pic a/d problems

[timmmmayyy!]

Has anyone else found this problem?

I have many pic18f452 chips and use the a/d converters. I have found that
there is a large variation in what x volts will give, varing between chip to
chip.

1.55 volts on one chip gave 79 (8 bit result) 83 on others.

It is quite strange this and quite anoying.

Cheers
Timmay!

 

[timmmmayyy!]

Reference is the vdd and vss.
It may be the circuit boards, i will put an ic holder on one pcb, and try
different chips within the same circuit.
impedance is 1k
aquasision time is 1ms.
thanks

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:25hkn0pg3sik7k7r9h3m5ggukcg3va1j11@4ax.com...

On Sat, 23 Oct 2004 11:13:50 +0100, the renowned "timmmmayyy!"
timmmmayyy@nospamplease.com> wrote:

Has anyone else found this problem?

I have many pic18f452 chips and use the a/d converters. I have found that
there is a large variation in what x volts will give, varing between chip
to
chip.

1.55 volts on one chip gave 79 (8 bit result) 83 on others.

It is quite strange this and quite anoying.

Cheers
Timmay!

Not at all, the ADCs meet all specifications when properly applied,
IME.

Perhaps your acquisition timing is off or your source impedance is too
high, as Frank suggests (2.5K maximum). Under some (improper)
conditions you can get voltage from a previously read input affecting
the current reading. What about the reference voltage?


Best regards,
Spehro Pefhany
--
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