In the process of testing my balanced bridge power amplifier, I managed to blow out channel 2 on my PC scope. I was trying the 'ol invert channel 2 method to look at the output of my amp. How do the pros (I know a bunch of you are working on X-amps!) hookup their balanced amps to the scope?
Is a differential probe the only way to go?
Also, I thought I was having HF rolloff problems with my design, but now I'm wondering if I was loading the amp with capacitance by floating the scope. Does this sound like a reasonable consequence?
Thanks for your advice.
Mike
Is a differential probe the only way to go?
Also, I thought I was having HF rolloff problems with my design, but now I'm wondering if I was loading the amp with capacitance by floating the scope. Does this sound like a reasonable consequence?
Thanks for your advice.
Mike
This is not advisable as if any fault develops the case of the 'scope could be at a potentially harmful voltage
Sorry you blew out one channel on your scope. Those can be tricky to fix (even impossible on some analog Tek scopes as the proprietary parts are no longer made).
Floating the scope is NOT a great idea because the chassis of the scope will have half the output voltage of your amp on it. You could zap yourself, or more likely, accidentally ground the scope and short things out and/or end up with the output voltage getting back into the input of your amp which can turn it into a very brief high-power oscillator (before something burns up).
Are you after really precise measurements with the scope or just an idea of when it's clipping? One option would be to use an inexpensive transformer. The ones sold for ground loop isolation of amplifier inputs will work and are about $8 for a pair of them. You can experiment with different resistor loading to get the flattest response out of it (they're usually around a few hundred ohms). They won't pass a 20khz square wave very well, however.
You might be able to limp along just measuring either half of the output circuit for critical measurements, and using the transformer when you need to view the bridged output.
Floating the scope is NOT a great idea because the chassis of the scope will have half the output voltage of your amp on it. You could zap yourself, or more likely, accidentally ground the scope and short things out and/or end up with the output voltage getting back into the input of your amp which can turn it into a very brief high-power oscillator (before something burns up).
Are you after really precise measurements with the scope or just an idea of when it's clipping? One option would be to use an inexpensive transformer. The ones sold for ground loop isolation of amplifier inputs will work and are about $8 for a pair of them. You can experiment with different resistor loading to get the flattest response out of it (they're usually around a few hundred ohms). They won't pass a 20khz square wave very well, however.
You might be able to limp along just measuring either half of the output circuit for critical measurements, and using the transformer when you need to view the bridged output.
Banned
Joined 2002
Banned
Joined 2002
PC scope isolation
I notice that you stated PC (personal computer) scope. Floating a normal scope is not a good idea as pinkmouse has stated. Floating a PC & PC scope is definitely not a good idea. If your scope probe ground is also the PC ground then it will be very difficult to float this - it will be attached via the PC itself and may also be referenced to ground by all the peripherals - monitors, printers etc. Your only chance to float a PC scope would be to use it in a laptop which is not connected to anything else and run the laptop off its internal batteries. Even then I don't think this is a good plan.
You can use a number of ways to measure a differential signal where both sides are at some arbitrary voltage above ground. A differential amplifier such a Tektronix AM502 with a TM500 series power frame can be picked up fairly cheaply at present on the second hand market. Alternatively isolated probes e.g. Tektronix A6902B or high voltage differential probes can be used e.g. Tektronix P5200, P5205 or P5210. Unfortunately all of these specialist probes are quite expensive, even second hand.
Can you give us a bit more information about the test setup you were using when the problem occurred? Ch1-Ch2 is a common technique for taking low bandwidth differential measurements. The critical point to note here is where the grounds of the two probes were connected.
James
I notice that you stated PC (personal computer) scope. Floating a normal scope is not a good idea as pinkmouse has stated. Floating a PC & PC scope is definitely not a good idea. If your scope probe ground is also the PC ground then it will be very difficult to float this - it will be attached via the PC itself and may also be referenced to ground by all the peripherals - monitors, printers etc. Your only chance to float a PC scope would be to use it in a laptop which is not connected to anything else and run the laptop off its internal batteries. Even then I don't think this is a good plan.
You can use a number of ways to measure a differential signal where both sides are at some arbitrary voltage above ground. A differential amplifier such a Tektronix AM502 with a TM500 series power frame can be picked up fairly cheaply at present on the second hand market. Alternatively isolated probes e.g. Tektronix A6902B or high voltage differential probes can be used e.g. Tektronix P5200, P5205 or P5210. Unfortunately all of these specialist probes are quite expensive, even second hand.
Can you give us a bit more information about the test setup you were using when the problem occurred? Ch1-Ch2 is a common technique for taking low bandwidth differential measurements. The critical point to note here is where the grounds of the two probes were connected.
James
a couple sources
EDN mag had an article on differentail measurement here's a link to one from 1996 http://www.e-insite.net/ednmag/archives/1996/092696/20df2.htm
EDN mag had an article on differentail measurement here's a link to one from 1996 http://www.e-insite.net/ednmag/archives/1996/092696/20df2.htm
When using two channels as shown in your diagram, you won't
need to float the PC or the scope. I use the same method with a
TEK TDS220 'scope alone and it works fine. The TEK also gives
you a math function that internally subtracts one channel from the
other and displays the difference as a single waveform. Which PC
scope are you using, and does it give you this ability?
I use a battery-powered PC based scope and laptop for floating
distortion measurements, but have trouble getting clean low-level
readings due to the large and variable EMI spectrum put out by
the laptop. Putting distance between the laptop and the test
probes helps, but doesn't eliminate the problem. I'd love to find a
solution to this other than buying a differential probe with the
high CMRR needed to do the same job.
need to float the PC or the scope. I use the same method with a
TEK TDS220 'scope alone and it works fine. The TEK also gives
you a math function that internally subtracts one channel from the
other and displays the difference as a single waveform. Which PC
scope are you using, and does it give you this ability?
I use a battery-powered PC based scope and laptop for floating
distortion measurements, but have trouble getting clean low-level
readings due to the large and variable EMI spectrum put out by
the laptop. Putting distance between the laptop and the test
probes helps, but doesn't eliminate the problem. I'd love to find a
solution to this other than buying a differential probe with the
high CMRR needed to do the same job.
I only see 3 methods:
-use an earth isolated scope-> a battery powered one (don't unplug yours from main's earth!!!)
-if you have a 2 channel scope, connect each channel to an output, and play with the A+B or A-B mode
-at school we have a differential scope, but don't even want to know it's price
-use an earth isolated scope-> a battery powered one (don't unplug yours from main's earth!!!)
-if you have a 2 channel scope, connect each channel to an output, and play with the A+B or A-B mode
-at school we have a differential scope, but don't even want to know it's price
Hi Joe:
I'm using the velleman PCS64i. I'm not really sure how I damaged the B channel. Maybe its just coincidental. The software does have add and subtract capability. It also has FFT, which is pretty cool. A battery pack is optional. I might have to look into a Picoscope now, but they're a bit pricier. Pico also sells a differential probe, but its something like 300 bucks.
In your experience does using the scope in this manner have any effect on HF performance?
Does the EMI travel through the parallel cable? If not maybe you could throw a thick copper pot over the laptop!
Thanks
Mike
I'm using the velleman PCS64i. I'm not really sure how I damaged the B channel. Maybe its just coincidental. The software does have add and subtract capability. It also has FFT, which is pretty cool. A battery pack is optional. I might have to look into a Picoscope now, but they're a bit pricier. Pico also sells a differential probe, but its something like 300 bucks.
In your experience does using the scope in this manner have any effect on HF performance?
Does the EMI travel through the parallel cable? If not maybe you could throw a thick copper pot over the laptop!
Thanks
Mike
mrothacher said:
I have a PICO and it works well, but the software does not yet
have add and subtract. You can do it manually by dumping the
data file into Excel, but yeesh...I have asked the mfr more than
once to add math functions, but no word on whether it's coming.
I've seen problems like that when trying to float the TEK scope,
even when using an AC isolation transformer, so I wouldn't risk
using your one remaining good channel in a floating setup. If the
Vellman is a kit that you can troubleshoot and fix yourself, I'd fix
the B channel and use the setup you drew earlier, but grounded.
In that case the scope should have no excuse but to work
properly and not break, if used within specs.
Or maybe throw a thick copper pot AT the laptop ...
butnow that you mention it, I think some of the EMI must be coming
in on the cable. Hmmm, maybe I can filter it out electrically.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.