Forums

Cute dpot hack for increased resolution

Started by Phil Hobbs July 22, 2020
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

On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote:
> 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.
Looks like an interesting graph, but I don't see any error bars. Won't the limitations of accuracy of the 6 bit dpots swamp out your monotonicity? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
On 2020-07-22 19:50, Ricketty C wrote:
> On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: >> 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.
> > Looks like an interesting graph, but I don't see any error bars. > Won't the limitations of accuracy of the 6 bit dpots swamp out your > monotonicity?
I'm not expecting it to be monotonic, for sure. The codes are completely scrambled, so just finding them is an issue. I'd expect the main effect of the DNL of the dpots will be to scramble them differently without making huge changes in the general outline of the plot. (The sort order will change, certainly.) 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
On Wednesday, July 22, 2020 at 9:05:23 PM UTC-4, Phil Hobbs wrote:
> On 2020-07-22 19:50, Ricketty C wrote: > > On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: > >> 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. > > > > Looks like an interesting graph, but I don't see any error bars. > > Won't the limitations of accuracy of the 6 bit dpots swamp out your > > monotonicity? > > I'm not expecting it to be monotonic, for sure. The codes are > completely scrambled, so just finding them is an issue. I'd expect the > main effect of the DNL of the dpots will be to scramble them differently > without making huge changes in the general outline of the plot. (The > sort order will change, certainly.)
I think my point is more that a 64 step pot is only going to have an accuracy close to that, so that won't go away by adding resolution. Or is resolution all you are looking for? I guess I'm not following what this gains? You get lots more steps, but you don't know any more about where they are. What am I missing? -- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
On Thursday, July 23, 2020 at 12:57:35 PM UTC+10, Ricketty C wrote:
> On Wednesday, July 22, 2020 at 9:05:23 PM UTC-4, Phil Hobbs wrote: > > On 2020-07-22 19:50, Ricketty C wrote: > > > On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: > > >> 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. > > > > > > Looks like an interesting graph, but I don't see any error bars. > > > Won't the limitations of accuracy of the 6 bit dpots swamp out your > > > monotonicity? > > > > I'm not expecting it to be monotonic, for sure. The codes are > > completely scrambled, so just finding them is an issue. I'd expect the > > main effect of the DNL of the dpots will be to scramble them differently > > without making huge changes in the general outline of the plot. (The > > sort order will change, certainly.) > > I think my point is more that a 64 step pot is only going to have an accuracy close to that, so that won't go away by adding resolution. Or is resolution all you are looking for?
Clearly.
> I guess I'm not following what this gains? You get lots more steps, but you don't know any more about where they are. What am I missing?
The steps that correspond to the coarse digital pot changing are going to be gross, but tolerably well defined. The trick is to use the fine digital pot to move the final output over a range which is larger than the biggest single change caused by a one bit change in the input to the coarse pot, so that you can be sure that you have over-lapping ranges of fine adjustment. What Phil seems to be talking about doing is mapping all the possible outputs for any particular example of the circuit and setting up a look-up table of a monotonic string of desired outputs, and the inputs to the coarse and fine digital pots that get them. It is an open question whether the outputs are going to be stable enough to let this work. A twiddling program that knew when it was getting close to running out of range on the fine digital pot, and started over from scratch after changing input to the coarse pot might be another possibility, but it does rather depend on what Phil is actually doing. It wouldn't fit neatly into a simple control loop. -- Bill Sloman, Sydney
On Thursday, July 23, 2020 at 1:18:30 AM UTC-4, Bill Sloman wrote:
> On Thursday, July 23, 2020 at 12:57:35 PM UTC+10, Ricketty C wrote: > > On Wednesday, July 22, 2020 at 9:05:23 PM UTC-4, Phil Hobbs wrote: > > > On 2020-07-22 19:50, Ricketty C wrote: > > > > On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: > > > >> 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. > > > > > > > > Looks like an interesting graph, but I don't see any error bars. > > > > Won't the limitations of accuracy of the 6 bit dpots swamp out your > > > > monotonicity? > > > > > > I'm not expecting it to be monotonic, for sure. The codes are > > > completely scrambled, so just finding them is an issue. I'd expect the > > > main effect of the DNL of the dpots will be to scramble them differently > > > without making huge changes in the general outline of the plot. (The > > > sort order will change, certainly.) > > > > I think my point is more that a 64 step pot is only going to have an accuracy close to that, so that won't go away by adding resolution. Or is resolution all you are looking for? > > Clearly. > > > I guess I'm not following what this gains? You get lots more steps, but you don't know any more about where they are. What am I missing? > > The steps that correspond to the coarse digital pot changing are going to be gross, but tolerably well defined. > > The trick is to use the fine digital pot to move the final output over a range which is larger than the biggest single change caused by a one bit change in the input to the coarse pot, so that you can be sure that you have over-lapping ranges of fine adjustment. > > What Phil seems to be talking about doing is mapping all the possible outputs for any particular example of the circuit and setting up a look-up table of a monotonic string of desired outputs, and the inputs to the coarse and fine digital pots that get them. > > It is an open question whether the outputs are going to be stable enough to let this work. > > A twiddling program that knew when it was getting close to running out of range on the fine digital pot, and started over from scratch after changing input to the coarse pot might be another possibility, but it does rather depend on what Phil is actually doing. It wouldn't fit neatly into a simple control loop.
For any use of this combination to be useful, there has to be a way of making changes to the combinations that map to a set of points with better error than what a single pot achieves. Unless the error of the pots at their defined values is significantly better than would be expected by the 64 step resolution that won't happen. Consider the case you describe. The coarse pot is at a setting and the fine pot reaches the end of it's range. To move to a new coarse setting the user/program/magic agent must know which coarse setting to switch to and what fine setting will go with that coarse setting to continue a forward adjustment. If the coarse device has a differential non-linearity that is not much better than expected for a 64 step pot, this won't be possible. It's no different than the problems of combining flash converters to obtain more resolution. One converter measures the coarse bits and the other measures the fine bits. But both must be accurate to the same level to be able to combine them. There may be fancy techniques used in the flash converter situation, but they are not in play in Phil's pot setup. He might be able to get a little more resolution, but I don't see how it will be much unless the pots are very much under specified. -- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
On Thursday, July 23, 2020 at 4:51:13 PM UTC+10, Ricketty C wrote:
> On Thursday, July 23, 2020 at 1:18:30 AM UTC-4, Bill Sloman wrote: > > On Thursday, July 23, 2020 at 12:57:35 PM UTC+10, Ricketty C wrote: > > > On Wednesday, July 22, 2020 at 9:05:23 PM UTC-4, Phil Hobbs wrote: > > > > On 2020-07-22 19:50, Ricketty C wrote: > > > > > On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: > > > > >> 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. > > > > > > > > > > Looks like an interesting graph, but I don't see any error bars. > > > > > Won't the limitations of accuracy of the 6 bit dpots swamp out your > > > > > monotonicity? > > > > > > > > I'm not expecting it to be monotonic, for sure. The codes are > > > > completely scrambled, so just finding them is an issue. I'd expect the > > > > main effect of the DNL of the dpots will be to scramble them differently > > > > without making huge changes in the general outline of the plot. (The > > > > sort order will change, certainly.) > > > > > > I think my point is more that a 64 step pot is only going to have an accuracy close to that, so that won't go away by adding resolution. Or is resolution all you are looking for? > > > > Clearly. > > > > > I guess I'm not following what this gains? You get lots more steps, but you don't know any more about where they are. What am I missing? > > > > The steps that correspond to the coarse digital pot changing are going to be gross, but tolerably well defined. > > > > The trick is to use the fine digital pot to move the final output over a range which is larger than the biggest single change caused by a one bit change in the input to the coarse pot, so that you can be sure that you have over-lapping ranges of fine adjustment. > > > > What Phil seems to be talking about doing is mapping all the possible outputs for any particular example of the circuit and setting up a look-up table of a monotonic string of desired outputs, and the inputs to the coarse and fine digital pots that get them. > > > > It is an open question whether the outputs are going to be stable enough to let this work. > > > > A twiddling program that knew when it was getting close to running out of range on the fine digital pot, and started over from scratch after changing input to the coarse pot might be another possibility, but it does rather depend on what Phil is actually doing. It wouldn't fit neatly into a simple control loop. > > For any use of this combination to be useful, there has to be a way of making changes to the combinations that map to a set of points with better error than what a single pot achieves.
No. All you need is piece-wise monotonicity, and a control procedure that can deal with the coarse steps between the monotonic pieces.
> Unless the error of the pots at their defined values is significantly better than would be expected by the 64 step resolution that won't happen.
Wrong.
> Consider the case you describe. The coarse pot is at a setting and the fine pot reaches the end of it's range. To move to a new coarse setting the user/program/magic agent must know which coarse setting to switch to and what fine setting will go with that coarse setting to continue a forward adjustment. If the coarse device has a differential non-linearity that is not much better than expected for a 64 step pot, this won't be possible.
All it needs to know is whether it needs the next coarse step to be the next one up or the next one down. It won't know how far it has moved, but it should be able to work out - once it has moved - whether it needs to make fine steps up or down to get closer to where it needs to be.
> It's no different than the problems of combining flash converters to obtain more resolution.
Not correct. All Phil needs is fine adjustment to let him eventually get close enough to where he needs to be. He doesn't care where that actually is - just that he can eventually get there.
> One converter measures the coarse bits and the other measures the fine bits. But both must be accurate to the same level to be able to combine them.
Been there, looked at that.
> There may be fancy techniques used in the flash converter situation, but they are not in play in Phil's pot setup. > > He might be able to get a little more resolution, but I don't see how it will be much unless the pots are very much under specified.
You are missing the distinction between resolution and granularity. He needs to get close enough to the right place (granularity), but he doesn't need to know exactly where it is (resolution). -- Bill Sloman, Sydney
Reminds me of using convergents for improving the resolution of PWMDAC.

For example, allowing the PWM counter's divisor to vary from 2^N - 1 to 
2^(N-1), improves the average error by several bits.

A long time ago, I computed, what was it, let me see, I think it was for a 
12-bit PWM register (max divisor 8000), when fitting to a 16-bit fractional 
input (1/65536th per unit), the RMS error was equivalent to a 20 bit 
counter -- that is, most outputs conform, within about 1/16th of an LSB, to 
the requested value.  (Not that a full and continuous 20 bit equivalent 
output is necessarily available!)

However, this was sampled over a subset of the total range (fractions 39000 
to 43000).  Over the whole range, there are of course unlucky codes (near 
small rational numbers, and especially near 0 and 1) where the error falls 
back to basic 12-bit accuracy.  For example, you simply can't have any more 
resolution than 1/4000th, 1/6000th, 1/8000th, etc., as the desired fraction 
goes towards zero.  But your only choices are 0 or 1/8000th for these 
parameters and you have to pick one.

The frequency range doesn't have to be restricted to an octave, of course, 
but we can always upscale the numerator with simple shifts, and that keeps 
the ripple, and error due to transitions, more stable.

Big downside: I'm not entirely sure how you'd match such an output to a 
signal of fixed sample rate...

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message 
news:84c330d0-8707-b234-d2f3-ec5c098e3390@electrooptical.net...
> 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 >
On Thursday, July 23, 2020 at 3:14:50 AM UTC-4, Bill Sloman wrote:
> On Thursday, July 23, 2020 at 4:51:13 PM UTC+10, Ricketty C wrote: > > On Thursday, July 23, 2020 at 1:18:30 AM UTC-4, Bill Sloman wrote: > > > On Thursday, July 23, 2020 at 12:57:35 PM UTC+10, Ricketty C wrote: > > > > On Wednesday, July 22, 2020 at 9:05:23 PM UTC-4, Phil Hobbs wrote: > > > > > On 2020-07-22 19:50, Ricketty C wrote: > > > > > > On Wednesday, July 22, 2020 at 6:43:57 PM UTC-4, Phil Hobbs wrote: > > > > > >> 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. > > > > > > > > > > > > Looks like an interesting graph, but I don't see any error bars. > > > > > > Won't the limitations of accuracy of the 6 bit dpots swamp out your > > > > > > monotonicity? > > > > > > > > > > I'm not expecting it to be monotonic, for sure. The codes are > > > > > completely scrambled, so just finding them is an issue. I'd expect the > > > > > main effect of the DNL of the dpots will be to scramble them differently > > > > > without making huge changes in the general outline of the plot. (The > > > > > sort order will change, certainly.) > > > > > > > > I think my point is more that a 64 step pot is only going to have an accuracy close to that, so that won't go away by adding resolution. Or is resolution all you are looking for? > > > > > > Clearly. > > > > > > > I guess I'm not following what this gains? You get lots more steps, but you don't know any more about where they are. What am I missing? > > > > > > The steps that correspond to the coarse digital pot changing are going to be gross, but tolerably well defined. > > > > > > The trick is to use the fine digital pot to move the final output over a range which is larger than the biggest single change caused by a one bit change in the input to the coarse pot, so that you can be sure that you have over-lapping ranges of fine adjustment. > > > > > > What Phil seems to be talking about doing is mapping all the possible outputs for any particular example of the circuit and setting up a look-up table of a monotonic string of desired outputs, and the inputs to the coarse and fine digital pots that get them. > > > > > > It is an open question whether the outputs are going to be stable enough to let this work. > > > > > > A twiddling program that knew when it was getting close to running out of range on the fine digital pot, and started over from scratch after changing input to the coarse pot might be another possibility, but it does rather depend on what Phil is actually doing. It wouldn't fit neatly into a simple control loop. > > > > For any use of this combination to be useful, there has to be a way of making changes to the combinations that map to a set of points with better error than what a single pot achieves. > > No. All you need is piece-wise monotonicity, and a control procedure that can deal with the coarse steps between the monotonic pieces.
If by "piece-wise monotonicity" you mean you need to pick your steps, you have no way to do that. Because you don't know where the thresholds are for the coarse pot, you have no idea what to set the fine pot to. If you think this can be done, you need to explain in detail how to do it. Are you talking about calibrating each part separately?
> > Unless the error of the pots at their defined values is significantly better than would be expected by the 64 step resolution that won't happen. > > Wrong. > > > Consider the case you describe. The coarse pot is at a setting and the fine pot reaches the end of it's range. To move to a new coarse setting the user/program/magic agent must know which coarse setting to switch to and what fine setting will go with that coarse setting to continue a forward adjustment. If the coarse device has a differential non-linearity that is not much better than expected for a 64 step pot, this won't be possible. > > All it needs to know is whether it needs the next coarse step to be the next one up or the next one down.
Wrong. Unlike you I explain my thinking. The problem is not setting the coarse pot, it's knowing when to change it and what to set the fine pot to so as to match the coarse pot. If the error in the coarse pot is large there is no way to know where to set the fine pot when you change the coarse pot. Coarse step of 1/64th of the range, nominally, can be 1/128th of the range or 3/128ths of the range or some such values as determined by the spec sheet. The relatively large error makes it impossible to know where to set the fine pot. It's that simple.
> It won't know how far it has moved, but it should be able to work out - once it has moved - whether it needs to make fine steps up or down to get closer to where it needs to be.
You mean make the coarse step and let your controller algorithm search for the right fine setting??? That means in the mean time the system is out of adjustment which is the point of using the fine adjustment. So that idea is a fail.
> > It's no different than the problems of combining flash converters to obtain more resolution. > > Not correct. All Phil needs is fine adjustment to let him eventually get close enough to where he needs to be. He doesn't care where that actually is - just that he can eventually get there.
Eventually? So this is not a real time control loop trying to maintain some parameter? This is just some setting he wishes to set once and forget?
> > One converter measures the coarse bits and the other measures the fine bits. But both must be accurate to the same level to be able to combine them. > > Been there, looked at that. > > > There may be fancy techniques used in the flash converter situation, but they are not in play in Phil's pot setup. > > > > He might be able to get a little more resolution, but I don't see how it will be much unless the pots are very much under specified. > > You are missing the distinction between resolution and granularity. > > He needs to get close enough to the right place (granularity), but he doesn't need to know exactly where it is (resolution).
I think you need to check the dictionary. lol -- Rick C. -+ Get 1,000 miles of free Supercharging -+ Tesla referral code - https://ts.la/richard11209
>I guess I'm not following what this gains? &nbsp;You get lots more steps, but >you don't know any more about where they are. &nbsp;What am I missing?
What it gets me is a combination of settability and bandwidth that I can't get in any single part at any price. I'll dump in a bit of pseudorandom DNL and see if that causes any issues. I don't expect it to--it'll get homogenized much like the regular steps. For an iterative adjustment (think LC filter tuning) I can get close by setting the fine pot to 0, adjusting the coarse pot, then walking the two together to find the optimum. Near full scale the possible search range is wider, but I don't think it'll take too long provided that I'm willing to settle for "good enough" rather than a global optimum. There are two mildly-interacting adjustments, but the response surface is simple--no local minima. It's for taking out the effect of extrinsic emitter resistance in the BJT diff pair that's the heart of the circuit. It works by applying a signal to one of the bases that cancels out the effective emitter degeneration and so restores the desired Ebers-Moll behaviour at higher photocurrents. If I feel like getting fancy, once I've found the right neighbourhood I can give the turd a final polish to get a bit closer. ;) There are non-negligible effects such as the tempco of wiper resistance that limit how good this approach can be. Sure beats a trimpot on a motor though! Cheers Phil Hobbs