My (borrowed) Tek DPO doesn't do differential inputs very well (got a math module a-b mode, too noisy plus crappy common mode range). So I think of doing a differential frontend end amp to take out up to say 100V common mode, for looking at switchmode signals on top of usually 24V (21V-30V) battery stuff. Sometimes need to see 100V signals on that, or at least tolerate the overdriven inputs to see signals of interest, So I expect to put in dynamic range limiting to stop the opamps saturating and getting 'stuck' in saturated mode. You know how they can take some time to come back out of saturation? Or is that something no longer happen with newer opamps?Equipment could be chargers, motor drives and so on, switchmode, dirty signals. Quite noisy, and I'm usually stuck with the ground reference on other side of the battery pack. Normally battery is run isolated from mains ground, so noise pickup is a problem. Also the safety issue of grounding wherever the CRO earth goes.So the topology seems most useful is the inverting front end for each input, feeding a diffamp. Ideas for one of these? I tend to use the DPO on 20MHz bandwidth filter, so I guess I'm not expecting too much speed from the thing, I have cheap 60MHz probes for a normal 1M input impedance, good enough? Ideas for which opamp would be nice. Is trying for 1M input too easy for noise pickup? I suppose I could have a couple flying leads with clips going into the box with this thing, a coax lead out into the DPO's input set to 1M input. Or you might convince me to use the 50R input, though I don't think I want to go there. Last time I mentioned doing that somebody tried to tell me my observed voltages would not be halved?Thanks,Grant.

100V common mode diff front end amp?

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Grant 15 years ago

Well if you clocked a flipflop inside the signal path? But it wouldn't be strict Manchester-like until you forced AC into the data stream to make the average zero requirement.

I didn't so that, and decoding Manchester has always remained a mystery to me, used a black box that does it ;) Which of course makes me wonder if you forced Manchester in the feedback path, would the analog data force correct digital sort of directly, automagically?

I'd like a decent current probe, Tek want anything north of $900 for one, so I made one from something I read on the 'net, it let me see a waveform, but of course no calibration. Some help, at the time I was investigating the nasty switching MOSFET drain to gate current injection, before I knew it was normal behaviour.

Grant.

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Grant 15 years ago

...

Thank you. Hmm, INA333 4000 hours operation from LI coin cell! Better than +/-15V

Grant.

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Grant 15 years ago

You have a block diagram for recovery? I never got my head around the clock extraction, how it worked.

And, you have to take care about the data edge setup time (right term? make sure you don't glitch output, latching moving input data) on the encoder?

Grant.

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Grant 15 years ago

Yes, I looked at datasheet yesterday, wont happen in my world. They did quote that 30V limit for ground leads, but they do that now for handheld multimeter leads I bought recently. Maybe some country has a silly law?

Grant.

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krw 15 years ago

The Manchester encoded signal has a transition in the middle of the cycle (falling edge of the clock). If that transition is positive, it's a '0', if negative, it's a '1'. The corollary to this is that if there is a transition at the beginning of the "cell", D(n) = D(n-1). A phase lock loop can pretty easily find the clock.

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John Larkin 15 years ago

Here it is in words:

Run the input stream into an XOR+RC edge glitcher. Use that to fire a non-retriggerable one-shot set to 0.75 of the bit period. That one-shot will wind up firing at the middle of every bit cell, where there's always a transition, and ignoring the other transitions. So the one-shot output becomes the recovered data clock.

Now use the one-shot output and a D-flop to recover the data.

I could draw it if it's not clear.

I invented this when I was just a kid! We did a lot of supervisory control systems using Manchester data over FSK modems, all TTL.

John

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Jim Thompson 15 years ago

Off the top of my head... clock CVSD data into 2-bit shift-register, X-OR the 2 bits to get the output.

Without spending any time to check it out, I believe that decoding it was essentially the same routine. ...Jim Thompson

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josephkk 15 years ago

just

:)

=20

It seems like we have a global population explosion of silly laws recently. Decent people have let down their guard, or maybe it was battered down by insane courts.

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John Larkin 15 years ago

Good bumper sticker:

QUESTION AUTHORITY

Better bumper sticker:

IGNORE AUTHORITY

John

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Phil Hobbs 15 years ago

I'd like to add to the first one

WAIT FOR THE ANSWER AND THINK ABOUT IT.

Mindless rebellion is, well, mindless.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net

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Jim Thompson 15 years ago

When our oldest son was in about 3rd grade, his teacher called. She was quite upset. She had confronted Aaron, "You should learn to respect authority". Aaron replied, "My parents taught me to question authority". ;-) ...Jim Thompson

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100V common mode diff front end amp?

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