After some initial testing with an ad202 I see that it can easily pull a negative on my adc. Any good ideas on how to clamp this to keep this off the protection diodes of an mcp3208? I believe that when the rail diodes are hit that all conversion is skewed to the direction of the hit.
You can put a resistor between the ad202 and the ADC input, and a Schottky diode from the ADC input to ground (cathode to the input pin). That limits the negative to -0.2 volt, which will be safe for the ADC. The resistor can be something like 1k or 10 k. The resistor might be enough to protect the ADC, but better with the diode too.
I have only tried it on my workbench with wires soldered to it. It works well as far as I can tell. I have a good feeling about it, but we will see when I get my boards made. (It better be good for $45!)
A little update on this ad202. I am a little disappointed with it in this respect, it has up to a 5mv input offset voltage, which gets right into the range I need to be measuring. I guess I should reconsider using a dual supplied op amp. Know of a good way to get a clean negative 15v or so supply?
I have figured out that this probe output runs about 100mv above ground potential, probably due to the galvanic nature of it.
There are some that are designed specifically for providing clean DC for op amps which need little or no extra filtering. Any filtering required is usually to clean up the input power.
Back on the ad202, can I use a voltage divider to calibrate my offset reliably on the output side? As of right now I am doing it in software. I really would like to use a small negative supply so I could do it both ways. I have though about using something like one of the many small dc-dc cap converters like the nju 7660, but I am not clear if those will be clean enough and stable enough to be reliable.
The problem I have been running in to is that with a 5mv offset on the wrong side of things I lose the bottom 2 parts oxygen sensing capability.
The AD202 is an isolation amp and the way it is set up I believe I could do that, as there are two power pins, and two output pins. I think I could offset the ground reference quite easily.
Comments
Is the ad202 working out otherwise?
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Tracy Allen
www.emesystems.com
I have figured out that this probe output runs about 100mv above ground potential, probably due to the galvanic nature of it.
The problem I have been running in to is that with a 5mv offset on the wrong side of things I lose the bottom 2 parts oxygen sensing capability.