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PicoByte and AC Coupling: When to use it and why

18/10/22

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AC Coupling on an oscilloscope is an often misunderstood feature; sometimes it's not used because it's not fully understood how it can help, and other times it's used when it does not need to be. This poses the following question: what is AC Coupling, and when can it be used?

This month's PicoByte offers a brief explanation of AC Coupling. Meanwhile, this article provides further information and examples on the subject...

 

What does AC Coupling do?

 

Simply put, AC Coupling uses filtering to remove the DC component of a signal, displaying just the AC component of the signal alternating around 0 VDC (Ground).

This coupling mode allows measurement of AC signals such as: ripple along a DC power supply, or measuring a biased signal through a transistor amplifier. AC Coupling will remove the DC component, revealing just the AC signal. 


What does that mean?


For every signal there are two main components, the DC and AC components. DC being the Y-Axis offset of the signal; how high the signal is on the display. AC is the alternating section of the signal, the bit that goes up and down on the display. 

What DC Coupling shows is both components at the same time, it will show the alternating signal, offset by an amount of DC volts. 

AC on the other hand removes this DC component (DC offset) showing just the AC component, showing the full AC signal, alternating around GND. 


Why use AC Coupling? DC works just fine!


AC Coupling comes into its own when measuring something such as ripple in a DC power supply. This is a brilliant example as the DC offset could be anything from 5V to 12V, and the ripple sits on top of the signal with an amplitude from 1-100 mVpk-pk. 

When zoomed into the ripple at a range of ±20 V that 1-100 mVpk-pk won’t show up that well, or at all. This is because the resolution of the majority of PicoScopes stands at 8 bits. This means that the ±20 V range is separated into 255 segments of around 150 mV making ripple of 1-100 mV almost impossible to measure.

To display this ripple as a measurable signal, switch to AC Coupling via the channel menu, this will bring the waveform down to ground. Decrease the voltage scale to around 20-100 mV revealing the alternating ripple signal. From here, any measurements can be made using the ruler or ‘Measurements’ from the tool-park menu on the left. 


How does AC Coupling work?


AC Coupling switches the input signal into a hardware high-pass filter to remove the DC component. Theoretically in an electronics application, a capacitor blocks DC and as it feeds into a 1 MΩ input, it becomes a high-pass filter, blocking the 0 Hz DC voltage, but subsequently blocks and attenuates some low frequencies as a result. 

AC coupling in a PicoScope uses a 1 nF capacitor with the 1 MΩ input impedance resulting in the circuit looking something like this:

Resulting in a frequency cut-off of around 1.6 Hz which can be verified using a frequency sweep on the Spectrum Analyser display. 

It’s important to take into consideration the added attenuation as this will be the key factor when determining whether to use AC or DC coupling, which leads onto the next point.


When not to use AC Coupling.


AC coupling is very important when measuring an offset AC signal, but the added attenuation and blocking adds problems for some AC signals, such as the Square Wave, or low-frequency sine wave as they can start to attenuate in weird, unpredictable ways. 

As a rule of thumb, stick to DC Coupling. When measuring a waveform such as a power supply ripple, or a biased transistor amplifier input where the AC waveform is offset by a large voltage, switch to AC Coupling to provide an aid to reading these AC waveforms effectively. 


Is there an alternative?


With some offset AC waveforms, where it’s not possible to use AC coupling. Such as summing two sine waves, the built in AC coupling can’t remove a sine wave offset unless it’s below the 1.6 Hz cutoff frequency mentioned previously. 

Waveforms like this can be viewed using higher resolution PicoScopes such as the FlexRes 5000 and 6000 series scopes. This will provide up to 16 bits of resolution at the trade-off of reducing the sampling speed. 

By using a 16-bit resolution, ±20 V range will have segments of around 600 μV, providing much more detail in the waveform capture allowing accurate measurement of smaller summed waveforms on much larger waveforms. 
Below is an example of a low-frequency, high-voltage sine wave with a high-frequency, low-voltage sine wave summed together in 8-bit resolution and 16-bit resolution showing the capability of these FlexRes scopes in this context. 

For more information on our FlexRes Scopes, check out our A-Z Library article on FlexRes, or  go to our FlexRes 5000 series products.