Transfering voltage from floating load into a ground referenced load - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: December 04, 2014, 09:25:01 pm
content: I have a pair of op-amps operating in a bridge arrangement so as to output a higher voltage than would be possible otherwise. However I want to use this voltage in following stages but it is floating, not ground referenced. The simple solution is to use a transformer, which I have done, but it is rather bulky even though HF, still larger than a few components.The final load is around 1K and the voltage into that load is +/-19V at 200KHz. I used the smallest transformer which I had in my drawer, an RM10, with 1:1 ratio and an inductance of around 250uH. It works fine but the transformer is large-ish. I could use a more miniature transformer, and probably I will.However if anyone has any knowledge of an alternative method, I would be grateful to hear it.

author: tom66
author_rank: Super Contributor
timestamp: December 04, 2014, 09:40:01 pm
content: Do you need a significant load on the ground-referenced side? Or is it just low power signal level stuff?

author: PChi
author_rank: Frequent Contributor
timestamp: December 04, 2014, 10:04:22 pm
content: Possibly a differential amplifier would do the job. But I guess this requirement is related to the earlier post about operational amplifiers where it is hard to find one that will have sufficient slew rate and high enough voltage to create +/- 19 V (is that peak or rms?) at 200 kHz. The Texas Instruments OPA552 is close or possibly something like the LME49830 might be suitable.

author: akis
author_rank: Frequent Contributor
timestamp: December 04, 2014, 10:44:19 pm
content: Yes it is the same project. I have a very small board housing an AD9850 DDS (oscillator), an Arduino Mini Pro, and a couple of op-amps for various analogue bits. The AD9850 generates the 200KHz (gets told to do so by the Arduino) which is +/-0.5V. I need it to be +/-19V being fed from a supply of +/-21V up to +/-25V.I have tried using the LM6271IN which is rated at +/-18V, I fed it +/-25V and it gets hot (over 70C), and unstable... It also started to smell. No smoke though.The high voltage OPA445 is useless at 200KHz.I have tried two LM6271IN in a bridge followed by a transformer - all good except transformer is a bit bulky. Of course the "load" is minimal (0.18W) so I could try smaller transformers. I will look to see something smaller than the RM10 which is the one I tried but is way overspecified for the job.In the meantime I have discovered the OPA2604. It has a good GBW (20MHz) and "OK" slew rate (25V/us) maybe it will be OK at 200KHz and accepts up to 48V, 2V short of what my max will be, but I am sure it will not smoke for just two extra volts. The problem is, will it work at 200KHz? The LM6172 has a slew rate of 3000V/us ! I have also tried the LM4562 which has a slew rate of 20V/us and GBW of 55MHz and it also works fine. The OPA445 which fails at 200KHz has a slew rate of 15V/us and 2MHz.So will try that and if it fails I will try the smallest transformer I can actually see without a magnifying glass

author: Marco
author_rank: Super Contributor
timestamp: December 04, 2014, 10:52:40 pm
content: Quote from: akis on December 04, 2014, 09:25:01 pmIf you didn't mind using a transformer why did you bother with the bridge? Couldn't you just have used the transformer for the voltage gain?

author: akis
author_rank: Frequent Contributor
timestamp: December 05, 2014, 12:07:44 am
content: Very correct. The transformer idea came in after a PCB prototype was built using an op-amp to amplify from 0.5V to 8V. Then it was decided that the supply rails would increase from +/-12V to +/-24V. The op-amp was no longer a viable option and since then I have been experimenting with different ideas. However I have not read the specs of the AD9850 to see if it can provide the necessary power. +/-19V into 1K is 0.18W. Maybe a simple transformer will not do and some sort of amplification will be needed.The current idea is to use the OPA2604 as a drop in replacement for the LM6172. If that fails, then back to the LM6172 and a tiny transformer at the end of it. Bridge will not be needed in that case. If however I can somehow use a bridge and avoid the transformer altogether maybe. So now we are back to my original post

author: MK
author_rank: Regular Contributor
timestamp: December 05, 2014, 08:33:27 am
content: AD815, an ADSL chip is fast enough and high voltage too, they recommend a step up transformer for 40VRMS at 500KHz, so it will be a suitable op amp I think, havent looked up the specs for a while though.

author: PChi
author_rank: Frequent Contributor
timestamp: December 05, 2014, 11:01:12 am
content: Quote from: akis on December 04, 2014, 10:44:19 pm

author: MK
author_rank: Regular Contributor
timestamp: December 05, 2014, 10:38:12 pm
content: AD815:900 V/µs Differential Slew RateLow Distortion-66 dB @ 1 MHz THD, RL =200 ?, VOUT = 40 V p-p0.05% and 0.45° Differential Gain andPhase, RL = 25 ? (6 Back-TerminatedVideo Loads)http://www.analog.com/en/high-speed-op-amps/high-output-current-amplifiers/ad815/products/product.htmlcertainly not marginal for your purposes.

author: richard.cs
author_rank: Super Contributor
timestamp: December 06, 2014, 10:58:25 am
content: So you want to drive +/- 19V into 1k from a +/- 0.5V source?If you had a transformer with a turns ratio of around 1:14 then the input side would look like 50 ohms. There are far more solutions already existing for putting a few volts peak-peak into 50 Ohms than there are for the same amount of power at higher voltages. Good video amps will manage it as will various discrete solutions and the monolithic amplifier lumps. Probably anything from 1:10 t 1:20 would be OK giving a impedance of 100 to 25 Ohms respectively and hopefully avoiding the need for custom transformers.