Designer’s Guide Community

RobG

Community Fellow

Offline

Posts: 570
Bozeman, MT

Re: Distribute bias current or bias voltages ?
Reply #15 - Feb 18^th, 2014, 8:11am

Horror Vacui wrote on Feb 17^th, 2014, 12:48am:

Here is my simple calculation:
If we use higher current to generate the same voltage, then the resistance would be proportionally lower.
v=i*R �& �vn^2=c*i*R^2 p=1/RC
v'=2i*R/2 �& �vn'^2=c*2i*(R/2)^2=c*i*R^2 /2 = vn^2 /2 --> vn'=vn/sqrt(2) p'=1/(R/2)/C=2/RC
So in the double current case we have 1/sqrt(2) voltage noise if there is no capacitance. But if there is a cap, then the bandwidth of the RC filter increases. This results in increased noise. But if the filtering pole is independent of the current-to-voltage generating resistor, then higher current means lower noise.

I think I know what you are talking about - it amounts to whether or not you are interested in the whole spectrum or just larger than 1/RC. Here's my math, all mistakes due to early morning mental fog.

Back to top

IP Logged

RobG

Community Fellow

Offline

Posts: 570
Bozeman, MT

Re: Distribute bias current or bias voltages ?
Reply #16 - Feb 18^th, 2014, 8:14am

harpoon wrote on Feb 18^th, 2014, 5:21am:

loose-electron wrote on Feb 17^th, 2014, 2:08pm:

In some big current array devices, I have done a two stage bias system of multiple current bias distributions to �local reference diodes which then provided a bias point for the local devices. How big that "bias circle" was a function of the proximity matching data. �

i like this idea ... distribute current globallly ... and then again locally.

This is what I and to some extent Carl where getting at, but you don't want too many daisy chained since the errors add.

Back to top

IP Logged

carlgrace Senior Member Offline Posts: 231 Berkeley, CA	Re: Distribute bias current or bias voltages ? Reply #17 - Feb 23^rd, 2014, 9:06pm RobG wrote on Feb 12^th, 2014, 9:33am: [quote And anyone familiar with Carl Grace's work knows he isn't a bonehead . Shucks, Rob, that is really nice of you to say. But when I make a bonehead comment it is still a bonehead comment no matter what I've done in the past.
Back to top	IP Logged

Lex

Senior Member

Offline

Posts: 201
Eindhoven, Holland

Re: Distribute bias current or bias voltages ?
Reply #18 - Mar 3^rd, 2014, 2:10am

In image sensors where there can be millions of amplifiers and ADC's on several micron pitch, I've seen mostly voltage routing. The two-stage-bias-system that is mentioned, I've seen on silicon as well, thought it has the disadvantage that it can create 'block patterns' as the part of the noise is correlated. Block patterns are more visible than the smooth gradient that would normally arise. (Hence more disturbing to the eye.) Of course, it depends on the application whether this is a problem.

Another obvious thing is whether changing from voltage to current routing costs power. In the image sensor example, the extra current draw would also be quite costly.

Back to top

IP Logged

RobG Community Fellow Offline Posts: 570 Bozeman, MT	Re: Distribute bias current or bias voltages ? Reply #19 - Mar 3^rd, 2014, 5:18am Great insight with the "block noise" Lex.
Back to top	IP Logged