Back to Basics: Oscilloscope Probes Part 4: Coupling Modes

Posted under Back to Basics, Educational, Hacks, Technology on June 18th, 2023 by
AC vs DC Coupling

Although not fully related to oscilloscope probes per se, I thought it was important to cover the aspect of AC/DC input coupling in scopes. It’s one of those things folks working with an oscilloscope for the first time randomly try to switch between the modes to see if the waveform comes properly. AC and DC coupling are two settings that determine how the input signal is coupled or connected to measurement circuitry.

DC coupling allows the oscilloscope to display both the AC and DC components of the input signal. Meaning if there is a DC shift in the AC signal it will also be shown. Let’s take an example, let’s say you want to measure the AC ripple/noise in a 24V DC power supply. If you connect the probes to the output of a supply and switch to DC coupling it will show the 24V signal + Ripple. But since the vertical screen resolution on a scope is small, you have probably put the voltage division set to 5V/div. Now you just see a flat DC at 24V towards the top, but the amplitude resolution of the ripple is lost as it might be in the range of say 300mV Vpp. Displaying this 300mV AC waveform on a large voltage division setting won’t give you any resolution and hence you can’t see the waveform.

This is where AC coupling helps. Switching to this mode, all it does is block the incoming DC signal(24V here)and you are left with only the ripple waveform, now you can switch to a voltage knob setting of say 100mV/div to clearly see 300mV Vpp waveform for further analysis. How do they implement this? In its basic form, all it is is a switch that puts a high pass filter with a cutoff ranging from 3Hz-10Hz(diff vendors) in the path of the incoming signal which blocks the DC. So here in lies a problem, only switch to the AC mode if you know that measurement freq is high or to block DC. For example, when measuring a 10Hz AC signal in AC coupling mode, the amp of measurement will be wrong as the high pass filter’s roll-off range will not be that sharp for lower freq ranges. So, switch to DC mode in these cases.

Remember the purpose of each mode and choose accordingly. A helpful way to remember is

DC Mode: Think “Direct Coupled/Connection” (Signal un-affected)

AC Mode: Think Only “AC-Coupled” through

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Back to Basics: Passive Oscilloscope Probes: Part 3

Posted under Back to Basics, Educational, Technology on June 11th, 2023 by
Probe Bandwidth

The next important thing to consider when using a probe is its Bandwidth. Fundamentally Bandwidth, from the probe’s perspective, is nothing but the range of frequencies a probe can reliably transport from the circuit being measured to the input of the oscilloscope. It’s defined as the range till the point amplitude of the signal falls to 0.707 times or -3dB from the input. This means that a probe with a BW of let’s say, 50MHz will have the amplitude 3dB lower at 50MHz. This will introduce a 30% measurement error if you are pushing a probe to its end limits. So there is usually a rule of thumb that says when trying to measure a circuit signal with Freq f, use a probe with around 5*f as the bandwidth. This will ensure that the amplitude error is less than 3% as you are way within the spec.

Again, for a square signal, the amount of freq(BW) content is not usually determined by the time period but rather by the rise/fall edges. Bandwith=0.35/RiseTime. So when you are trying to measure a signal with a fast rise time, if the probe BW is not high, the value of the rise time measured will be longer. The same 5 times rule above applies for the rise time case also just to be well within the spec for low measurement error.

You have to always ensure that the probe bandwidth is larger than the oscilloscope input bandwidth. If your scope is limited, even if your probe transports the entire signal, the scope can’t display it without some error. The rule of thumb I use is, Probe BW>=1.5*Scope BW. But do note that some manufacturers underrate the values of probe BW, so you may be able to get away with using a 50MHz probe with 50MHz scope if it’s by the same manufacturer and is matched.

When using the 1x/10x probes do read the probe’s specs carefully, BW is drastically different for 1x and 10x. 10x mode has an order of magnitude higher BW than 1x(Why? That’s a long explanation. Maybe for the future if there is interest). So please do not use them interchangeably when measuring if you don’t know the BW of the signal you are measuring. It’s very easy to screw up on this point.

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