Multiple current mirrors
in [Design]
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From: C Egernet on 5 Jan 2010 10:50 Gentlemen, I'm working on the concept for an instrument that needs to take a number of current sources (photodiodes, 44 off) and find the ratio between each of the 43 currents with the 44th (the largest). The currents are in the range of, say, 100 nA to 10 uA. Electronics design is a bit out of my skill set but I'd like to give it a try. I could try to A/D-convert as soon as possible and divide digitally but I suspect that it is more sound to do the following: Mirror the 44th current with BJTs and use Hobbs' laser noise canceller to form the differences of the logarithms of the currents and A/D- convert these. Now, my question is: is it at all feasible to replicate a current so many times with any kind of bandwidth and precision? I will have to do this with discrete components so matching is going to be a problem. I anticipate using small signal RF transistors to get a reasonable beta, and having a calibration procedure to get rid of remaining errors. That said, I don't want calibrations to be a crutch for doing things wrong in the first place. The measurement bandwidth is going to be quite low (not fixed yet but probably less than 200 Hz) but it feels right to do the ratioing out to a few MHz. Comments and suggestions are most welcome Chris Egernet
From: Phil Hobbs on 5 Jan 2010 14:37 On 1/5/2010 10:50 AM, C Egernet wrote: > Gentlemen, > > I'm working on the concept for an instrument that needs to take a > number of current sources (photodiodes, 44 off) and find the ratio > between each of the 43 currents with the 44th (the largest). The > currents are in the range of, say, 100 nA to 10 uA. > > Electronics design is a bit out of my skill set but I'd like to give > it a try. > > I could try to A/D-convert as soon as possible and divide digitally > but I suspect that it is more sound to do the following: > > Mirror the 44th current with BJTs and use Hobbs' laser noise canceller > to form the differences of the logarithms of the currents and A/D- > convert these. > > Now, my question is: is it at all feasible to replicate a current so > many times with any kind of bandwidth and precision? > > I will have to do this with discrete components so matching is going > to be a problem. I anticipate using small signal RF transistors to get > a reasonable beta, and having a calibration procedure to get rid of > remaining errors. > > That said, I don't want calibrations to be a crutch for doing things > wrong in the first place. > > The measurement bandwidth is going to be quite low (not fixed yet but > probably less than 200 Hz) but it feels right to do the ratioing out > to a few MHz. > > > Comments and suggestions are most welcome > > Chris Egernet 43 noise cancellers in one box? Cool. Noise cancelling nanoamps isn't impossible, but it's hard to do well. Manyfold current mirroring at megahertz bandwidths with only 100 nA is going to be very tough. I'd suggest using a TIA plus and 43 biggish resistors--big enough to drop a few volts at your comparison photocurrent value--each one driving the emitter of a small RF BJT such as a BFT25A, whose collectors goe to the noise canceller diff pairs. Putting a similar BJT in series with the feedback resistor of the TIA will get you a nice first-order temperature compensation and reduce the nonlinearity. Keep the transistor dissipation down to something reasonable--well below 1 mW--so that the temperature tracking doesn't get screwed up. Also, make sure you use a diode-connected transistor (CB shorted) for the compensation, because the BE diode doesn't have the same characteristics as the transistor in normal bias. As far as the noise cancellers go, the bandwidth of the integrating loop needs to be wide enough to cover all the important modulation of the photocurrent. This is because of the pronounced nonlinearity of the logarithmic Ic vs Vbe characteristic of the BJTs. If you don't need to be too close to the shot noise, you can use an LM13700 instead of the diff pair--by using the linearizing diodes (see the data sheet), you can get rid of the loop nonlinearity and have a straight ratio output rather than a log ratio, which reduces the effect by a lot. That'll cost you probably 8-10 dB in SNR, but you may not mind that--you'll still be within shouting distance of the shot noise. It's possible to do this other ways and get better performance, but it takes real work. (I've been designing a follow-on to the original noise canceller over the last 6 months or so, so I've been reminded of it.) The temperature tracking of the mirror devices is the biggie. If you keep their Ic and Vce the same, and mount them identically, it will help, and of course you aren't going to get much heating down at a few microwatts of dissipation. If you don't need to noise-cancel all the photocurrents them at once, you might want to look at multiplexing them into a single noise canceller. The MAT04 that I used in my published circuits was discontinued earlier this year, unfortunately. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: C Egernet on 5 Jan 2010 17:38 On Jan 5, 8:37 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > 43 noise cancellers in one box? Cool. When I first read about the noise canceller idea, I knew that I had my hammer. I just needed to find a suitable nail. > Noise cancelling nanoamps isn't impossible, but it's hard to do well. > Manyfold current mirroring at megahertz bandwidths with only 100 nA is > going to be very tough. In fairness, I would mirror the several uA current but still. Noise cancelling in a much wider bandwidth than I plan to measure in is maybe also the wrong approach? It is essentially a DC signal I am after. > I'd suggest using a TIA plus and 43 biggish resistors This is a simple, idea even I can understand. Seems to make a lot of sense. > Putting a similar BJT in series with the feedback resistor of the TIA > will get you a nice first-order temperature compensation and reduce the > nonlinearity. The temperature compensation sounds very neat, however, which nonlinearity are you referring to? > ... you can get rid of the loop nonlinearity and have a straight > ratio output rather than a log ratio, which reduces the effect by a lot. But getting a linear signal rather than a logarithmic signal will move problems to the A/D-conversion. I am just as interested in knowing whether the signal from the 23rd photodiode, say, is 1% or 1.01% as I am in knowing whether the 37th photodiode gives 100% or 101% of the reference. > The temperature tracking of the mirror devices is the biggie. If you > keep their Ic and Vce the same, and mount them identically, it will > help, and of course you aren't going to get much heating down at a few > microwatts of dissipation. Exactly. And the instrument is going to live in a carefully air conditioned environment with plenty of airflow. I plan to keep power dissipation down. Best regards, Chris Egernet
From: Phil Hobbs on 5 Jan 2010 17:53 On 1/5/2010 5:38 PM, C Egernet wrote: > On Jan 5, 8:37 pm, Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> > wrote: >> 43 noise cancellers in one box? Cool. > > When I first read about the noise canceller idea, I knew that I had my > hammer. > I just needed to find a suitable nail. > >> Noise cancelling nanoamps isn't impossible, but it's hard to do well. >> Manyfold current mirroring at megahertz bandwidths with only 100 nA is >> going to be very tough. > > In fairness, I would mirror the several uA current but still. Noise > cancelling > in a much wider bandwidth than I plan to measure in is maybe also the > wrong approach? It is essentially a DC signal I am after. > >> I'd suggest using a TIA plus and 43 biggish resistors > > This is a simple, idea even I can understand. Seems to make a lot of > sense. > >> Putting a similar BJT in series with the feedback resistor of the TIA >> will get you a nice first-order temperature compensation and reduce the >> nonlinearity. > > The temperature compensation sounds very neat, however, which > nonlinearity > are you referring to? The nonlinear emitter impedance and temperature drift of the common-base device that you need between the resistor and the emitters of the noise canceller's diff pair. The feedback network of the TIA would be a resistor plus a diode connected transistor, which would match those of each of the split outputs. There would be some small offset voltage if the currents weren't identical in the feedback and output branches, but if you're really making 43 1:1 mirrors with 44 identical resistors and 44 transistors, they'd match very well, as long as you're dropping a few volts in the resistors. > >> ... you can get rid of the loop nonlinearity and have a straight >> ratio output rather than a log ratio, which reduces the effect by a lot. > > But getting a linear signal rather than a logarithmic signal will > move > problems to the A/D-conversion. I am just as interested in knowing > whether the signal from the 23rd photodiode, say, is 1% or 1.01% > as I am in knowing whether the 37th photodiode gives 100% or 101% > of the reference. Terrific. Most folks don't like the log output because the gain depends on the signal level, but your application is very good. Watch out for base current errors, though--you need transistors with really good beta linearity for that job. Have a look at the HFA3046 array. > >> The temperature tracking of the mirror devices is the biggie. If you >> keep their Ic and Vce the same, and mount them identically, it will >> help, and of course you aren't going to get much heating down at a few >> microwatts of dissipation. > > Exactly. And the instrument is going to live in a carefully air > conditioned > environment with plenty of airflow. I plan to keep power dissipation > down. > Lots of airflow is going to produce a certain amount of low frequency noise, from microphonics and temperature variations due to turbulence. I'd want to put a bit of insulation around the transistors--a 1 mK fluctuation will give you 2 uV of drift, which is ~100 ppm in collector current--not particularly subtle. Please keep me posted on how it's going--that's the largest noise canceller instrument I've heard of. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: C Egernet on 6 Jan 2010 10:38
> The nonlinear emitter impedance and temperature drift of the common-base > device that you need between the resistor and the emitters of the noise > canceller's diff pair. Okay, so it is the action of the common-base transistor I don't understand: The TIA will convert the comparison photodiode current into a voltage which is shared among the 43 noise cancellers, right? The large-ish resistors will then convert that voltage into currents for each noise canceller, right? So what does the common-base BJT do? Act as a current-follower, pinning the voltage of the emitters of the differential pair? Sorry for asking elementary questions, but I'd very much like to understand this clearly. > Lots of airflow is going to produce a certain amount of low frequency > noise, from microphonics and temperature variations due to turbulence. > I'd want to put a bit of insulation around the transistors--a 1 mK > fluctuation will give you 2 uV of drift, which is ~100 ppm in collector > current--not particularly subtle. It's good that you mention it. It seems that in physical science, it is always easy to make a thermometer. The hard part is making something that is either _not_ a thermometer or _only_ a thermometer. > Please keep me posted on how it's going--that's the largest noise > canceller instrument I've heard of. So far I am only toying with the concept so I am increasingly convinced that this is the way to go. Best regards, Chris Egernet |