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100V common mode diff front end amp?

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.

Reply to
Grant
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In the mid '80's I had a Tektronix probe device that provided complete isolation so I could look at off-line switchers. Surfing I can't locate such an animal anymore. You might look on ebay. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed
Reply to
Jim Thompson

We have a TPS2024, with all four channels and the trigger input isolated. It's amazing for probing power stuff. You can hook a probe ground clip anywhere, like on one side of a 240 volt AC line, and just not care.

John

Reply to
John Larkin

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Yes but real noisy at mV levels, an old tek7000 with a 7a13 is so much better.

Reply to
cbarn24050

Does yours make you sign a disclaimer that you won't put more than 30V on the probes every time you power it up? And with silly yellow stickers on the probes reminding you... (until they get cut off).

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Check out the INA117. It has 200Vcm input range. Downside is the input impedance is not high Z. Art

Reply to
Artemus

This is another case of "trivial in concept, but the devil is in the details."

Take a look at the TEK 7A13 vertical amplifier. Download the operator and service manuals. There are some interesting CMRR vs frequency graphs in the manual. This designed by people who knew what they were doing.

And just getting the signals into the amp without messing up the CMRR is a challenge.

Also check out the TEK A6902 isolation amplifier. It's more a dual-channel isolation amp, so you can use one probe and stick the common lead anywhere you want. I've found it very useful in debugging switching power supplies safely. Never did find the service manual tho. If anybody has a link to the schematic, I'd like to have one in the archives.

I've never tried it, but for stuff that doesn't have to be that accurate, you might be able to make something like a wideband LM3900. I've always been intrigued by the simplicity of huge voltage compliance you can get with that topology.

Reply to
mike

No, we must have an older, pre-lawyer model.

John

Reply to
John Larkin

If you have something like a fet whose source is a couple hundred volts off ground, and you want to see the gate waveforem, the TPS is great.

The 7A13 zooms like nothing else, but you can't take out the common mode and zoom simultaneously.

One nice combination is an AM502 working as a preamp for a modern digital scope. It has switchable bandwidth and a pretty good combination of differential inputs and offset at the same time. It can get you down to things like 2 microvolts/div.

John

Reply to
John Larkin

It wouldn't be hard to do a matchbox-sized battery-powered probe sort of thing with fiberoptic output. That would work a million volts off ground.

John

Reply to
John Larkin

Divide down and use an instrumentation amp to amplify. You'll need

0.1% or better resistors for the divider. I've used this trick to get 100mV signals from several hundreds of volts common mode noise. 
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------
Reply to
Nico Coesel

Damn, I should ask for a downgrade in the firmware.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

...

I looked and looked for the comms module*, somebody in Hong Kong offered me one for USD800 a few years ago, I hate to think what they'd want for a real diff-amp input module. I sort of know what you mean, one job we had a good HP (I think) with a plug-in diff module, was wonderful.

*Anybody got hardware interface spec and schematic for Tek TDS3034?

Only outside comms it has is a parallel printer port, yes I think about emulating a printer to capture an image, for when the internal floppy drive breaks, as they all do eventually in fan cooled equipment.

Grant.

Reply to
Grant

...

Is that when running off battery? Cos surely the mains earth would change things? We got 240V mains, a little more exciting than yours :)

I'm wondering if I can power the DPO from a 12V SLA battery, instead of the original NiCd battery pack. Seems ballpark voltage might be okay, if I don't expect the DPO to charge the SLA battery.

Ah, re-reading what you write, you got isolated probes? I've got cheap USD6 Chinese probes for my TDS3034. No budget for 'real' probes.

Grant.

Reply to
Grant

...

I don't need hi Z, and yes, it the one I keep coming back to, $10 one off, not bad. Just had another look at the spec +/- 500V CM and diff voltage protection, so it's likely best I'll find? Just make a quiet* +/- 15V supply for it, or run from a pair of 9V batteries?

  • As in not add crap from the mains, be nice if I could find an electrostatic shielded mains transformer from an old tape recorder or something? Or wind my own earth shield plus secondaries onto a mains toroidal xformer? Wonder how bad the turns/volt is?

Grant.

Reply to
Grant

[ideas for a diff amp?] ...

That's one issue, a couple flying leads, with miniature clip? If I could some really flexible coax I'd use that for the input leads.

But then you start into guard drivers and stuff, how far does one go?

I'm now tempted by the INA117 since it's been seconded by someone here.

"Performance as a dc amplifier in a single-power-supply mode is not as precise as a standard integrated-circuit operational amplifier operating from dual supplies." --TI datasheet.

I'm surprised the things are still stocked. Never did find a use for them, like you I've always thought they might be good for something.

But what?

Grant.

Reply to
Grant

...

What diff amp plus ADC -> optic to a receiver, DAC to DPO? I'd have to run everything through such little boxes to match up the times... or somehow delay all other signals, or, or?

Nice, but it has issues..

Reply to
Grant

It doesn't need to be a diffamp if it's battery powered and floating.

Maybe you could do FM at some huge carrier frequency, with a tiny commercial VCO as the modulator.

John

Reply to
John Larkin

At the differential output, run an other amplifier stage driving an optoisolator. To linearize the opto output, put an identical optoisiolator in series on the input side and use the output from it as feedback to the second amplifier stage.

By the way, if you are measuring ripple voltages riding on some large DC voltages, do you actually have to have a frequency response down to DC ?

If frequency response down to DC is not needed, why not use some DC blocking capacitors, either directly at inputs or after a floating, battery powered differential amplifier and the scope ?

Of course, some resistors and voltage swing limiting diodes may be needed to handle the capacitor charge/discharge currents during startup and power down. to drive the signal to equipment at ground potential. On the optoisolator input side, put an other optoisolator in series and return the second optoisiolator output to the floating

Reply to
upsidedown
[snip]
[snip]

I did that with isolated sensing on switching power supplies in the early '80's. I vaguely remember issues with linearity over a large dynamic range. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed
Reply to
Jim Thompson

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