my capacitance can not make ac coupled!!??

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I use Tektronix TDS3034B to measure the output of amplifier, which has a 6v dc voltage. so i use a 10uf capacitance to stop the dc voltage,but maybe dc voltage has a deep friendship with the ac voltage,it still goes through the capacitance as hard as possible. but in the simulation,it really stops the guy(dc v).i don't know what is the problem? is it the problem of my oscillograph?

Our Martians are beginning to learn AVR

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There is leakage in most real capacitors. You need to bleed off that leakage; 100K to GND is usually enough.

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Why not simply put the scope into AC mode???

I have a TDS3034 (at work), great scopes 8)

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The cap could be defective too. also polarity on electrolytics needs to be observed

Why not put the scope in AC mode?

Jim

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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At 10uf, it is probably electrolytic. They have much more leakage. Especially if you don't have the polarity correct.

Analog oscilloscipes typically have used 0.1uf film for AC coupling. But, you have a good Tek scope that surely must have its own AC coupling. Use it!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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At 10uf, it is probably electrolytic. They have much more leakage. Especially if you don't have the polarity correct.

Analog oscilloscipes typically have used 0.1uf film for AC coupling. But, you have a good Tek scope that surely must have its own AC coupling. Use it!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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How have you connected the 10uF cap? Is the negative side towards the scope and positive side towards the 6V+AC?

If there is no other components than the 10uF cap, remember that your scope has a input impedance of 1Mohm, which with a 10x probe would be 10Mohm. So it would take 10 or 100 seconds from the RC constant to charge the cap so that there is 63% of 6VDC on the +ve side and 0V on the -ve side. And that is only the 63% portion, so let's take 50 or 500 seconds before it is at 95% or something.

So basically if you connect a 1k resistor from scope tip to gnd, the capacitor charges almost instantly compared to just the scope impedance.

- Jani

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oh,i know the AC coupling , and i know how to use it. the point is ,i don't want to cheat myself.what i need is to get the real wave ,not a beautiful wave!so i refuse to use AC coupling. for example, an IC connecting 5V and GND,and it outputs a square wave,of course the hign level should be 5v and the low level should be 0. if i use AC coupling, wa~~,it shows me 2.5v and -2.5V,and i believe it is its true wave,then i will be cheated like a fool.
then,i use 10k resistance ,it really works,the wave falls slowly to AC coupling mode.but...
after i place such a resistance, it changes the circuit .in my circuit, the amplitude of the wave is reduced greatly!!
so...
the above methods you have mentioned, still can't give me the real true world.
i am still fighting with SaTan in the deep and dark hell

Our Martians are beginning to learn AVR

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Come on, are you serious? Not using AC mode on your scope, fiddling with external parts that surely will distort your beautiful wave, just because you forgot you left the channel on AC mode?! There's a little mark on the display that says it's in AC or GND mode, bandlimited mode etc!

Common sense will tell you the channel is in AC mode if you see waves between -2.5V and 2.5V on a 5V single supply system, or when your signal falls/rises then settles when you put the probe on a signal.

I only use AC mode to measure ripple and noise on supply lines mostly.

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If you are trying to view a low frequency signal in the presence of dc, the use the vertical offset adjustment on your scope. (not the vertical position adj.) If you need to use a dc free signal for additional processing, use a differential amp with an offset adjusment.

Rick

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AC coupling is AC coupling, whether it is inside an oscilloscope or not.

A square wave between 0V and 5V will come out +2.5 and -2.5 with AC coupling. Period. Even with your 10uf cap. It will simply take longer to reach this state with the big cap. If you think otherwise, you are only fooling yourself.

AC coupling requires the user to comprehend a few things about the circuit and the signals in the circuit. This is no different from using a 10X probe or when you use vertical position to move "ground" to the top or bottom of the screen. You need to be able to take these factiors into account. Same with AC coupling.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net