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Old 28th April 2010, 03:48 PM   #651
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Quote:
Originally Posted by jackinnj View Post
In fact, Art Kay demonstrates the thermal effects in one of the papers -- the thermal effect can seem as if it's noise. This is why Williams mounts the JFETs in (I guess) some kind of potting compound.
It would be lf or 1/f-noise-like, then?

jd
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Old 28th April 2010, 03:49 PM   #652
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It would be lf or 1/f-noise-like, then?

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Old 18th April 2011, 02:31 PM   #653
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Hi folks the specs of the error amp and pass transistor are widely discussed, but maybe someone can resume all this together what are the specs to have a look on? Something like this? 1) Opamp : wide bandwidth (closed loop bandwidth), high speed, fast settling, low noise. Do I missed something? 2) Pass transistor: (here i get more problems) Fast, low On-resistance, high beta ???? Ill try to make low current (only 100mA) super reg and for this want to replace the out-transistor (and maybe the error amp). There are much faster lower current types as DH44 (FZT851) there, so what specs of a tr. are important for this use? Best Regards Dimok
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Old 18th April 2011, 02:34 PM   #654
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Hi folks the specs of the error amp and pass transistor are widely discussed, but maybe someone can resume all this together what are the specs to have a look on? Something like this? 1) Opamp : wide bandwidth (closed loop bandwidth), high speed, fast settling, low noise. Do I missed something? 2) Pass transistor: (here i get more problems) Fast, low On-resistance, high beta ???? Ill try to make low current (only 100mA) super reg and for this want to replace the out-transistor (and maybe the error amp). There are much faster lower current types as DH44 (FZT8519) there, so what specs of a tr. are important for this use? Best Regards Dimok
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Old 23rd July 2011, 08:53 PM   #655
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Default It oscillates, now what (part III)

I've been comparing the super-regulators, actually about a dozen regulators of varying parentage. After wiring up another set of Walt Jung's design I put them on the HP3577a spectrum analyzer to see what's really going on. When you look at the results with a scope (a Tek 2465) there are no apparent issues whether the error amplifier is an AD797 or an AD825, but the spectrum analyzer tells a different tale.

The regulator output is connected to the spec analyzer via a pair of low leakage 10uF capacitors. (3577 gets unhappy with more than a 1.1 volts on the input.) I ran the analysis from 10kHz to 10MHz. It's not actually oscillating, the current would be a lot higher than what we're seeing. Le voila:
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Last edited by jackinnj; 23rd July 2011 at 09:18 PM.
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Old 23rd July 2011, 09:41 PM   #656
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. . . but the spectrum analyzer tells a different tale.
OK, you have me stumped. That's definitely NOT the kind of subtle difference I'd expect between two well-regarded, high performance opamps.

Those all appear to be discrete spectral lines - such as every AM radio station within 10 miles, and every microcontroller clock oscillator within 50 feet. And I'd probably start by looking for a grounding or shielding problem that found its way into the AD797 unit, but NOT the AD825 assembly. (I presume these are two separate units, rather than a case where something got disturbed in the process of changing out the opamps.)

Based on a cursory review of the Data Sheets at < http://www.analog.com/static/importe...eets/AD797.pdf >, and < http://www.analog.com/static/importe...eets/AD825.pdf >, the two chips seem to have roughly equivalent performance in the 0.5MHz to 5 MHz range, although the AD797 DOES have some finagling of the internal compensation in that area.

Any guesses as to what the underlying cause is?

Dale
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Old 23rd July 2011, 09:52 PM   #657
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Yes, the AD797 seems to do a better job of picking up the ambient noise -- the line ~700kHz is probably WABC which is a clear-channel station (although I don't think they run it up to 50kW). I'll take a peak tomorrow with the HP 3577 and the HP 3586, perhaps get a loop antenna going.
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Old 24th July 2011, 08:47 AM   #658
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Originally Posted by jackinnj View Post
Yes, the AD797 seems to do a better job of picking up the ambient noise -- the line ~700kHz is probably WABC which is a clear-channel station (although I don't think they run it up to 50kW). I'll take a peak tomorrow with the HP 3577 and the HP 3586, perhaps get a loop antenna going.
I wonder if it is actually the chip picking up this stuff, or the circuit with it's wires and loops? Are this physically two different units?

jan didden
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Old 24th July 2011, 12:19 PM   #659
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They are the same regulator PCB with the same connections -- uses the LM317 pre-regulator. Thus, you're just seeing the gain (and rectication) of the two device up through the a.m. broadcast band etc., etc.

I took care not to move the device on the device on the table top, nor any of the connecting wires. It is NOT in the box I've constructed to test the regulators.

For the comparisons, I've built a bunch of these boxes to house the regulators. The regulator snaps into its power supply via a bunch of banana plug/jack. The DUT or measurement device snaps into the regulator on the other side, and it's provisioned for Kelvin connection of the error amplifier. I was going to use an umbilical, but it was easier and less expensive to drill holes. Regrettably, I couldn't find sufficient cookie tins this time of year.
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Old 24th July 2011, 04:10 PM   #660
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I suggest a very simple check that could be helpful. With the spurious signals displayed, switch off the power to the reg, and see what changes.

All instances of oscillation I've seen with these regs have always been with tens if not hundreds of mV of sine wave output. I do agree with the observation that a time domain plot won't show this lower level stuff.

wj
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