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How does AC coupling work? 3206

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How does AC coupling work? 3206

Postby ariaaudio » Tue Mar 27, 2007 3:31 pm

I'm trying to view an AC waveform riding on a DC voltage. The DC voltage is about 400 volts, and the AC is less than 100mV. I'm using a /10 probe. With my standalone LeCroy digital scope, and my creaky old analog Tektronics scope, AC coupling completely blocks the DC component so I can view the AC component with no visible DC offset. However, the 3206 seems to work differently, perhaps by subtracting the DC component in software? Anyway, I can't view the AC signal because the trace is offset to the top of the scope window and the little "!" indicates overload. Should I just use a blocking capacitor on my scope probe?
-- mike elliott
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Postby ziko » Wed Mar 28, 2007 11:31 am

Hi and thank you for your post. You say you are measuring 400V and using a /10 probe, well this will attenuate it to 40V which is beyond what the 3206 can measure (+/-20V). In any case at such high voltages we recommend using a differential oscilloscope probe due to the safety aspect.

http://accessories.picotech.com/oscillo ... robes.html

(please scroll to the end)

You can get rid of the DC component with our software.

Hope this helps.
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Postby ariaaudio » Wed Mar 28, 2007 5:30 pm

Thank you. I think I understand. A standalone oscilloscope blocks DC using a coupling capacitor. The Picoscope products do no use a capacitor, instead they use software to remove the DC component.

I have the Picoscope differential probe. Also I will make some adapters with blocking capacitors.
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Postby alan » Thu Mar 29, 2007 6:18 pm

Hi Mike,

Our scopes work in the same way as benchtop scopes - when you switch to AC a capacitor is used to block the DC signal.

Although the signal you are measuring is technically an overload I am surprised it does not work. Please can I ask you to try measuring say the output from a 9V battery using a scope probe on its x1 setting. On the DC setting you should see 9V on the scope on the AC it should be 0V.

If not then either there is a software problem not switching from AC to DC or possibly a hardware fault. If this does not work properly on CHA try B, if the 2 channels work differently then its going to be a hardware issue.

Best Regards
Alan
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Postby ariaaudio » Fri Mar 30, 2007 5:45 pm

Thanks, Alan. I set the inputs to AC coupling, +/- 100mV sensitivity, and shorted the X1 probe together and nulled out Channel A. Viewing with Current/Filtered (to reduce noise), the trace set itself smack on the 0V line. I then connected the probe to a 9V battery and gave it a minute to settle. The trace rode a smidge higher, so that the dashed 0V line is visible below it. If I had to guess, it looks like maybe 1mV. Same with Channel B.

This looks like the coupling capacitors have some leakage. With 9V of DC this is hardly an issue, but going to 400VDC and using the X10 probe setting, I reckon the probe is presenting 40VDC to the scope, which appears to be less than the +/-100V maximum I see in the user's guide for this model. If blocked, of course, it would be zero volts, so all the ADC should see is any AC riding on the blocked DC.

But what I get with a 400VDC signal (AC coupling, X10 probe, +/-20V sensitivity) is a +8.2VDC offset on CH A, +7V on CH B.

With a external blocking capacitor (0.1uF/600V film) at the scope input, the leakage DC offset drops to +10mV. I reckon that my scheme of providing an external DC blocking capacitor would work, but since the 3206 has blocking capacitors inside, it might be possible to replace them with something that has less leakage?

We're quite capable of doing a simple capacitor replacement (he said, not having opened the 3206 to look within) -- is it possible to get a service manual or other documentation?
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Postby alan » Mon Apr 02, 2007 3:26 pm

Hi,

From your tests it sounds as if the AC/DC coupling in the scope is actually working - thanks for checking this.

The problem here really comes down to feeding a 400V signal into a scope with a 20V range. When the scope is set to DC coupling, both the AC and DC parts of your signal will be attenuated, using a x10 probe you will still see 40V which is an overload for the scope.

When the scope is set to AC, then the AC part of the signal will be attenuated by a factor of 10, but your DC signal will not be attenuated by your probe, the scope input will see a 400V signal which is an overload.

You have three options here - one is to use an external capacitor as you have done. This has obviously to be rated at > 400V and be of a low leakage type.

The second option is to attenuate your signal before the scope - for automotive use we sell a 20:1 external attenuator. http://www.picotech.com/auto/attenuator.html This has a max input of 300V, but using this at 400V is going to be much less of an overload that putting 400V into a 20V scope input.

The third option is to use an active probe such as the x10/x100 probe from http://accessories.picotech.com/oscillo ... robes.html

I hope this helps.

Regards
Alan
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Postby ariaaudio » Sun Apr 08, 2007 5:54 am

Hi Alan, thanks for the reply.

As I understand it, switching the probe to X10 places a 9Meg ohm resistor in series with the signal. This forms 10:1 voltage divider when working against the scope's 1M input impedance -- for AC as well as DC. That does, as you say, exceed the scope's 20V max input level. But if the input of the scope consists of a 1M resistor to ground followed by a coupling capacitor (when in AC coupling) then the DC component should be of no consequence, as the capacitor will block it. However, if there are protection devices directly at the BNC inputs which limit voltages to +/-20, then yeah -- the 40V would be a problem.

What is the input circuit?
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Postby alan » Tue Apr 10, 2007 8:29 am

Hi,

In DC mode the 9M resistor in the probe forms a 10:1 attenutor with the 1M input impedance of the scope. In DC mode both AC and DC signals will be attenuated by a factor of 10.

In AC mode the blocking capacitor is switched in before the 1M input impedance of the scope. AC signals will be attenuated by a factor of 10, but from a DC point of view the input impedance of the scope is (ideally) infinite due to the capacitor. In other words the input to the scope will see 400V not 40V. I would not worry about a overload at 40V, but 400V could cause problems.

The reason we (and all other scope manufacturers) have the capacitor at before, not after the attenuator is to present a high impedance (ie not load) to DC signals when in AC mode.

Regards
Alan
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Postby ariaaudio » Tue Apr 10, 2007 2:07 pm

Thank you, Alan.

So if I am visualizing thing correctly, in X10 AC mode, the signal passes first through the probe's internal 9M resistor, into the scope's BNC, then to a series coupling cap (which blocks the DC component), then it sees a 1M resistor shunting to ground.

If that's the case, then the external blocking capacitor I used in the experiment (above), is essentially just in series with the scope's internal capacitor, and that the results I got in that experiment suggest that the scope's internal cap is more leaky than the film cap I used for the experiment.

So if I wanted to watch that small AC signal riding my my DC signal, the low-leakage part could be used external to the BNC (as in my experiment), or inside the scope, in place of the stock part?
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Postby alan » Tue Apr 10, 2007 2:20 pm

Yes you are correct in your understanding.

If you have a suitable low leakage capacitor that will withstand the 400V and then it would be fine to connect this in series with the input of the oscilloscope. The scope can then either be set in AC or DC mode.

I would not recommend opening up the scope to change the internal capacitor as apart from invalidating the warranty you risk changing the AC/DC cut off frequency, input capacitance etc etc.

Regards
Alan
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Postby ariaaudio » Tue Apr 10, 2007 3:39 pm

Thanks, Alan. Since the capacitor in the scope is the first thing the signal hits, I reckon that as long as it is rated for the (unattenuated) voltage that the probe is connected to, then high DC voltages can't hurt anything, unless the charging current that occurs when the probe is first connected to the signal can cause a dangerous voltage spike at the output of the capacitor which the scope can't handle. What's the voltage rating of the internal capacitor?
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Postby alan » Tue Apr 10, 2007 4:05 pm

Hi Mike,

I hope it does not sound as if we are being difficult here, but we would rather not give such data on components within the scope. As a simple example the voltage rating of the capacitor is only one factor - if it is charged to too high a voltage and then the scope is switched to DC then the current rating in the relay used to switch from AC to DC may be exceeded as the capacitor discharges. There are also issues with input protection and safety ratings. The published specification of the scope takes all this into account.

As mentioned feel free to use an external blocking capacitor but please do not feed more than the 100V mentioned in the specification for max overload into the scope.

Regards
Alan
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Postby ariaaudio » Wed Apr 11, 2007 2:52 pm

Thanks, Alan -- I quite understand: proprietary is proprietary. I work on a lot of old-school equipment, like tube guitar amps, that have high DC voltages and am used to scopes that were built to handle those voltages. Just switching over to newer USB scopes now and am trying to get familiar with their performance criteria. I'll put a big "100VDC Max" sign by the scope's BNCs and build up some external blocking capacitor thingies for when I need to look at higher voltage stuff.
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Re: How does AC coupling work? 3206

Postby tomfield » Wed Aug 10, 2011 3:08 pm

This discussion sounds kind of scary concerning high voltages. As the differential isolating probes are kind of expensive, my intended approach for measuring high voltages wrt ground was to go with 100x (100meg input impedance) probes purchased elsewhere (like the CS1002 Analog Probe that the Cleverscope people sell rated at 1.5Kv).

I assume that that should be safe, provided that I only set my PicoScope 4227 for DC input, but what if I by accident select AC input on the PicoScope 4227? That could apply a very high DC voltage to the internal series AC capacitor on the PicoScope 4227 which is rated at who knows what voltage.......maybe real small. Does your 20x 300v attenuator help with this?
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Re: How does AC coupling work? 3206

Postby PeterF » Fri Aug 12, 2011 9:30 am

Hi Tom,
For safety, when measuring high voltages we would always recommend our active differential probes. If you must use a passive x100 probe make sure that you leave the scope set at DC input (the default setting) if there is DC present greater than the voltage rating of the scope. By starting with DC and auto ranging, you can measure the DC present and, if safe, select AC which will allow you to increase the input sensitivity to view the AC component. You are unlikely to accidentally select AC as the DC/AC selection pull-down is rarely used and is a "Two-click" process.
Regards,
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