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Old 26th October 2003, 09:49 PM   #171
andy_c is offline andy_c  United States
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Hi Fred,

On the subject of line regulation, if we assume no preregulator, it seems that the AD825 is not the best choice for line regulation performance at 120 Hz. Because the dominant open-loop pole of this op-amp is at about 10 kHz, the open loop gain at 120 Hz is not nearly as high as it could be, given its gain-bandwidth product. In looking at my 90 Volt version of this thing, which has no pre-regulator, I found that (in simulation at least) one can get about 40 dB better line regulation at 120 Hz by using an op-amp having about the same gain-bandwidth product as the AD825, but with an open-loop pole below 120 Hz (giving about 40 dB higher open-loop gain at this frequency).
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Old 26th October 2003, 10:03 PM   #172
Bricolo is offline Bricolo  France
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Quote:
Originally posted by johnferrier



Scheme 2 gets my vote. Using a sense ground connection allows the star ground point to be moved from the "regulator" block to the "circuit" block.


JF
I'm not sure I understand what you mean.
Are you good for drawings?
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Old 26th October 2003, 10:18 PM   #173
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Hi guys,

Read Fred carefully...it's a little gem thrown your way...

It never hurts to go just a little overboard on regulation if that's your cupper...

Well put, Fred.

Cheers,
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Old 26th October 2003, 10:45 PM   #174
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Stick with the AD825. (a friend)
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Old 27th October 2003, 02:49 AM   #175
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Quote:
Originally posted by Bricolo

Are you good for drawings?
No, but I'll try...

Click the image to open in full size.

Here's something off the internet:
Click the image to open in full size.

First, I'm not sure what part you don't understand. You don't know why I choose scheme 2 or you don't know why I indicated that the sense lines allow you to move the star connection to the circuit block. So, I'm not sure which question I'm trying to answer.

As far as scheme 1 and scheme 2, it really depends on where the sense circuitry is connected (where the Kelvin connections are located). At first scheme 2 looked right to me, but it's dependent on the sense connection.

As in a four-wire ohm measurement, there are two "Kelvin" connections: upper Kelvin (Ku in diagram) and lower Kelvin (KL). These are the sense lines, of the greatly simplified Regulator circuit. The regulator will regulate the voltage across those two points. Because of series resistance, the voltage at other points in your whole circuit will be not as well regulated (though perhaps the difference is small).

The reason this helps is because the current in the relatively low impedance conductors of +Reg and +Reg Return have voltage drops (again perhaps small). The sense lines +Reg Sense and +Reg Return Sense are relatively high impedance connections to the voltage reference. The Regulator is looking/sensing/regulating across the Kelvin connections.

To the best of my knowledge, the best place to place the Kelvin sense connections is where your most sensitive circuitry is (ie the input to the amplifier circuitry). Someone at diyAudio wrote that the most sensitive circuitry should be closest to the power supply. That way fluctuations in the power section of your circuitry are downstream of your sensitive circuitry. Rather than the sensitive circuitry being down stream of the power section.

Simply put using the two sense lines properly compensate for the series resistance loss of the power and return lines.

I would expect that the Kelvin connections are the place to have star connections. The +reg side is starred as the +reg return is a star.

Comments welcome.

Also, if anyone is familiar with something called a 16-wire Kelvin connection (where each of the 4 traditional Kelvin connections each have 4-wire connections), I'll appreciate some information. I'm sure that ran across that once, but can not find any reference to it (thought it was from a Texas Instruments page).


JF
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Old 27th October 2003, 07:14 AM   #176
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Default Re: Just extreme design I guess......

Quote:
Originally posted by Fred Dieckmann

Using a 1.8 mA JFET current source like the J507 will give about 1 Megohm impedance which is a couple orders of magnitude better than a 10K bias resistor for the current source LED voltage reference.
What does this mean in real life, I mean at the regulator output? Hardly 10-100 times better?
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Old 27th October 2003, 07:45 AM   #177
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Default Re: Re: Just extreme design I guess......

Quote:
Originally posted by peranders

What does this mean in real life, I mean at the regulator output? Hardly 10-100 times better?
Maybe spending a few minutes with pen and paper or a Spice
session can give a clue to this.

No, I have not been thinking about this particular problem, so
I do not know the answer.
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Old 27th October 2003, 07:48 AM   #178
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Question That's not helping your sales.

"What does this mean in real life, I mean at the regulator output? Hardly 10-100 times better?"

Build it and see! Who do you think I am?
Your mother?

Hint: it depends on the frequency, and the loop gain of the op amp used. When do you think you will have the measurements to share with us? We are all extremely lazy and I thought maybe you would do the work for us for a change.
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Old 27th October 2003, 09:19 AM   #179
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Fred, what about using a pre-regulator instead? Then you can use a plain resistor with good concience.
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Old 27th October 2003, 09:27 AM   #180
Dave S is offline Dave S  United Kingdom
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Default LM317s and level shifting

There are 2 aspects of the ALW reg which concern me: the LM317 pre reg and the level shifter.

ALW in his application note states that decoupling the ref pin on the LM317 pre-reg is a must do (BTS!!). This indicates that the SR (downstream of the LM317) is somewhat sensitive to what it is fed with. If this is the case then surely some of the "greyness" people attribute to the 317 will afflict the SR?

Fred has obviously thought about this and made some excellent suggestions to improve the PSRR if ommiting the pre-reg. The level shifter is the target of these improvements (although I would have to think about the merits of using a FET for the level shifter and if splitting the 10K sink resistor into 2 X 5K I would decouple the mid point to the +ve supply rail not to 0V).

My approach would be to get rid of the LM317 and the level shifter, if you don't have the level shifter the pre-reg is (possibly?) not required.

The clever bits of the Jung topology are that the voltage reference and the op-amp supply are powered from the regulated o/p. I agree that the reference will benefit from this approach but the op-amp should be quite happy working off a zener/emitter follower with local decoupling for the op-amp (eg. 10R and 100uF) off the raw dc supply. The op-amp can then drive an emitter follower (with suitably high beta) directly without the need for the level shifter. The reference can still be powered by the output as per the Jung topology.


Dave
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