P
Phil Hobbs
Guest
So I\'m doing a simplified version of the differential laser noise
canceller, which in the spherical cow universe looks like it does very
well out to about 10 MHz, thanks to the amazing properties of BFP640s
and some new photodiodes with reduced series resistance. (At least
according to Hamamatsu.)
One thing I need for this is an adjustable resistance with good
bandwidth. The fastest dpot I can find is the AD5273BRJZ1 (1k, 64
steps, ~6 MHz bandwidth at half scale).
The resolution is too coarse for my application, but as it\'s pretty well
set-and-forget, I don\'t mind some algorithmic complexity.
Turns out that if you make a sort of Darlington connection, with one
dpot connected as a rheostat in series with the wiper of the other
(which is connected to one end), you can get the approximate resolution
of a 10-11 bit dpot.
1k
0-*----R1R1R1---------------0
| A
| *-------*-----*
| | |
| V | 5k
*------------R2R2R2--*
It works best if R2 is about 5 times R1, but the bandwidth may be better
if I stick with the 1k version.
Neglecting switch resistance, calculating the total resistance as a
function of the codes, sorting into a single 1-D array to get a
monotonically increasing resistance function, and taking the first
finite difference reveals a step size nearly always less than 0.1%
except near the low-resistance end, which I don\'t care much about.
There\'s a plot at
<https://electrooptical.net/static/oldsite/www/sed/FightingDPOTs.png>
Fun.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
canceller, which in the spherical cow universe looks like it does very
well out to about 10 MHz, thanks to the amazing properties of BFP640s
and some new photodiodes with reduced series resistance. (At least
according to Hamamatsu.)
One thing I need for this is an adjustable resistance with good
bandwidth. The fastest dpot I can find is the AD5273BRJZ1 (1k, 64
steps, ~6 MHz bandwidth at half scale).
The resolution is too coarse for my application, but as it\'s pretty well
set-and-forget, I don\'t mind some algorithmic complexity.
Turns out that if you make a sort of Darlington connection, with one
dpot connected as a rheostat in series with the wiper of the other
(which is connected to one end), you can get the approximate resolution
of a 10-11 bit dpot.
1k
0-*----R1R1R1---------------0
| A
| *-------*-----*
| | |
| V | 5k
*------------R2R2R2--*
It works best if R2 is about 5 times R1, but the bandwidth may be better
if I stick with the 1k version.
Neglecting switch resistance, calculating the total resistance as a
function of the codes, sorting into a single 1-D array to get a
monotonically increasing resistance function, and taking the first
finite difference reveals a step size nearly always less than 0.1%
except near the low-resistance end, which I don\'t care much about.
There\'s a plot at
<https://electrooptical.net/static/oldsite/www/sed/FightingDPOTs.png>
Fun.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com