micro inputs, voltage clamps

[BeeBop]

Hello,

I am using an MCP3204 analog to digital converter to measure the output of an OP amp. The A/D converter has a maximum input voltage of 5v, however the OP amp is powered by + and -12v. I need to ensure that the voltage into the ADC will never exceed 5 volts without changing the linearity of any voltage below that point. I think I can use two diodes for input protection: one cathode to+5V, anode to input, the other, cathode to input, anode to GND. I also have a 3.9k series resistor from opamp output and a 10uF cap, between the signal and GND.

My question is: Do these need to be 5v zeners, or can I use a 1N4148 or similar?

Any advice would be so nice!

Thanks,
Robert

 

[Nigel Goodwin]

Hello,

I am using an MCP3204 analog to digital converter to measure the output of an OP amp. The A/D converter has a maximum input voltage of 5v, however the OP amp is powered by + and -12v. I need to ensure that the voltage into the ADC will never exceed 5 volts without changing the linearity of any voltage below that point. I think I can use two diodes for input protection: one cathode to+5V, anode to input, the other, cathode to input, anode to GND. I also have a 3.9k series resistor from opamp output and a 10uF cap, between the signal and GND.

My question is: Do these need to be 5v zeners, or can I use a 1N4148 or similar?

Any advice would be so nice!

Just 1N4148's, many devices (such as PIC's) already have them internally, so you only need the series resistor - check your datasheet!.

 

[BeeBop]

Thanks Nigel,

The data sheet doesn't say much about it, other than just one line which says the inputs are protected, and the same Analog Input Model which is in the AD PIC data sheets. (I scoured it again, after reading your reply last night.)
It was everything else I read, which claimed 'be sure and protect the inputs from spikes,' which caused me to fret. No one ever gave a practical method, although one or two suggested a very fast diode - 1N4150 or 1N5908.
Anyway, thanks again, now I can stop procrastinating, and finish the thing!

Thank you so much,
Robert