shouldn't have any problems with your sampling frequency of
 300 Hz.  All you have to do is have the processor tell the ADC when
 to start, then let it finish.  I believe there is an END OF
 CONVERSION flag on the ADC that indicates when conversion is
 complete.  Have your host processor watch this flag to tell when
 conversion is complete.   As soon as this flag is activated, you can
 start another conversion.

 One other thing you may want to think about.  That is a sample and
 hold circuit on the front end of the ADC.   Otherwise, the ADC will
 be trying to convert a moving target.  You want the input voltage to
 the ADC to be steady while the conversion is going on.  Otherwise,
 you'll get erroneous results.

 Hope this helps you out.   Let us know the results of your project.


                                           Regards,

                                              Jiom



On Fri, 28 April 2000, Saurabh Sinha wrote:

>
> Hi,
>
> I need to use the ADC0804LCN to sample an analog signal and store the output
> in an MCS51. I wanted to find out how the sampling frequency (say 300Hz) can
> be set (can it be done in hardware?). Also, I am slightly forgetting the
> concept of cut-off frequency, am i right in saying that would most-likely be
> the -3dB points. Is there any way I can set this....(Sorry if this is a
> stupuid question...)
>
> Regards,
>
> Saurabh

jim@jpes.com