Hello,
The datasheet does not mention anything about the voltage
reference max input voltage. If you see in "electrical characteristic",
states that you cannot operate the device above 0.8V over VDD in IO pins:
VIH Input High Voltage
I/O ports:
D040 with TTL buffer 0.25 VDD + 0.8V
Thats all i've found about it, because in the ADC chapter doesn't
mention anything about it.
By the way: If I use two series resistor, say, I would divide the
voltage. If I divithe the voltage, say by 2, every bit in the
conversions must be muplitplied by two.
Example:
5V Voltage reference, at 1024 bits (ADC resolution) gives me 4,8 mv /bit.
Lets imagine I connect a 10V batt with two 100k res. VIN will be 5V,
same as Vref, adc result all '1' . Here, every ADC step means 4,8 mv * 2
(voltage divider) = 9,6 mV . Thats not accurate enough, because the PIC
(and I think any ADC) specs says +- 1 LSB in the conversion. so, the
conversion could be at least +- 9,6 mV wrong, which is not suitable for
say Nimh batts, that require the charge to be cuttof when a 10mV curve
occurs.
So, to summarize it, If I divide the voltage, I loose resolution
(inverse proportional). Same applies to the Op Amp, which will reduce
the voltage too, but with higher part count and cost ;-)
Regards.
-------- Mensaje original --------
Asunto: Re: [Sdcc-user] [OT] - ADC Ideas
Fecha: Thu, 8 Mar 2007 11:46:51 -0300
De: C C Budden <[EMAIL PROTECTED]>
Responder a: [email protected]
Para: <[email protected]>
Referencias: <[EMAIL PROTECTED]>
Hi Markos. Did Yoy take a look on the PIC datasheet? If you have a Voltage
reference (2 pins) on the IC apart power supply, and in the datasheet is
specified that can support 12 V with 5V on power supply, of course you can.
I didn´t see that PIC datasheet, but in case that you cannot use that
voltage reference range, you will have to make a conditioning signal circuit
with an operational amplifier.
Take care to not exed the specified ranges or damage of the device may
ocurr.
On a simple way, if you use a couple of resistors use metal film resistor
diu to the lower inductance they have compared with others. On most AD
imputs its not recommended to use caps, inductors exept you have the
impedances well separated.
I hope this help you.
Regards, Casiano.
----- Original Message -----
From: "Markos" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Thursday, March 08, 2007 5:48 AM
Subject: [Sdcc-user] [OT] - ADC Ideas
> Hi guys,
> maybe this is a little offtopic, but i'm sure you could
> help me. I'm implementing a simple battery charger, and I need to
> messaure voltages ranges from 0 to 12 V.
> ¿Could I use the PIC integrated ADC to messaure above 5 V (provided that
> I use the voltage reference pins tied to 0 and 12 V, for example)? Most
> external ADC have a 0 to 5V input range....
> How would you guys solve this? Any Ideas?
>
> Thanks in advance.
>
>
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