100V common mode diff front end amp?

Practically, yes. In the sense that subtracting two AC coupled signals in the scope doesn't work very well, the DC is moving about, and I want to see the small and large signals on top of that.

Older scopes I worked with had enormous input offset ranges so one could amplify a pair of signals and have the boring bit miles off screen, this modern DPO thing has 8 units of vertical, and barfs at only 5 units of offset, putting up a signal out of range error.

Well, isn't that the same as switching to AC coupled on the scope?

Hmm, tick tick, tick, what I need is isolated ground, failing that, a CRO input lead where the -ve connector is a local 'ground' for the wanted signal on the +ve connector. With a fairly good frequency range, some MHz.

So I describe that as needing a diff amp front end with a believable unity gain and very good CM rejection up to +/-100V.

Yes, and at some point one wonders if they seeing the signal of interest, or artifacts of the signal collection circuitry. That's why I like to be careful how I do this, the result needs to be trustworthy.

Seems like too much in the signal path, some unknown delay times involved.

Grant.

ally

that,

,

ating

some

r happen

y

nce

d from

of

the

sy

h

O's

ut,

ng

ed?

True but the Tek/7a13 will load it much less.

Zoom? what are you talking about?

but you can't take out the common

I dont know what you mean here.

happen

Maybe not. If the probe ground clip is connected to the fet source, the gate sees only the probe load relative to the source, even when the source is flailing around hundreds of volts p-p. With a 10x probe, the loading will be tiny.

A 7A13 will load the gate *to ground*, so if the source is moving, it will jam lots of current into the gate, often enough to make trouble.

I have some products that have opamps riding on a 200 volt power rail, rectified 3-phase with ripple. The TPS floating channels let me ground clip to that power rail and then probe tiny signals in the opamp circuits. I couldn't do that with a 7A13 very well.

Looking at a small signal after subtracting a big offset.

7A13 is a diffamp. You can select the low side to be an external input, or the Vc internal offset supply, but not both. 7A22 is a nice compromise, simultaneous diff input and decent offset, but only up to 1 MHz. On the other hand, it goes to microvolts/div.

John

If the requirement isn't for high frequency, there are isolation amplifiers mass-produced for this purpose. A DigiKey search for "isolation amplifier" brings up seven pages...

t a

=A0

to

usually

on that,

est,

turating

ke some

nger happen

irty

erence

ated from

ue of

ach

use the

ch

put

easy

with

DPO's

input,

doing

alved?

e

ust

h

Not allways, you have attached a 4 foot antenna to your fets source.

How so? Normaly you would use 10M probes (P6055).

Yes it would. I used it for that very thing before I got the TPS.

Ah now I see, your using it as a single ended amp instead of a differential amp. Do you have the manual?

Yeah, back then with slow signals I had better luck transferring the analog value as a duty cycle though the optocoupler, transfer was very accurate.

I suppose now you'd call the technique self-clocked delta-sigma, or something like that? Signal into an integrator that forced output duty cycle to match, therefore recovery on other side of opto was very close to original signal.

I used CMOS inverters for rail-to-rail buffers to get the accuracy.

Grant.

Sorry, it is for high frequency, well the sort of signals you see when switching ion the 20MHz filter on the 'scope.

I still think the INA117 idea is winning here. Like John L. said upthread, I want to get rid of a large DC offset to see the interesting part of the signal, with spending heaps, and not get a significant delay. Hmm, unless I build these for all four inputs? That'll stop ground loops for starters.

Probing around power stuff is hairy, and accidentally shorting out stuff with the ground leads a constant worry for me. Even when working with isolated battery bank. As soon as I clip scope ground to something, I have a potential Bang! waiting to surprise me. And if it wasn't mains earth, it's the common ground between probes, sometimes shorting current sense resistors.

So a diffamp for each probe seems like it would suit me, and all channels suffer same delay, even better. Suit most of what I'm doing at the moment.

The INA117 claims +/- 500V input protection, so it looks very good for this problem.

Grant.

Grant.

That's what makes the INA117 attractive, resistors onboard trimmed to

0.01%, +/-200V CM, and +/-500V peak input protection on chip. Unity gain for the diff signal, that'll suit me, it would have to have reasonably low noise as well? Datasheet says: 25uV

Am 23.04.2011 17:31, schrieb Grant:

Why not buy a commercially available differential probe? They start about 200 Euros, for example from Testec. They usually have a wide common mode range and a switchable differential mode range (eg. 10:1,

100:1). I'd at least consider buying one of these before frying my scope's frontend or even myself with a self-tinkered circuit...

Am 24.04.2011 02:12, schrieb Grant:

The TPS scopes run from integrated LI-ION batteries or from an isolated bench power supply (which is so crappy that it will make the scope display several millivolts of noise when it is connected).

Ronnie

Am 23.04.2011 22:19, schrieb John Larkin:

Good idea - the Yokogawa ScopeCorders have something like this built in. Unfortunately the bandwidth is quite limited at these models...

Ronnie

Looks nice but I don't think the (creapage) distance between the pins is large enough to actually work reliably at 200V (or more). I also like to add a capacitor between the inputs of the instrumentation amp to filter unwanted signals.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

Well creepage distance is for diff voltage which is usually lower but can be accidentally high, but unlikely over 100V for my stuff.

Conformal coating for creepage distance?

Or perhaps the safer three opamp topology using the inverting inputs for high voltage? That's the other way to do it. Use single opamps each nulled out, and 0.1% resistors instead of the laser trimmed ones in the INA117...

Response is already down to 200kHz, how much less? I'm already suffering loss of bandwidth ;)

Grant.

The common mode voltage is also close to the supply pins!

I like to use an instrumentation amp like the INA332 or INA333 and have an external divider. If you use 3 seperate opamps the precision resistors will cost you more than a single instrumentation amp.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

Each of the five BNC inputs (four channels and trigger) is fully isolated. You can use any probes, or no probes.

John

Which reminds me. In thinking delta-sigma modulators... I just got an inquiry... I vaguely recall of a trivial way to turn sigma-delta into a Manchester-like stream, then restore easily on the receiving side... nice for passing thru opto-couplers.

When you first posted, I thought of using current probes... I have some good from DC to 50MHz, but the front-end would end up being power hungry :-( ...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

Something resembling SACD ?

You just xor the data stream with the clock, no? Recovery is almost as simple.

John

I don't remember what it was called. Last did it in late '80's :-) ...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