C-multiplier again

dB.

is.

OK this is still a sim. However I spiced the C-multiplier (aka Hobbs filter at teachspin) I use to get down to ~1nV. I haven't every bumped into the Early effect (I don't do much on the transistor level.) But this shows attenuation down near 90dB at 100kHz. (Which is where the switcher I use lives.)

George H.

Version 4 SHEET 1 1140 1108 WIRE -608 -448 -688 -448 WIRE -448 -448 -528 -448 WIRE 80 -448 -448 -448 WIRE 112 -448 80 -448 WIRE -448 -432 -448 -448 WIRE 112 -400 112 -448 WIRE -448 -352 -512 -352 WIRE -400 -352 -448 -352 WIRE -304 -352 -320 -352 WIRE -224 -352 -304 -352 WIRE -112 -352 -224 -352 WIRE 48 -352 -32 -352 WIRE -688 -320 -688 -448 WIRE -512 -320 -512 -352 WIRE -224 -304 -224 -352 WIRE -448 -288 -448 -352 WIRE -304 -288 -304 -352 WIRE 112 -288 112 -304 WIRE 208 -288 112 -288 WIRE 256 -288 208 -288 WIRE 112 -256 112 -288 WIRE 208 -256 208 -288 WIRE -688 -224 -688 -240 WIRE -512 -224 -512 -256 WIRE -448 -224 -512 -224 WIRE -304 -224 -448 -224 WIRE -224 -224 -224 -240 WIRE -224 -224 -304 -224 WIRE -448 -208 -448 -224 WIRE 208 -176 208 -192 WIRE 208 -176 112 -176 WIRE 112 -160 112 -176 FLAG -448 -208 0 FLAG -688 -224 0 FLAG 112 -160 0 FLAG 80 -448 Vin FLAG 208 -288 Vout SYMBOL npn 48 -400 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -688 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -512 -448 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0

Oh, and thanks again Mike for showing me how to post and read the LTspice stuff... I feel so empowered.

SSRs make interesting overload limiters too, especially if you can turn them off in a reasonable amount of time after an overload, or during powerup transients.

I'm using a Clairex part for that just now, 4 ohms typ.

John

Seems to be missing some components. I got the 2N4401 and what appears to be a AC source in series with 15V. Everything else is just gaps. I browsed the file but could not find any references.

You are welcome.

Mike

I'm not sure how that relates to protecting the inputs of the AD797.

Mike

OK, I see the problem. Maybe the file is missing a portion near the end.

Can you copy the original to an empty folder and zip it, then post the zip to abse?

Thanks,

Mike

This is a discussion group. It doesn't.

But come to think of it, an SSR could be a 4-ohm replacement for the

John

SSR - Solid State Relay? Too slow to protect the inputs, even if you could design a circuit to detect a fault instantly.

Back-to-back schottkys across the inputs, with perhaps 2 ohms in series to the electrolytics, might be the best solution. Hopefully the schottkys would turn on fast enough to do the job.

Noise adds as the sum of the squares. A good op amp will have about the same noise as a 50 ohm resistor. A 2 ohm resistor would add negligible noise to the system:

En = sqrt(50^2 + 2^2) / sqrt(50) = 1.00079968026 = 0.0069dB

Hard to measure that small an increase.

Mike

sorry, typo:

En = sqrt(50^2 + 2^2) / 50 = 1.00079968026 = 0.0069dB

Mike

Leakage current is usually Poissonian, perhaps with some 1/f noise on top. The main problem with the LEDs being photovoltaic is 120 Hz from inductive ballast fluorescents and 40 kHz - 1 MHz hash from electronic-ballast fluorescents. The ugliness of electronic ballasts has to be seen to be believed.

Cheers

Phil Hobbs

Let's start over.

A 50 ohm resistor has 9.07312516170696E-10 v/sqrt(Hz) at 25C A 2 ohm resistor has 1.81462503234139E-10 v/sqrt(Hz) at 25C

The total is

sqrt(9.07312516170696E-10^2 + 1.81462503234139E-10^2) = 9.25280844976e-10

Cross Check: the noise of a 52 ohm resistor at 25C is 9.2528084497627E-10

The increase due to adding 2 ohm in series with 50 ohm is

That is much easier to measure.

So every ohm counts!

Mike

ch

Well that figures, It's my first attempt to post a ltspice circuit.

Not much to it. a two pole low pass RC filter (R-C to ground. feeds R- C to ground) and this feeds the base of a npn. the output has a minimum 1 k ohm to ground with anoter C. R is 1k and C is 100uF Al electro. There's also 10 ohms between the final R-C cap node and the base, but this is only to stop oscilations and doesn't change the spice sim much. I'm going to play hookey (sp) from work tomorrow and get the homestead ready for our annual Memorial Day party. (Else I'd try and repost) Summer's here early in Buf. NY. If you're in the area I grill up some great Polish sausage with Walt's mustard. (Walt lives down the road.)

George H.

bs

hich

to

sed

Geesh lots of typos there, please forgive me (I'm a terrible speller)

George H.

The Clare parts I'm using - to protect electronics - turn off in anout

Big ones would probably add noise. Little ones wouldn't protect.

John

What are you protecting, and against what? MOSFET thermal overload?

What's the time constant? Seconds? You have plenty of time.

How long does it take to destroy a junction in a 1GHz op amp? Nanoseconds? Picoseconds?

The AD797 input bias current is 250nA, so it's probably bipolar. Perhaps the problem is not so much destroying the junction as it is breaking it down in the reverse direction. I understand this increases the device noise. If it's reverse breakdown, it may only take a few nanoseconds.

Zero volts across them. Zero current. Probably high capacitance, maybe

Little ones may be fine. The 11DQ05 will take 150 Amp surge, so all it needs is a high enough value resistor in series to keep the voltage drop under the AD797 spec:

Or use a more modern op amp that has lower noise and higher max input differential voltage. Or put a 10 / 0.025 = 400 ohm resistor in series with the input. That should keep the current under 25mA:)

Mike

It's a 12-channel isolated 4-20 mA input/output board. It's got a bunch of resistors, transzorbs, and mosfets in harm's way. We're allowing the customer to apply up to 50 volts at either polarity. There are a couple of 1206 resistors that dissipate something like 20 watts each at max overload, with the current limited by the fets inside the SSR. An ARM processor (one per channel!) is digitizing everything at about 100 KHz, notices the overload, and turns off the SSR. I figure the resistors are OK for a millisecond maybe, so we've got to turn things off before then.

Microseconds at least, if the mechanism is thermal. You could vaporize metalization a lot faster, with enough energy.

That opamp has back-to-back diodes across its inputs and no current limiting resistors. The problem is too much current from charging those big caps.

Leakage resistance maybe, and capacitance adding a pole to the feedback loop. I did say "probably."

Might work. 100 pF, maybe more, at 0 volts.

Mr Johnson would object.

John

GAAA! If you want low noise from AN OPTICALLY OPEN DIODE you need to shield from light. Incandescent will cause 120 Hz input, fluorescent 60 Hz and 120 Hz, electronic ballasts and CCFL can go from kilohertz to megahertz.

Use a 1.25V bandgap reference like real folk do.

So you now agree that a 49.9 ohm resistor is not enough to prevent damage, as I have claimed from the beginning.

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 chages 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.

That was just a quick pass in google. There are probably much better choices.

Mike

Thanks. When you get some time, I'd like to see your circuit.

I do love Polish sausage, but I'm up in Ontario near the Great Lakes. A bit too far for a weekend trip. If I still had my plane it would be trivial. But I gave that up after an engine failure at 25,000 ft at midnight over the Sierras. After my copilot declared an emergency, she amended her message to ATC with the words, "And We Only Have One Engine!"

Walt who? There has to be some reason you would mention this name. Can I guess? I hope it's who I think. If so, please tell him I think he is one of the finest engineers I have had the pleasure to study. And decades later, his papers are still among the most valuable on the web.

Thanks,

Mike

GAAA? I said I didn't do it.

Bandgaps are really noisy. The amplify the small difference between two junction drops. That's a lot noisier than the unamplified drop of two junctions in series.

John