ADC protection pins

[xar]

Hi,
I would like to know if this could be feasible to protect ADC inputs on Isopod.
A series resistor of about 100Ohm with a couple of skottky diodes tied to the ADC IN pins and Vcc / Gnd.
Or is there a better way ?
TIA,

Giuseppe Marullo

[Dave]

Hi Giuseppe,
It might work using the diodes as suggested, however this might clip ranges of voltages that perhaps were meant to be measured. It seems a simple voltage divider would be adequate in most cases to scale a larger voltage swing to the level of the IsoPod ADC. If the input voltage to be measured connects to R1, with the other side of R1 then connects to R2 and the ADC pin, other side of R2 then to ground, your voltage at the ADC pin would be V in * R2 / (R1 + R2). If the voltage to be measured doesn't go beyond the levels the ADC uses, then no scaling is necessary.

[RMDumse]

Protection wise, yes, the back biased Schottky diodes are a good idea, and way to protect the A/D input. They add a little capacitance to the input, but keep voltage excursions from blowing the chip.

[xar]

Randy,
yes I was going to reply to Dave but I was not able to post the reply.
The skottky solution will allow normal adc range, at the nominal resolution, on DC coupling.
I got 100Ohm 2W resistors, the pack is bigger than the DSP so I will protect just what I will use(2 for the Sharp).
I am searching for skottky smd type, but I was unable to find them into hobby stores.
The skottky will be 1A, they should suffice.

[RMDumse]

If you are interfacing to a Sharp distance sensor, such as the GP2D12, then you don't need much of any protection. The maximum voltage the device puts out is still below the upper limit of the A/D maximum. I wouldn't bother with the diodes. While the diodes would perhaps protect you from a miswiring, they themselves require wiring and increase the change of a mistake, too. I'd go straight in from the Sharps (in fact that's exactly what Dave and I did on our Mini Sumo's).