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Simple schematic for a/d in 18f452

 
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JohnE
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Simple schematic for a/d in 18f452
PostPosted: Wed Oct 20, 2004 9:54 am     Reply with quote
Hi All,

Have you any simle schematic how to use a/d in 18f452. As far I know we need a serial resistor and capacitor in parallel. Please correct me if I'm wrong.
Many thanks and regards
JohnE
Ttelmah
Guest
Re: Simple schematic for a/d in 18f452
PostPosted: Wed Oct 20, 2004 10:22 am     Reply with quote
JohnE wrote:
Hi All,

Have you any simle schematic how to use a/d in 18f452. As far I know we need a serial resistor and capacitor in parallel. Please correct me if I'm wrong.
Many thanks and regards
JohnE

You don't need any resistor or capacitor.
You need a voltage source, that has a characteristic impedance low enough to charge the internal capacitor, in the sample time required (below something like 2.5KR typically). The _internal_ circuit, comprises a MOSFET 'switch', that presents a resistance of typically about 5KR, in series with other internal resistances, totalling about 1KR, which together charge the internal capacitor which is normally around 120pF. When the ADC channel is selected, the MOSFET is switched to make the connection, and the capacitor starts to charge. When the reading is actually made, the MOSFET is disconnected, and then a successive approximation converter measures the voltage on the internal capacitor. The charging time for the capacitor, is dependant on these internal resistances, the value of the capacitor, and the source impedance (together with how far the voltage has to 'slew' from the last reading). The resistances of the source, and the internal resistances are summed when working out this time.

Best Wishes
Mark



Joined: 07 Sep 2003
Posts: 2838
Location: Atlanta, GA

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PostPosted: Wed Oct 20, 2004 10:32 am     Reply with quote
That depends on what voltage source you are trying to measure. Typically a 0-5v is not a problem. If you want to measure higher than 5v then you will need to scale it down.
Guest
PostPosted: Thu Oct 21, 2004 12:18 am     Reply with quote
many thanks for your answer
BR JohnE
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