Re: Measure 10-15 volts ***accurately*** (was Re: AD with 12F675 fl...

Started by Wouter van Ooijen April 9, 2004
> The actual value of the resistors in the voltage divider is
> unimportant. The stability of the ratio of the values is
> important. Most resistor networks have far better value and
> temperature coefficient matching between resistors than the
> actual absolute value of the resistance. So, to accurately
> read a 0-15VDC voltage with a PIC, I've used a 10k resistor
> network, putting three resistors in series. The top of the
> bottom resistor goes to the PIC analog input pin.

So you have source impedance of 10k // 20k, which might be on the high
side. Could you try with a 1k pack?

Wouter van Ooijen

-- -------
Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products


I'm not going to do the math if I don't need to, but all the formulas
are in the spec sheet. The lower the input impedance, the more
accurate the reading for the same sample time. The higher the input
impedance the higher the error will be caused by leakage current and
lastly, the minimum sample time suggested by Microchip is probably for
less than 1 bit error at 10k ohms. The setup time is proportional to R
* C the R being the input resistance, the C being chip dependant.

#1. Keep the input impedance as low as practical.
#2. Make the setup/sample time as long as practical.

Chad

--- Harold Hallikainen <> wrote:
>
> The actual value of the resistors in the voltage divider is
> unimportant. The stability of the ratio of the values is important.
> Most resistor networks have far better value and temperature
> coefficient matching between resistors than the actual absolute value
> of the resistance. So, to accurately read a 0-15VDC voltage with a
> PIC, I've used a 10k resistor network, putting three resistors in
> series. The top of the bottom resistor goes to the PIC analog input
> pin.
>
> Harold > FCC Rules Online at http://www.hallikainen.com -
> Manufacturers, consultants, engineers, attorneys and labs can
> advertise on http://www.hallikainen.com >
> -- "d1camero" <> wrote:


=====
My software has no bugs. Only undocumented features.

__________________________________



Oops also if you are dividing the measured voltage, any reference
errors(VDD) will be effectively multiplied by the same ratio.

Chad

--- Harold Hallikainen <> wrote:
>
> The actual value of the resistors in the voltage divider is
> unimportant. The stability of the ratio of the values is important.
> Most resistor networks have far better value and temperature
> coefficient matching between resistors than the actual absolute value
> of the resistance. So, to accurately read a 0-15VDC voltage with a
> PIC, I've used a 10k resistor network, putting three resistors in
> series. The top of the bottom resistor goes to the PIC analog input
> pin.
>
> Harold > FCC Rules Online at http://www.hallikainen.com -
> Manufacturers, consultants, engineers, attorneys and labs can
> advertise on http://www.hallikainen.com >
> -- "d1camero" <> wrote:


=====
My software has no bugs. Only undocumented features.

__________________________________




The actual value of the resistors in the voltage divider is unimportant. The stability of the ratio of the values is important. Most resistor networks have far better value and temperature coefficient matching between resistors than the actual absolute value of the resistance. So, to accurately read a 0-15VDC voltage with a PIC, I've used a 10k resistor network, putting three resistors in series. The top of the bottom resistor goes to the PIC analog input pin.

Harold FCC Rules Online at http://www.hallikainen.com -
Manufacturers, consultants, engineers, attorneys and labs can advertise on http://www.hallikainen.com
-- "d1camero" <> wrote: