[sdiy] Divider or op-amp ?

[jbv]

WeAreAs1 at aol.com a *crit :

> In a message dated 5/10/04 12:19:42 PM, jbv.silences at club-internet.fr writes:
>
> << I have a voltage that can vary from 0 to +15V,
> and I need to send it to the input of ADC that
> is limited to 0 to 5V.
> Can I use a simple divider with 2 res. or do I
> need an op-amp ? >>
>
> I think the simple 2-resistor voltage divider should be just fine.  That is,
> assxuming that your ADC expects to see voltage at its input and not current.
> Nevertheless, you may need to take into account the input impedance of the ADC
> and the output impedance of the source of the 0 to +15V.  Make sure that the
> values of the resistors of your divider are dimensioned properly so as to not
> put too much load on the source of the voltage.  Using an opamp makes this a
> bit easier because it would effectively isolate the ADC from the source
> voltage, especially if you used a high-impedance FET-input opamp.  However, depending
> on particulars of your circuit, an opamp may just be overkill.
>
> Michael Bacich

Well, the source is the output of a 741 (used as BPF) and the ADC is 1 of the 8
ADCs of a 80C535 Siemens uC.
Inbetween I might also add an anlog multiplexer such as the 4051.
BTW how do I calculate the value of the 2 resistors for the divider ?

Thanks,
JB