[sdiy] Divider or op-amp ?

[WeAreAs1 at aol.com]

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