carlgrace wrote on Feb 11th, 2014, 2:36pm:Aaron's right about voltage distribution being noisier but also think of crosstalk. If you have one voltage node connecting all your amplifiers together it is much easier for them to talk to each other.
One problem with distributing voltages is that you typically end up with errors because the "grounds" are far apart and there is some finite impedance between them.
One idea is to send a single current to amplifier 1, then generate an output current to amplifier 2, and so on. That way you are distributing currents but you don't need to route 100 wires from a master bias circuit.
Also, if you're designing an array, I highly recommend you do the layout so that it is "routed by construction". Try your best to limit top-level routing. It will save you time and the array will match better.
A couple of observations - in addition to IR drops causing problems when matching voltages, process and temperature mismatches between components are issues if the distance between the devices is large. Beware, this can also bite you if you route current to create a voltage. I know of a group that tried to create local voltage references routing a current and ran into trouble because the local resistance didn't match the master one. Temperature, process, and packaging stress all conspire to make devices different across the die.
Avoid errors caused by the conversion device: if you are trying to create a voltage then you should generally route a voltage. (You might have to Kelvin connect things to avoid IR drop problems.) On the other hand if you want to create a current (for example, by biasing the gate of a local current source) you should route a current and mirror it locally.
When you have a huge array of devices it may make sense to do a little of both - make the layout tall and skinny so the biased devices are adjacent and have them all share the same local bias. You might be able to make this work for (guessing) 10 amplifiers without the end amplifiers debiasing. This means you will need ten of these sub-blocks to get 100 amplifiers, but you will only have to route ten bias lines. As Carl pointed out, you will have to be careful that the amplifiers don't crosstalk via the common bias line.
And for giggles I'm going to make Carl regret suggesting daisy chaining 100 biases since even a 5% error in the mirror gain will cause a 13,000% error in the last one.
I will leave it as an exercise for the reader to calculate the random mismatch and thermal noise errors of the last device (hint, ~sqrt(200)x).