Re: Low noise current source
- From: Phil Hobbs <pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx>
- Date: Fri, 07 Aug 2009 00:32:37 -0400
MooseFET wrote:
On Aug 6, 5:15 pm, Phil Hobbs <pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx>Yep. BJTs are really good, so above audio frequencies it's usually best to leave them alone to get on with the job.
wrote:
MooseFET wrote:On Aug 6, 7:53 am, John LarkinWith a BJT running barefoot, you'll get full shot noise. For 500 mA,
<jjSNIPlar...@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 6 Aug 2009 06:59:11 -0700 (PDT), MooseFET <kensm...@xxxxxxxxx>The current is 0 to 500mA. The noise needs to be down by about 10^8
wrote:
The circuit I have is basically:What are the numbers?
U1
Vref----!+\ R1 !!-------- Current out
! >--+-/\/--!! N MOSFET
--!-/ ! !!--
! C1=== !
! ! R2 ! R3
--------+---/\/\---+---/\/\--- GND
The noise current of R3 is the one I can't avoid. U1 is a lowish
noise op-amp and R1 and R2 are low enough that they are below its
noise. Other than increasing R3 and the voltage drop on it, is there
any way to reduce the noise? I am looking for a factor of something
near 10.
The Vref is very well filtered because I couldn't find a reference
with a noise number anywhere near that of the op-amp etc.
or 9 from there.
that gives you a 1-Hz SNR of I/(2e) = 181 dB. That's in the range of
the better op amps. If you want N times less than full shot noise, you
have to drop about N*kT/e in the emitter resistance, and choose an
amplifier whose noise is smaller than the Johnson noise of that resistor.
Basically, are you suggesting a BJT with a base voltage and an emitter
resistor?
If the emitter resistor drops N times kT/e and the base voltage is constant, ideally your output current noise is 20 log N dB below shot noise. This performance is limited mainly by the Johnson noise of the extrinsic base resistance Rbb' and the shot noise of the base current, so you want a high beta or else some scheme to return the base current to the collector. (Don't use a Darlington, or you'll be limited by the noise of the driver stage.) Parallelling smaller devices can help both beta and Rbb', and that will also distribute the dissipation among several emitter resistors.
Thermal drift will be a serious worry, but if you use a cascode you can reduce that a lot by reducing the active device's dissipation and keeping V_CE nearly constant. The cascode contributes its own base current shot noise but is otherwise completely benign.
There will still be a thermal settling time after you change the current. For small changes, biasing at the maximum power point helps, but it's hard to make it work over 0-500 mA!
In a laser noise canceller design, I once tried using a fairly clever feedback loop that adjusted V_CE to keep the device dissipation exactly constant. The dissipation was constant, all right, but unfortunately the current error caused by the Early effect was almost as big as the drift I was trying to get rid of!
Since you have an op amp looking after the DC performance, you might be able to make something like that work in this case. (Of course if you really need to go down to zero current, you'll need a fair amount of voltage headroom.) ;)
You'll probably at least want to make sure that the total dissipation on the heat sink is constant--besides speeding up the thermal settling a lot, that'll help keep the temperature drifts of everything else in the box under control. Thermocouple offsets are a bear at this level.
Getting the current to be sub-Poissonian at the bottom of the range might need range switching, or else degeneration by diodes instead of a resistor. The good news is that diode-connected transistors work over the full output range, and have a lower noise temperature (Tj/2). The bad news is that you effectively only get kT/e worth of degeneration per Vbe of drop, so (taking the lower T_noise into account) they wind up taking about 15 times more voltage drop for the same noise reduction.
It's usually the low, low baseband where the trouble lies. Also watch out for the capacitor tempcos (see below).Most voltage references are dramatically noisier than transistors, and
BJTs tend to be quite a lot quieter than MOSFETs, especially at low
frequency.
I have the reference very seriously filtered. Since it is a constant
voltage, the time constant here can be very long.
A quiet reference can make a big difference. For instance, at low
frequency the LM329 is about 10 dB quieter than just about any other
reference out there (~1.2 uV vs. 4-5 uV, p-p), including the fancy
Intersil floating gate ones. (It does drift with temperature, so you
have to figure out something for that problem.)
I am using the LT1021 with a two pole filter. Unfortunately
temperature drift looks a lot like low frequency noise to my
application. I need it to hold still for minutes.
The LT1021 is a nice part but I'm a bit worried by the low frequency noise spec--they use 0.1 Hz as the LF cutoff, vs 0.01 Hz for the LM329, and the noise is a bit worse even so, at least going by the plots. There are also the Intersil floating gate parts (they can source and sink current, so you can stack them to get higher SNR), and the expensive but very good LTZ1000. It's a pity that the LM329's TC is 50 ppm/K. Dirty old National Semi discontinued the LM399, which was the bee's knees for that sort of thing. (*)
If holding still during a measurement is more important than absolute accuracy, you can just charge up a huge metallized plastic capacitor periodically from your reference via an analog switch, and hang a good quality op amp buffer on it. Since the charge on the cap is fixed, that'll be limited by the tempco of the dielectric constant, so it isn't a complete solution either, but the thermal diffusivity of plastic is so very low that it takes a physically big cap a long time to drift much. Polyester and polypropylene have opposite TCs around room temperature, so you could get a first-order temperature compensation by putting two in parallel--probably good for a factor of 10 without trimming, once you've characterized them.
Temperature control will be your friend here. The one major second-order imponderable in the use of BJTs for this is the temperature coefficient of beta, and at some point it's just easier to keep T from varying than to keep dialling in more tweaks.
Cheers
Phil Hobbs
(*)I wish someone would build little temperature controlled modules with a few LM329s and a bunch of 0603 resistors and SOT23 transistors sitting on a 2-oz copper ground plane, and one of Jan's little PICs adjusting all the dissipations to keep the temperature fixed and the first and second order gradient terms zero. That would make an amazing reference and still be a lot cheaper than an LTZ1000.
--
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
.
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