I see opamps that specify "differential" and "common-mode" input capacitance, but don't say what that really means. An extreme case is ADA4817-1, where Ccm is
1.3 pF and Cdiff is 0.1!
Any ideas? If I use this as a classic inverting amp, with the + input grounded, what would the capacitance look like at the - input?
(I'm being pressured to get a proposal out, with specs and pricing, so I don't have time right now to measure this... not that it would be easy.)
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
They call Cm the "common mode capacitance of the amplifier" (between equations 14 and 15) so I would guess 1.4pF, as George's not-unduly-pessimistic reading of the ds.
I was moving wood after work today, (and thinking), it's not the most 'pessimistic number'. I'm not sure how they measure the CM Cap. but they could tie the inputs together, measure that, and then divide by 2. In which case most of the CM cap could be in one input or the other.
So my pessimistic number would be 2.5, (someone may have measured it.)
Generally pin capacitance isn't measured on automatic test equipment. There is too much capacitance just in the handlers, relays, cables, etc. You can try to null that capacitance, but really nobody (based on places I worked) bothers to measure it at test at all.
Pin capacitance is a GBD (guaranteed by design) spec. Here is the reasoning behind this. The first step once wafers leave the fab is to test them parametrically. You insure the basic components meet specifications. There will be test structures in the test pattern than will have enough capacitance to make it easier to measure component capacitance. Once you believe the wafer is within specifications, the pin capacitance should also be within specifications.
Now the final product (packaged chip) is bench tested, and that will include testing pin capacitance. It will be done powered. This has to be the case since protection diodes need to be reversed biased. Since the parameter is not measured at final test, it tends to be very much guardbanded. Much like leakage specifications, the actual product will be much better than the specification.
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