Analog optocoupler - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: June 20, 2020, 07:55:42 pm
content: Hi,any suggestions for parts or circuits for opto-isolating two analog signals, one 0-5V and the other -/+ 5V?I am looking for a solution without distortion at up to 30KHz (non-repetitive signal - i.e. non-sinewave).It can amplify the signal or not, I can work around it if it does (attenuating it before or after opto-isolating).Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2020, 07:57:25 pm
content: What accuracy?Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: June 20, 2020, 08:09:49 pm
content: Hi Tim,Do you mean accuracy in terms of output amplitude vs input amplitude? If so then not that bothered because I can amplify/attenuate before or after.If you mean distortion or how "identical" the o/p is to the i/p then I have no idea how to quantify/specify. My background is mostly digital and have been getting into analog only recently.

author: Benta
author_rank: Super Contributor
timestamp: June 20, 2020, 08:13:22 pm
content: Search for Broadcom (ex. HP, Avago etc.) "isolation amplifier". TI ISO124 is also an option. None of them cheap, though...Perhaps an audio line signal transformer could be an easier solution (like earlier used for line inputs to audio amps).

author: ogden
author_rank: Super Contributor
timestamp: June 20, 2020, 08:24:34 pm
content: SI8920 could be used as well. Question here is: what will be done with that analog signal on the other side of isolation? Default approach in case of digital/DSP systems - converto to digital domain as soon as possible, isolate digital signals.

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2020, 09:25:01 pm
content: The classic way of doing this is an opto-isolator that uses an LED coupled to a CdS cell. I don't know if that will be fast enough for your application though, and I don't know how linear it is. They were used in audio applications.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: June 20, 2020, 09:40:05 pm
content: Quote from: Benta on June 20, 2020, 08:13:22 pmI used the Avago HCNR201 some time ago. Around $5 on Digi-Key. 1 MHz bandwidth according to the datasheet.

author: TimFox
author_rank: Super Contributor
timestamp: June 20, 2020, 10:03:56 pm
content: An old-fashioned way is with a voltage-to-frequency (VFC) converter driving an LED through a light pipe (if needed) to a photo sensor driving a frequency-to-voltage (FVC) converter. The TI (former Burr-Brown) VFC32 can serve as both functions (see data sheet). The max VFC frequency can be set to 200 kHz, which should work for analog signals from DC to 30 kHz. The accuracy suffers some at higher full-scale frequency, but the VFC spec at 200 kHz is better than 0.1% linearity (check conditions). The part is still active (Feb 2020). Depending on the application, you may need an analog input offset voltage. This can be very useful if you need to transmit across a very high voltage, with a suitable light pipe.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 20, 2020, 10:31:10 pm
content: Quote from: TimFox on June 20, 2020, 10:03:56 pmThose chips look cool, but they seem stooopid expensive these days for what they do?

author: TimFox
author_rank: Super Contributor
timestamp: June 20, 2020, 10:52:20 pm
content: Mouser lists 81 VFC ICs from $2 to $32 in singles. Obviously, higher spec units will cost more. The VFC32 is a precision part with a good app note in the data sheet.

author: profdc9
author_rank: Frequent Contributor
timestamp: June 20, 2020, 10:55:56 pm
content: How about using a IL300 optocoupler with a op-amp as a voltage-to-current driver for the LED and another as the current-to-voltage driver for the photodiode?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2020, 06:20:27 am
content: Quote from: ricko_uk on June 20, 2020, 08:09:49 pmWell... okay, what's the signal doing, what's it for?Being able to adjust amplitude is a big gimme:- Optos have terrible manufacturing accuracy (CTR 50-200% say)- It changes over time (mainly the LED fades with use, over some years/decades)- And of course temperature tooDistortion is where the gain depends on the signal's instantaneous amplitude (or some past history thereof, but that gets even more complicated). We can express distortion in many useful ways: peak, average or RMS error from a best-fit line; distortion of a sine wave, or more complicated wave (THD); mixing products of two or more sine tones (IMD); etc.Sine distortion is simply that, when a sine wave A sin wt is passed through a nonlinear, one-to-one transfer function f(x), we can substitute it into the Taylor series of the function, f(A sin wt) = f(0) + (A sin wt) f'(0) + (A sin wt)^2 f''(0) / 2 + ..., and reduce the powers of sin^n using trig identities. Some terms will go to DC (e.g., sin^2 x = (1 - cos 2x) / 2 has a constant term 1/2), some will go to the fundamental frequency, others go to harmonics (like the cos 2x term).If you've not taken calculus, Taylor series isn't going to mean anything to you, but suffice it to say, it is an equivalent form to represent a function (given some constraints, which amplifiers obey so we're good), f'(x) means the derivative of the function (well, also not going to mean much, but if nothing else, it's a basic operation we can apply to functions), and the "..." means it's an infinite series (actually N terms with an error term absorbing the difference; if the error term happens to go to zero as N-->infty, great, it's an exact representation).And a function is simply the curve corresponding to pairs of input and output values, so, voltages or currents or whatever.If you were using this for a DC application like setting a power supply's output, you'd most likely be interested in the statistical error approach: how closely does it fit a straight line? Also, what is the zero intercept of that line, do we need to trim that out as well or is it fine as is?For signals, audio for example, THD is the more useful measure. It's not the easiest to calculate, but to be fair, it's not the easiest to measure the transfer curve anyway; at best you can sample some points on it. Easier to measure it with a signal generator and, say, sound recorder, then let a computer do the hard work (Fourier transform perhaps).There's also frequency response, how flat it should be over time -- does the A factor vary with w (frequency) as well? Or for a time domain response, say you apply a step, how fast does it settle down within some threshold bound of the target value?Tim

author: Wolfgang
author_rank: Super Contributor
timestamp: June 21, 2020, 09:48:14 am
content: Quote from: TimFox on June 20, 2020, 10:52:20 pmThere are some IL300 circuits in the IL300 app notes. I built some of those, but they are- not very accurate- a bit noisyThey have better ICs today, I would guess.

author: Zero999
author_rank: Super Contributor
timestamp: June 21, 2020, 09:50:35 am
content: Again, not exactly cheap but Analog Devices make some decent isolation amplifier ICs. The ADuM3190 is the most cost effective one I could find in RS Components.https://uk.rs-online.com/web/p/isolation-amplifiers/7863151/https://docs.rs-online.com/941a/0900766b814bac4c.pdf

author: exe
author_rank: Unknown
timestamp: June 21, 2020, 10:55:53 am
content: I think I tried all the analog optocouplers with feedback on the market (such as il300, etc). They are only precises in ideal conditions. Under mechanical stress they start to noticeably drift. Also there is gain non-linearity*. "precision" versions are also expensive. If taking your requirement "without distortion" literally, then analog optocouplers are not for you. Output is also like 0-500mV, so you need to scale it up. I'd definitely wouldn't use them if accuracy is required.*Datasheets mention "0.01 % servo linearity", and "High gain stability, ± 0.005 %/°C typically", which made me thinking those devices have quite some precision, but then they say "Transfer gain linearity" is +-0.5% typically, which is roughly what I got under ideal conditions (i.e., constant temperature and no mechanical stress).

author: StillTrying
author_rank: Super Contributor
timestamp: June 21, 2020, 02:49:33 pm
content: If I read this with theverysimilar thread:https://www.eevblog.com/forum/projects/optically-transparent-electrical-isolation-of-analog-signal/Opto-isolation might not be needed to get enough RF shielding.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2020, 07:38:16 pm
content: My first recommendation is the Analog Devices ADUM3190 which is inexpensive for a complete isolation amplifier. My second would be circuit based on the IL300 linear optocoupler. Anything else would be a lot more involved.

author: ricko_uk
author_rank: Super Contributor
timestamp: June 21, 2020, 11:19:53 pm
content: Woow, thank you all for so much info!!I need to go through it all and get back with answers to the various questions.In the meantime thank you again!

author: vk6zgo
author_rank: Super Contributor
timestamp: June 22, 2020, 12:09:18 am
content: Such devices were fairly commonly available back in the 1980s, as an alternative to the classic, "coax wound around a ferrite ring" 'hum bucking coils' often used in TV OBs.The disadvantage for that use compared to the latter is that they need a power source.The ones I saw were good to well above 5MHz (PAL video), & maintained the DC component of the signal.

author: ricko_uk
author_rank: Super Contributor
timestamp: July 02, 2020, 03:04:55 pm
content: Thank you all!Tim,Apologies for the delay in replying but have been quite busy.Ref that is it for: I want to achieve two things 1) optocouple two boards and 2) transfer the analog signal from the more noisy (digital and SMPS) PCB to the more silent analog PCB reducing the amount of noise transferred as much as possibleAnd thank you for the long detailed explanation! Always appreciated!!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 04:33:51 pm
content: Does the frequency response have to go down to DC?Can you use a simple transformer?And why would you want an analog signal on the secondary side?It's much more common to have some signal conditioning and feed an analog signal to an ADC and then push a digital data through an isolation barrier, and never convert it to analog anymore.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2020, 04:53:38 pm
content: What is the common mode voltage, DC and AC?If it's nearly zero (common ground, just noisy), use chokes (CMCs) and diff amps!Tim

author: ricko_uk
author_rank: Super Contributor
timestamp: July 02, 2020, 05:56:42 pm
content: Yes. Thank you Tim!

author: fcb
author_rank: Super Contributor
timestamp: July 03, 2020, 06:51:37 pm
content: HP/Broadcom/whoever still make an optocoupler used for video isolation - it’s pretty linear and there are some nice app notes. HPCN4582 or some such.There used to be a nice optocoupler with two photodiodes in it (2nd was used to linearise the LED) - it was designed for DC accurate circuits.

author: Zero999
author_rank: Super Contributor
timestamp: July 04, 2020, 01:07:59 pm
content: I tried Googling for HPC4582 but couldn't find anything.How about using a digital isolator and PWM? Here's an application note from Silicon Labs.https://www.silabs.com/documents/public/application-notes/AN614.pdf

author: exe
author_rank: Unknown
timestamp: July 04, 2020, 01:30:51 pm
content: Quote from: Zero999 on July 04, 2020, 01:07:59 pmAfaik that's how isolated amplifiers work, like those:https://www.silabs.com/isolation/isolated-analog-and-adcs/si892x-isolated-amplifier

author: fcb
author_rank: Super Contributor
timestamp: July 04, 2020, 05:19:11 pm
content: Video bandwidth optocouplerHCNW4562http://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/plastic-wideband-analog-video-optocoupler/hcnw4562Super linear optocouplerHCNR201http://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/high-linearity-analog/hcnr201

author: Warhawk
author_rank: Frequent Contributor
timestamp: July 04, 2020, 07:55:13 pm
content: AMC1311