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--- Quote from: jimmc on April 06, 2010, 11:03:09 pm ---(...)
There is an interesting quirk on the MAX4329 data sheet, the common mode input voltage range (over the full temperature range) is quoted as  GND-0.05v to Vcc-1.4V; so using a mid point ground at the minimum supply voltage of 2.7v means that the common mode range is exceeded (by 50mV).
Possibly an asymmetric split supply (say +1.6v, -1.4v) would be better.

--- End quote ---

I think you had the wrong data sheet, the one from the Maxim site quotes Vee to Vcc. Even the input offset voltage is specified @CM=Vee and Vcc
Yes I looked at the full temperature range.

My fault, I mistyped the part number it should  be MAX4239 as in Dave's blog.
The figures are correct for the MAX4239.


If anyone's interested, this month's Circuit Cellar has an article on picoammeter design, which is a bit different to Dave's design.

A guard ring would be a good idea for any future evolution if an order for 10,000 came in (cheap but a minor effect on the performance) but it certainly isn't worth re-designing the PCB for another run of 100. Chances are that EMI noise from the room is going to swamp that current anyway. Equally, if you were after ultimate performance then an offset power supply might be appropriate but I tend to be of the opinion "if it ain't broke, don't fix it". It might seem a simple matter of changing the resistive divider ratios, but seemingly simple things like that have a horrible habit of coming back to bite you in the form of obscure bugs.

This raises an interesting point about engineers. We seem to be wired differently (ho ho) to other people. Whilst a road mender, a doctor, or a lawyer will tend to stand back at some point and say "that'll do", an engineer will tend to want to design the most perfect piece of equipment possible. The trouble is that it takes exponentially more time to produce. Then, having got a brilliant design, we want to make it better... and better... and better, long after our changes would ever be noticed by the customer.

What separates a student or run-of-the-mill engineer from a professional is knowing when to stop, and that is not easy to learn. I wonder if female engineers have this problem too or if it is just a testosterone thing?


I'm fairly new to PCB layout, but was there any reason why the top side (where the labels are) couldn't have a ground plane? And also, having a ground polygon on the component side?



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