how to build an isolation amplifier input?

[Martin.M]

My little oldie scope allows high voltage between the both ground clips 
There are isolation amplifiers inside.



I want to know please how they have build them.

greetings
Martin

[mk_]

My little oldie scope allows high voltage between the both ground clips 
There are isolation amplifiers inside.



I want to know please how they have build them.

greetings
Martin

use google to find the schematics of this scope and you will see how they did a long time ago

Michael

[Martin.M]

Tek have never printed the shematic for the 222, only as block diagram

greetings
Martin

[HighVoltage]

It is probably the same principle as in the THS-720P scope. I have never seen the schematics of that one either but I always suspected it to work like a differential scope probe.
May be similar to this attempt:
http://electronics.stackexchange.com/questions/29363/is-this-a-good-design-layout-of-an-active-differential-scope-probe

Next time I open my THS-720P, I will have a closer look.

[Martin.M]

I thought that also at first but the isolation is extremly, also with high input sensitivitys.

[bktemp]

There is no schematic, but a description in the service manual:
The ADC is floating. Only the digital data from the 512 sample FIFO is transferred using an isolation transformer.

The Fluke scope meters use transformers for isolating the analogue signal. They split the signal into a DC/LF part and a high frequency part before transferring them using transformers.

This should give a much better CMRR than using a conventional differential amplifier.

[TurboTom]

The THS700 series of handheld scopes transfer the analog signal after the input attenuator / amplifier to the ground-referenced side where the A/D converter and digital circuitry resides. The insulation barrier consists of a combination of a transformer for the high frequency components of the signal and an analog optocoupler (within a feedbach loop) for DC and low frequency components. On both ends of the isolation barrier, TEK placed an ASIC so it's hard to tell how exactly they are doing it, especially how they are managing the frequency crossover between the two signal paths without noticable phase distortion.

Depending on what you intend to do and what kind of frequency raqnge you would like to cover, there are off-the shelf solution for analog isolation couplers (actually consisting of a digital coupler and a modulator/demodulator at the input/output).

Cheers,
Thomas

P.S. BTW, here's a link to a schematic of the THS700 series http://kazus.ru/forums/attachment.php?attachmentid=25865&d=1315575619

[Someone]

Opto's and transformers are great when you need kV of isolation, for a few hundred volts a differential amplifier is possible. I've built a low impedance design at 100k which is good for 200V, and with 1M input impedance it could go up for 500V operating range.
https://www.eevblog.com/forum/projects/diy-100mhz-differential-probe/

[tautech]

I'm fairly sure Dave did a teardown on a scope with isolated channels but I can't remember which one.

Was it a handheld, a Fluke maybe.....somebody will remember and link it here.

IIRC it showed the isolation componentry of the signal path and PSU supply to the front end, that's all that was isolated.

[HighVoltage]

P.S. BTW, here's a link to a schematic of the THS700 series http://kazus.ru/forums/attachment.php?attachmentid=25865&d=1315575619

Thank you!

[Martin.M]

Was it a handheld, a Fluke maybe.....somebody will remember and link it here.

I have only oldies in my collection, with CRT. The only one with isolation inputs is the 222 (the right one)
Actually I am searching to get a Tek211 to be complete with the 5  they have made

For a actually handheld I have to save money, would like to have the brandnew R+S