[sdiy] Divider or op-amp ?

[Scott Gravenhorst]

Given R1 and R2, where R1 is the "top" resistor, Vout = Vin * (R1+R2)/R2

jbv <jbv.silences at club-internet.fr> wrote:
>
>
>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
>

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