Attenuating a 12Vpp signal to drive a 50ohm load with low current - Page 1
Forum section: electronics / projects

author: DaJMasta
author_rank: Super Contributor
timestamp: September 30, 2016, 08:09:36 pm
content: This may be more on the beginner side of things, but it is technical and I'm more having trouble with choosing the best option than just coming up with some solution.So I have an audio signal that for the purposes of processing in my circuit, ends up at up to 12Vpp centered around 6v (single supply), because of auto-gain and such, I expect it to be around 11Vpp most of the time. I want to read out this signal after the amplification stage and have a non-inverting buffer amp to drive the output, but I want to drive a standard 50 ohm terminated line. I also have a later stage (output of a comparator) that may not need to be buffered, but which again I want to drive a 50 ohm line. The trouble is, the opamps I'm using to keep the fidelity of the audio really high (.00003% THD in the audio band) can only source 31mA continuously, and I'd like to keep it under that to be safe, and that's not great for driving a 50 ohm load at 12V.So I need an attenuator of some sort, what I came up with was a 550 ohm resistor in series with the output of the amp (so up to a 1Vpp signal when attached to a 50 ohm load) which sinks 20mA, which is within the opamp spec, but it seems higher than I'd like, and I don't want to bring the signal amplitude too low to keep my triggering options open for various instruments I'd like to hook it up to and to stay as far above the noise floor as possible - ideally 2Vpp would be my target, but I don't know of a way to do it with these opamps.Does it seem like my inline resistor solution seem sufficient? Should I just be AC coupling the outputs (good caps large enough for ~20Hz aren't super cheap, but it's doable)? Is there an attenuating buffer amp configuration that would be a good option (I have a spare since they're two per package, so I could put one on the comparator output as well)? Should I just 'require' high impedance termination from my instruments and not worry about my output buffer currents so much?Looking for the solution that has the most utility without sacrificing output fidelity, thanks!

author: Marco
author_rank: Super Contributor
timestamp: September 30, 2016, 10:26:43 pm
content: Do you connect your equipment with km long wires? If not I'd go high impedance.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 30, 2016, 11:19:36 pm
content: No, I'm expecting around 1/1000th of thatPoint taken, I suppose there's not necessarily a big benefit to being able to drive a 50 ohm line, and there are not many instruments that only accept a 50 ohm input.I've got a 15k resistor in series with the output of the buffer amp and a 3.3k to ground, giving me about 18% of the original signal now (2.1Vpp or so). I think I will AC couple the audio out because that will take out the virtual ground offset, but I'll just use another of the same caps I've got on the inputs to that signal path. Sound reasonable?

author: David Hess
author_rank: Super Contributor
timestamp: October 02, 2016, 06:35:23 pm
content: Lots of circuits use a series resistor to drive a terminated 50 ohm transmission line from a high voltage source without problems.If you are paranoid, then the simple alternative is to use a series and shunt resistor pair (L attenuator) to provide a high input impedance to the high voltage source, attenuate the signal, and provide a terminated source to drive the 50 ohm transmission line but it is probably not necessary.