Why should diodes be noisy here? While the ADC input is within operating
range they aren't conducting.
I am assuming you want to protect the ADC input against the ESD spike
from hell or something like that. This is how it's done:
a. Resistor from input signal to diodes. 1k or so, depends on how much
you can tolerate. More is better but make that a longer body resistor to
reduce the chance that a spark jumps across. That limits the spike
current in general.
b. Diodes to the positive and negative rail of the ADC. You can use
SOT23 duals (series) or even SC75 if space is tight.
c. Resistor 200 ohms from there to the ADC input.
This results in the lion's share of the ESD pulse going through the
external diodes instead of the parasitic substrate diodes in the ADC.
Even if the "pulse from hell" drives the diode 2V beyond a rail the
current going through the ADC substrate would remian under 10mA.
Of course, you can also generate helper voltages lower than the rails
and leave out that 200 ohms resistor but that is more effort.
By low noise, I presume they can't tolerate a lot of series resistance.
If you look at ESD testing, there is human body model and machine model.
Machine model is for assembly, and human body model is real life. (Of
course plugging a sensor into an instrument can look a lot more like a
machine model than a human model.) If you look at the human body model
circuit, just adding capacitance can greatly reduce the ESD spike.
It isn't clear to me why a transistor will be any quieter than a
discrete diode, other than there is less capacitive coupling. But if
this low noise circuitry, I have to presume the impedances involved are
small as well, so the cap can't couple in that much noise, given a
voltage divider with the cap at omega*C.
The chips generally have nfet ESD protection, i.e. body diode in the
negative direction and snap back in the positive direction. Maybe a
small nfet with low breakdown voltage on the input will do the trick.
But this isn't all that different from using a transorb diode.
Inducing latchup is another story. Seems to me series resistance and
diodes to the rails is the only solution. But I never saw the depletion
mode fet circuit. If that circuit goes high-Z in both directions, that
would reduce latchup.