I've done that before. One issue is that the chopamp spikes are mostly current noise, so you have to bypass the chopamp inputs to keep the spikes out of the rest of the circuit.
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 https://hobbs-eo.com
You can look at it either way. Inside the op amp loop's control bandwidth, the voltage across the emitter resistor is held still by overall feedback, so regardless of the noise voltage drop across the base resistor, all the 1/f noise has to come out the collector.
Outside that bandwidth, there's a narrow region where the base resistors contribute, but then the bypass caps take over and hold the voltage across the emitter resistance constant again. So basically the noise comes out the collector at all frequencies as long as the emitter resistor is dropping several times kT/e.
Didn't help, for reasons given above.
Using an MPS4250 Darlington helped a lot with the 1/f noise, but the flatband noise is now only about 6 dB below shot noise, which is a pity. Good enough for this rev, though. Now I just need to find a suitable Darlington in SOT89 or SOT223.
Next time round I'll use another of the power devices as the input transistor, and add some extra base current to it, but not enough for its 1/f noise to dominate.
-- 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 https://hobbs-eo.com
I don't know. If the 1/f noise is an areal effect, chopping the die up into smaller ones wouldn't make much difference, other things being equal. If it's a perimeter effect, parallelling devices would make it worse. It would need to be some sort of collective effect for a single die to be worse. I can't think of a mechanism for that offhand, but that doesn't mean it's impossible.
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 https://hobbs-eo.com
Right. The input bias current specs are all DC!
I've seen weird voltage offsets, beyond the spec limits, that depend on the capacitances that the inputs see. CMOS analog switches have very interesting dynamics. I'm sort of grateful that your problem is not just now my problem!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Yeah, be that way. ;)
An MPS4250 tacked onto the ZXTP25020 knocked it down pretty well, so I'll probably use that plus a bypass from the big guy's base to the supply to try to recover the quiet HF performance. Should work as long as the 4250 isn't running enough current to actually oscillate, but of course it'll dump the full shot noise of the 4250's I_C into the diode laser. That shouldn't be a gigantic issue.
Hopefully the noise will be okay for Customer #1, because Customer #2's board is quite different and hasn't been fabbed yet.
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 https://hobbs-eo.com
Thanks, I thought that, but it's good to check. I use the (pfet) VP2106, I < = 100mA. I never looked at the noise closely. the loose spec we quote is 50 nA (in 100 kHz?) which is .. ~20X the shot noise.
George H.
Driving home.. you know I did measure the noise once. I had two lasers running. I locked one to the edge of an atomic absorption line, (sharp edge). Tuned the other to the same spot.. but left it free running. sent them both into a ~50/50 beam splitter,(mixed) and onto a fast photodiode, I'd get a nice beat note ( ~10 MHz) the 'scope FFT would give a BW of ~100 kHz.. maybe 200, mostly instrument limited.
100 kHz out of ~3.8x 10^14 Hz (~780 nm) ~is 10^-9 territory. There's some factor relating current to wavelength, I'll have to look it up.Anyway the beat note was intermittent, and would disappear with bumps and other disturbances, I wonder if 1/f current noise was part of the problem.... (This would have been in pfet's.)
George H.
Turned out that most of the 1/f noise was due to the HP 50-ohm termination I was using as a test load--which turned out to be made of cermet--plus the temperature coefficient of the leakage of a 22-uF 50V aluminum polymer cap I was using to AC-couple the noise measurement amplifier.
I found this out by replacing the laser driver with an HP precision power supply (set to 80V) and a 2k wirewound resistor.
The two effects were almost the same size at the lower frequency limit of my measurement, so nothing I did changed it much, until I fixed both at once.
The 1/f corner is now down around 20 Hz where it belongs, and the box is way sub-poissonian outside the op amp bandwidth.
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 https://hobbs-eo.com
You should have caught that long ago when I asked if you had run a noise vs device parameter simulation in LTspice ?>:-} ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
Eggcellent. (Thanks for sharing.) When looking at noise, I made the same mistake twice! And only found it when I went back to my notebook.. saying to myself, "I did this two years ago and it worked.." Big dope slap.
So what do you use for a coupling cap? And do you guesstimate the 20 Hz corner.. (since 50 ohms and 22 uF is 1^10-3 sec, ~100 Hz.)
George h.
The noise amp is an LT1028 (1985 date code, PDIP) running noninverting gain of 100. It has a 10 ohm/990 ohm feedback network, and on the noninverting input I have 10k to ground and the aforementioned 22 uF alpo. IIRC JL said that alpos get less leaky after being run for awhile at highish voltage.
When I come to build the real test setup for these (assuming we sell any) I'll use a CPH3910 FET buffer running directly into an ADA4898 plus buffer. That way I can use a big resistor and a film cap for the AC coupling, and not have to deal with the 300 kHz noise peak of the LT1028. Might even do an analogue cross-correlation measurement.
I really need to make a bunch of amp/filter boards, because I'm continually doing these sorts of measurements. If I got 20 of them made, I could build them into test setups.
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
Alpos? Like the dog food? I like that.
I have some leakage measurements, in both directions, if anyone is interested.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Of course, sorry I wasn't thinking clearly. We've got bag fulls of
10 uF panasonic film caps I use for such stuff.Oh I've always wanted to do that, I don't really have anything useful to look at.
I'm always hacking up old pcb's for the next project. I've got a drawer full of 'not quite right' pcb's. (I'm the opposite of JL, each board may take several spins.)
George H.
99-44/100% pure dog food, or something like that. ;)
I saved the ones you posted earlier, but any other data are welcome!
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
I got thanked once for suggesting that someone replace an LT1028 with an AD797, which does seem to be a better behaved part.
It's a long time ago - about twenty years - so I can't remember exactly why it was better in the application, or what the application was.
Element14 (Newark) still seems to stock it, and while it's not cheap - around $10 - it's not prohibitively expensive. Apparently it is made with the their advanced complementary bipolar process (whatever that is) which offers decent PNP transistors.
-- Bill Sloman, Sydney
As done this way...
Note that the individual contributions from each noise source, resistor, device noise, whatever, are tabulated in the .OUT file, where it'd be a piece-a-cake to spot the extra noise... had you included the termination in your simulation in the first place ;-)
Oooooops! LTspice doesn't have a .OUT file!! Nor does it have a .NET file. Sonofa gun!
Wonder why that may be?
Maybe to hide the idealized components Mikey replaces your real components with to enhance simulation speed?
But Hey!!! LTspice is FREE ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
As done this way...
I have a noise report in SS where I order the noise of each component and state its % to the total.
-- Kevin Aylward
"%" would be nice... would make the noise report quicker to scan for "culprits".
What always surprises me, no matter how many times I've done it, is the "culprit" is something _other_ than anticipated ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
It's what you learn, after you know it all, that counts.
It does have netfiles, and stuff you print gets put in the log file.
But none of that works if the model doesn't include the noise correctly. For instance, the ADA4898 model from AD overestimates the 1/f noise by some orders of magnitude, which is irritating. The 4899 model is much more realistic, but that one is ten times as fast and only runs on +-5V.
And in the present case, I would have needed a correct model of the random 30-year-old HP termination I was initially using as the load. ;)
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
Oscillator phase noise is somewhat bizarre. It acts like FM capture effect. You can have a noise plot, find and eliminate a dominant noise noise, yet still have the "improved" topology have an essentially overlaid new phase plot. A new noise source just pops up as the dominant, and the noise don't change.
Kevin Aylward
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