If you hang an emitter follower on the LED, the tempcos cancel out to leading order, so you can make a pretty reasonable voltage reference--much quieter than a bandgap, though less accurate.
It's pretty unlikely for a LED to detect more than a microamp in any indoor setting, but it could be as much as 10 uA in sunlight.
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
What you mean is that means you're trying to post a binary (encoded as text) to a text-only (that's plain text) newsgroup, which is a big no no.
Just stop your silly news reader from wrapping long lines, simple!
Grant.
-- http://bugs.id.au/
Yes :)
Grant.
-- http://bugs.id.au/
Shoot, I wasn't saying that--I'm not using the thing. I thought the cap bootstrap was pretty cute, and that the circuit might be a useful data point for John, that's all. 1.25nV/rtHz at the output is pretty decent, vs. 15nV/rtHz (typ) for the LM8261 alone.
As far as exceeding the manufacturer's spec, it's quite possible that Walt, the manufacturer, was telling us from personal knowledge that this was okay. Or not.
Anyway, if you dislike the 1rst stage you'll hate the 2nd--that's got
10 ohms and 1,500uF instead of 49.9 ohms and 10uF. Walt used ordinary wimpy signal diodes to protect that stage, so it looks like he recognized a pitfall, and thought two wimpy diodes sufficient.By his attention to it in the circuit, it looks like Walt thought about input protection considerations, and thought these measures appropriate. Were they? I don't know, I'm not Walt.
ISTM R3 offers some protection against whatever danger Walt saw and wanted to protect. If you'd rather ditch it and go naked as you wrote up thread, by all means rock on -- you can get under 1nV/rtHz that way.
-- Cheers, James Arthur
My proposal was to add back-to-back schottky diodes across the input pins, then reduce the series resistance R3 to the electrolytics.
It turns out that low noise op amps set an absolute maximum voltage across the input pins to keep the the internal diodes off. There may or may not be a corresponding current limit specified.
The following is a short list of the maximum input voltage of some low noise op amps. Note "nV" stands for "nV/sqrt(Hz)," and some op amps have a minimum stable gain.
AD797 : 0.90nV +/-0.7V 25 mA AD8099 : 0.95nV +/-1.8V +/-10mA G >=2 ADA4898 : 0.90nV +/-1.5V LMH6624 : 0.92nV +/-1.2V LT1128 : 0.85nV +/-1.8V +/-25mA LT6200 : 0.95nV +/-0.7V +/-40mA OPA687 : 0.95nV +/-1.2V G >= 12 OPA847 : 0.85nV +/-1.2V G >= 12
The +/-0.7V of the AD797 and LT6200 are the toughest requirements to meet.
An IRF 11DQ05 was one of the first schottkys that looked suitable:
If you look at Fig. 1, "Maximum Forward Voltage Drop Characteristics" on page 3, you can see the forward voltage is 0.7V with a diode current of 2A at 25C.
Since the instantaneous voltage across the input pins is 10V with a hard short, we can set the series resistor to
R = E / I = 10 / 2 = 5 Ohms
The highest forward voltage shown on the graph is 1.1V, corresponding to a forward current of 10A. This means the rest of the op amps could use a series resistance on 1 Ohm and still be below spec.
John Larkin raised the issue of diode leakage and added capacitance adding a pole at the input which could make the amplifier unstable.
Here is a portion of my reply: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are large electrolytics at both inputs. These would swamp any extra capacitance from the diodes. There are 10k and 1k resistors at the inputs. These resistors, plus the electrolytics, would swamp any noise from diode leakage.
Since I may need to use a similar approach, I modeled this circuit in LTspice and uploaded the archive 3CBC2208.ZIP to abse.
The Transient Analysis is shown in 3CBBA0E7.ASC. I modeled the AD797 as a single pole op amp with Avol=15Meg, GBW=110Meg, Slew=110Meg.
I added RX=7.5K across the input pins to match the AD797 datasheet, and also added CX=1nF across the input pins to account for any stray capacitance from the schottky diodes. I evaluated the response with CX and RX both included and excluded in the feedback from the pertubation source V2.
As shown in the Transient Analysis, there is considerable ringing at the output of the op amp after a step change at the input. This appears to be caused by driving the large electrolytic cap C5 at the output. Reducing the value of C5 causes the ringing to increase.
Increasing the value causes the slew rate limiting to increase.
Changing the value of CX from 1pF to 10nF has little or no effect on the response. Changing the value of the series input resistor, R3, from 2 ohms to 49.9 ohms has little or no effect on the response.
Thus any extra capacitance from adding schottky protection diodes at the input will not change the loop characteristics.
The Open Loop Gain is shown in 3CBC2036.ASC. The response changes considerably depending on how CX and RX are located at the input.
However, in either case, the value of CX either has no effect on the open loop response, or improves it. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Next, to confirm the expected diode turnon, Jack Smith of Clifton Laboratories posted a page titled "Diode Turn-on/off Time and Relay Snubbing" at:
He used a HP 8012B set to +/-3.9V. The 50 ohm series resistance gave a measured output current of about 65mA. The 11DQ05 is shown at:
As expected, the diode turns on and off quickly, and shows a forward drop of 0.4V at this current.
Finally, some datasheets may recommend adding a small resistor in series with the input pin, presumably to prevent oscillation due to the high bandwidth of the input transistors. The extra resistor increases the overall noise, which defeats the purpose of the amplifier.
It may be possible to substitute a one or two turn ferrite bead to kill any high frequency oscillation, and retain the low noise performance below 1MHz.
Thanks,
Mike
"Can you say bootstrap. Sure you can."
If you add a digital pot to the design so that you can trim it, the accuracy can be made darn good. You need to subtract the tempco away but a silicon diodes drop is a good match to a non-superbright LEDs tempco.
A current saving idea:
Vcc ! --------!+\ ! ! >-------+----------Ref out ! ---!-/ ! ! ! ! ! ---------+ [R] ! ! ! ! [LED] ! ! ! ! ! GND ! --------------+---[R]---GND
The operating current of many op-amps is nearly constant with voltage. I don't know if it is noisy or not. The op-amp is only given just enough gain to make the output above the Vee pin enough.
Temperature compensation could perhaps be done by putting a thermistor into the divider chain.
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