Interesting TIA design
in [Design]
|
From: Phil Hobbs on 24 Oct 2009 14:02 Joerg wrote: > Phil Hobbs wrote: >> I just finished up an interesting new (to me) TIA design for a Far >> Eastern customer. (There's no NDA, so I can talk about it.) It made a >> pretty interesting study. >> >> It works up to about 30 uA photocurrent, but manages to be shot >> noise limited above 20 nA at 400 kHz and 100 nA at 1 MHz with 30 pF of >> photodiode capacitance, which is a factor of about 20 better than I >> originally expected. (A narrower current range device can be shot >> noise limited at 20 nA at 1 MHz.) >> >> It uses a pair of parallelled BF862s in a bootstrapped bootstrap, >> connected to another pair in a cascoded common-source configuration. >> It avoids the 300 kelvin resistor noise by having a main signal path >> with no resistors! It uses capacitive feedback on the common-source >> stage and a differentiator going into the second stage--really an odd >> design, but it works great. (Bootstrapping the drains of the >> bootstrap FETs was good for 3 dB of SNR, interestingly.) LTSPICE said >> I could get a bit better performance by running the bootstrap a bit >> above IDSS, but I wasn't that brave. >> > > As John said, be brazen there, go for it. Just like we used to run power > amp tubs with plenty of control grid current in the old days. I've never > made a grid glow but I did run plates well past cherry-red. "When men were real men, women were real women, and small furry creatures from Alpha Centauri were real small furry creatures from Alpha Centauri." >> It's for a relatively narrowband application, so in principle it could >> run at lower frequency, except that it has to work around fluorescent >> lights. >> >> Electronic ballast fluorescents produce not only EMI but also strongly >> modulated light. With a 40 kHz switching frequency the harmonics go >> up to above 1 MHz. The phosphor isn't fast enough to do that, of >> course, but there are mercury and argon emission lines all over the >> place, and the plasma responds pretty fast, especially near the ends >> of the tube, so you have to pick an operating wavelength that avoids >> the emission lines. 940 nm is pretty good, fortunately--there are >> lines at 870 and 1010 nm that are inconveniently close, but relatively >> weak. >> >> What I wound up with was a glorified AM radio--the above-mentioned TIA >> stage running into a 74VHC4053 analogue mux connected as a double >> balanced mixer, with a 1455 kHz LO and two 455 kHz ceramic IF filters, >> cascaded so as to increase the stopband attenuation. >> >> Besides the TIA, the two most challenging parts were: >> (1) Achieving a 1% linearity spec over the full range, including the >> gain error from a 1x-128x programmable gain amp. Doing a built-up PGA >> with good accuracy at 1 MHz is surprisingly hard with jellybean parts, >> I discover--the on-resistance specs of muxes are the pits. (2N7002s >> to the rescue.) >> > > If you do a low-volume (meaning non-consumer) design check out canned > PGAs. One of my favorites is the AD603. They raised the price a couple > of bucks over the years but it is a great chip. Nice part. It won't make the 1% accuracy spec, though. > > Another option is to get a chip with at least two uncommitted > N-channcels on there, such as the CD4007 (cheap). Unbuffered logic can > work as well, even cheaper. Now servo them by running one of them in a > current source with opamp feedback. So your DAC commands a voltage and > this sets the exact Rdson because it fudges the gate until that > condition is met. The other FET that is part of your RF divider will > follow, as long as this is monolithic and not a multi-chip part. But > even the Rohm multi-chips are pretty good because the devices usually > come off the same wafer. No guarantees with multi-chip, of course. That is a truly gnarly idea. One for the tricks file. > > >> (2) Getting rid of the DC and low frequency photocurrent without using >> resistors. I made a BJT current sink degenerated with 4 diodes in >> series with the emitter. Using N diodes reduces the transconductance >> by a factor of N+1, and increases the equivalent shot noise voltage by >> sqrt(N+1), so the resulting current sink runs 10*log(N+1) dB lower >> than full shot noise--7 dB for 4 diodes. So far, so vanilla. The >> problem was that the capacitance of the diodes dominated at low >> photocurrents, which essentially doubled the 1 MHz noise at 40 nA. I >> used BFT25A C-B junctions for the diodes, but that was the best I >> could get. I'd love to figure out how to make fast, wide range, >> sub-Poissonian current sources below 100 nA. >> > > No chance to chop DC and LF the old-fashioned way, with inductors and > capacitors? Unfortunately not at these impedance levels. I'd need a 10 H inductor. > > >> So I learned something about a corner of the design space that I'd >> never worked in before, which I thought was pretty neat. >> > > Always nice to enter a new field. I just entered that of injectors and > high pressures. It does command some respect, if something flies off in > a 4500 psi situation things can get ugly. My son has a 4500 PSI carbon fibre air tank for his paintball gun. I explained the importance of keeping it in one piece, but I'm not sure I made an impression. Cutting a broomstick would do it. 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: Jamie on 24 Oct 2009 16:34 Phil Hobbs wrote: > I just finished up an interesting new (to me) TIA design for a Far > Eastern customer. (There's no NDA, so I can talk about it.) It made a > pretty interesting study. > > It works up to about 30 uA photocurrent, but manages to be shot noise > limited above 20 nA at 400 kHz and 100 nA at 1 MHz with 30 pF of > photodiode capacitance, which is a factor of about 20 better than I > originally expected. (A narrower current range device can be shot noise > limited at 20 nA at 1 MHz.) > > It uses a pair of parallelled BF862s in a bootstrapped bootstrap, > connected to another pair in a cascoded common-source configuration. It > avoids the 300 kelvin resistor noise by having a main signal path with > no resistors! It uses capacitive feedback on the common-source stage > and a differentiator going into the second stage--really an odd design, > but it works great. (Bootstrapping the drains of the bootstrap FETs was > good for 3 dB of SNR, interestingly.) LTSPICE said I could get a bit > better performance by running the bootstrap a bit above IDSS, but I > wasn't that brave. > > It's for a relatively narrowband application, so in principle it could > run at lower frequency, except that it has to work around fluorescent > lights. > > Electronic ballast fluorescents produce not only EMI but also strongly > modulated light. With a 40 kHz switching frequency the harmonics go up > to above 1 MHz. The phosphor isn't fast enough to do that, of course, > but there are mercury and argon emission lines all over the place, and > the plasma responds pretty fast, especially near the ends of the tube, > so you have to pick an operating wavelength that avoids the emission > lines. 940 nm is pretty good, fortunately--there are lines at 870 and > 1010 nm that are inconveniently close, but relatively weak. > > What I wound up with was a glorified AM radio--the above-mentioned TIA > stage running into a 74VHC4053 analogue mux connected as a double > balanced mixer, with a 1455 kHz LO and two 455 kHz ceramic IF filters, > cascaded so as to increase the stopband attenuation. > > Besides the TIA, the two most challenging parts were: > (1) Achieving a 1% linearity spec over the full range, including the > gain error from a 1x-128x programmable gain amp. Doing a built-up PGA > with good accuracy at 1 MHz is surprisingly hard with jellybean parts, I > discover--the on-resistance specs of muxes are the pits. (2N7002s to > the rescue.) Yeah, got a whole draw full of those FETS, nice components to have about. > (2) Getting rid of the DC and low frequency photocurrent without using > resistors. I made a BJT current sink degenerated with 4 diodes in > series with the emitter. Using N diodes reduces the transconductance by > a factor of N+1, and increases the equivalent shot noise voltage by > sqrt(N+1), so the resulting current sink runs 10*log(N+1) dB lower than > full shot noise--7 dB for 4 diodes. So far, so vanilla. The problem > was that the capacitance of the diodes dominated at low photocurrents, > which essentially doubled the 1 MHz noise at 40 nA. I used BFT25A C-B > junctions for the diodes, but that was the best I could get. I'd love > to figure out how to make fast, wide range, sub-Poissonian current > sources below 100 nA. I would think you being critical of noise, wouldn't using diodes in such a manner introduce some Johnson noise ? Just curious, maybe it don't have any bases here. > > So I learned something about a corner of the design space that I'd never > worked in before, which I thought was pretty neat. > > Cheers > > Phil Hobbs >
From: Phil Hobbs on 24 Oct 2009 16:39 Jamie wrote: > Phil Hobbs wrote: >> (2) Getting rid of the DC and low frequency photocurrent without using >> resistors. I made a BJT current sink degenerated with 4 diodes in >> series with the emitter. Using N diodes reduces the transconductance >> by a factor of N+1, and increases the equivalent shot noise voltage by >> sqrt(N+1), so the resulting current sink runs 10*log(N+1) dB lower >> than full shot noise--7 dB for 4 diodes. So far, so vanilla. The >> problem was that the capacitance of the diodes dominated at low >> photocurrents, which essentially doubled the 1 MHz noise at 40 nA. I >> used BFT25A C-B junctions for the diodes, but that was the best I >> could get. I'd love to figure out how to make fast, wide range, >> sub-Poissonian current sources below 100 nA. > > I would think you being critical of noise, wouldn't using diodes in > such a manner introduce some Johnson noise ? > Just curious, maybe it don't have any bases here. >> At any given current, diode degeneration is quite a bit noisier than resistor degeneration with the same voltage drop. The kicker is that if you want it to work over a wide current range, diodes are better than shot noise everyplace and a resistor isn't. 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: Nico Coesel on 24 Oct 2009 18:35 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 24 Oct 2009 09:48:32 -0700, Joerg <invalid(a)invalid.invalid> >wrote: > >> >>Always nice to enter a new field. I just entered that of injectors and >>high pressures. It does command some respect, if something flies off in >>a 4500 psi situation things can get ugly. > >Pinhole leaks, like from bad welds or defective hoses, can make >invisible fluid jets that cut like a sword, or inject hydraulic fluid >like a hypodermic. Bad stuff. Really bad indeed. Someone once told me a story about a farmer putting his hand on a oil leak. The oil came out of his elbow. The farmer lost his arm and his farm. A few years ago I sollicited for a job at a company that does airplane simulators and so on. Huge amounts of hydraulics. Hoses just running on the floor, no shielding whatsover. Way too dangerous. I declined the job for that and other reasons. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" --------------------------------------------------------------
From: Joerg on 24 Oct 2009 20:12
Phil Hobbs wrote: > Joerg wrote: >> Phil Hobbs wrote: [...] >>> Besides the TIA, the two most challenging parts were: >>> (1) Achieving a 1% linearity spec over the full range, including the >>> gain error from a 1x-128x programmable gain amp. Doing a built-up >>> PGA with good accuracy at 1 MHz is surprisingly hard with jellybean >>> parts, I discover--the on-resistance specs of muxes are the pits. >>> (2N7002s to the rescue.) >>> >> >> If you do a low-volume (meaning non-consumer) design check out canned >> PGAs. One of my favorites is the AD603. They raised the price a couple >> of bucks over the years but it is a great chip. > > Nice part. It won't make the 1% accuracy spec, though. > Well, you need to linearize it a bit or servo it with an out-of-band pilot tone. But for a mass product it ain't the ticket, demasiados Dolares. >> >> Another option is to get a chip with at least two uncommitted >> N-channcels on there, such as the CD4007 (cheap). Unbuffered logic can >> work as well, even cheaper. Now servo them by running one of them in a >> current source with opamp feedback. So your DAC commands a voltage and >> this sets the exact Rdson because it fudges the gate until that >> condition is met. The other FET that is part of your RF divider will >> follow, as long as this is monolithic and not a multi-chip part. But >> even the Rohm multi-chips are pretty good because the devices usually >> come off the same wafer. No guarantees with multi-chip, of course. > > That is a truly gnarly idea. One for the tricks file. > You can also use this trick with RF dual N-channels or fast arrays for super-fine delay tuning. First time I did that it was met with some disgust in the design review but worked like a champ. It made a TMS320 almost unemployed and obsoleted some expensive and terribly noisy digital delay lines. Long story short they could have never made it work reliably with those delay lines. >> >> >>> (2) Getting rid of the DC and low frequency photocurrent without >>> using resistors. I made a BJT current sink degenerated with 4 diodes >>> in series with the emitter. Using N diodes reduces the >>> transconductance by a factor of N+1, and increases the equivalent >>> shot noise voltage by sqrt(N+1), so the resulting current sink runs >>> 10*log(N+1) dB lower than full shot noise--7 dB for 4 diodes. So >>> far, so vanilla. The problem was that the capacitance of the diodes >>> dominated at low photocurrents, which essentially doubled the 1 MHz >>> noise at 40 nA. I used BFT25A C-B junctions for the diodes, but that >>> was the best I could get. I'd love to figure out how to make fast, >>> wide range, sub-Poissonian current sources below 100 nA. >>> >> >> No chance to chop DC and LF the old-fashioned way, with inductors and >> capacitors? > > Unfortunately not at these impedance levels. I'd need a 10 H inductor. Jim would build you a gyrator on silicon :-) >> >> >>> So I learned something about a corner of the design space that I'd >>> never worked in before, which I thought was pretty neat. >>> >> >> Always nice to enter a new field. I just entered that of injectors and >> high pressures. It does command some respect, if something flies off >> in a 4500 psi situation things can get ugly. > > My son has a 4500 PSI carbon fibre air tank for his paintball gun. I > explained the importance of keeping it in one piece, but I'm not sure I > made an impression. Cutting a broomstick would do it. > Be careful. A woman from our village was killed by a paintball tank that flew off. Hit her in the head, she slumped into the arms of her husband and was dead within seconds. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |