how to build an isolation amplifier input? - Page 1
Forum section: electronics / testgear

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 25, 2016, 06:08:21 am
content: My little oldie scope allows high voltage between the both ground clipsThere are isolation amplifiers inside.I want to know please how they have build them.greetingsMartin

author: mk_
author_rank: Regular Contributor
timestamp: June 25, 2016, 08:04:47 am
content: Quote from: Martin.M on June 25, 2016, 06:08:21 amuse google to find the schematics of this scope and you will see how they did a long time agoMichael

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 25, 2016, 08:11:24 am
content: Tek have never printed the shematic for the 222, only as block diagramgreetingsMartin

author: HighVoltage
author_rank: Super Contributor
timestamp: June 25, 2016, 08:23:26 am
content: 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-probeNext time I open my THS-720P, I will have a closer look.

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 25, 2016, 09:06:11 am
content: I thought that also at first but the isolation is extremly, also with high input sensitivitys.

author: bktemp
author_rank: Super Contributor
timestamp: June 25, 2016, 09:17:13 am
content: 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.

author: TurboTom
author_rank: Super Contributor
timestamp: June 25, 2016, 10:58:40 am
content: 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,ThomasP.S. BTW, here's a link to a schematic of the THS700 serieshttp://kazus.ru/forums/attachment.php?attachmentid=25865&d=1315575619

author: Someone
author_rank: Super Contributor
timestamp: June 25, 2016, 11:10:23 am
content: 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/

author: tautech
author_rank: Super Contributor
timestamp: June 25, 2016, 11:51:21 am
content: 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 25, 2016, 11:56:35 am
content: Quote from: TurboTom on June 25, 2016, 10:58:40 amThank you!

author: Martin.M
author_rank: Frequent Contributor
timestamp: June 25, 2016, 12:46:21 pm
content: Quote from: tautech on June 25, 2016, 11:51:21 amI 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 madeFor a actually handheld I have to save money, would like to have the brandnew R+S