Hi,
I built a digitizer with an adc0831. I thought I had read a datasheet
saying that the max. on the input was 5 volts but read later that its
the supply voltage (6.3) plus 0.3 volts. I have a 1k resistor between
the signal source and the input pin. Fearing to smoke it i put in a
4.7v zener before the resistor. With the zener the pin is getting 4.25
volts. Without the zener its 5.7v. The source is was a 741 which
proved too noisy so I replaced them with TL071s.
1) What is actually the maximum voltage limit on the adc input?
2) Is the voltage without the zener likely to rise above the adc input
limit?
3) Please confirm for me (or not) that the resistor and zener are really
making up a voltage divider?
Thanks,
tango2

Tango2 wrote:
Hi,
I built a digitizer with an adc0831. I thought I had read a datasheet
saying that the max. on the input was 5 volts but read later that its
the supply voltage (6.3) plus 0.3 volts. I have a 1k resistor between
the signal source and the input pin. Fearing to smoke it i put in a
4.7v zener before the resistor. With the zener the pin is getting 4.25
volts. Without the zener its 5.7v. The source is was a 741 which
proved too noisy so I replaced them with TL071s.
1) What is actually the maximum voltage limit on the adc input?
2) Is the voltage without the zener likely to rise above the adc input
limit?
3) Please confirm for me (or not) that the resistor and zener are really
making up a voltage divider?
Thanks,
tango2

If the Opamp and ADC are powered from the same power supply then there
is no problem and you don't need the zener. If the power supply on the
opamp is separate from the power supply on the ADC then there is a
concern and some protection is needed

Dan

"Dan H" wrote in message
oups.com...

Tango2 wrote:
Hi,
I built a digitizer with an adc0831. I thought I had read a datasheet
saying that the max. on the input was 5 volts but read later that its
the supply voltage (6.3) plus 0.3 volts. I have a 1k resistor between
the signal source and the input pin. Fearing to smoke it i put in a
4.7v zener before the resistor. With the zener the pin is getting 4.25
volts. Without the zener its 5.7v. The source is was a 741 which
proved too noisy so I replaced them with TL071s.
1) What is actually the maximum voltage limit on the adc input?
2) Is the voltage without the zener likely to rise above the adc input
limit?
3) Please confirm for me (or not) that the resistor and zener are really
making up a voltage divider?
Thanks,
tango2

If the Opamp and ADC are powered from the same power supply then there
is no problem and you don't need the zener. If the power supply on the
opamp is separate from the power supply on the ADC then there is a
concern and some protection is needed

Dan

.. the reason being that the op amp can't output a voltage greater than
the supply.
Based on the given clues, I would say that the maximum converter input is
the applied supply voltage plus 0.3 volts, but thats the fry-point max,
not the useful max necessarily.

elvis