I'm new to electronics and have a question about oscilloscopes, if I understand this correct when attaching probes, one of the leads of the probe needs to be grounded, my question is why? Can't I use a oscilloscope just as a voltage-meter to measure the potential difference across the two leads ? What would happend if I attached the "ground" lead to something non-ground ?
Electronic – Oscilloscope probes, measurement
measurementoscilloscope
Related Solutions
The problem is that you are using a MEMS digital accelerometer, and what you are reading is the SCK (serial clock) pin of the serial interface. In order to function, that sensor needs to be interfaced with a microcontroller, that sets it for the sampling frequency, the range and so forth.
So you don't have to expect a square wave with 100Hz frequency, but a fast (depending on the bus bitrate) spike, corresponding to a transmission. Expanding the spike, if the scope is fast enough, you should then see the clock square wave inside the spike.
Moreover, if you don't set the SPI interface correctly, the uC will not generate the clock (the sensor operates in slave mode), and you won't read any value.
If you want to see a 100Hz signal, you could probe the Int pin, which sends an interrupt to the microcontroller every time a measure is available. Then, if you handle the interrupt from the microcontroller properly, you wil see the pulse corresponding to the transmission every 10 ms (100Hz).
But make sure that you're not using motion detection; in that case, only when an acceleration is measured, it will generate the interrupt.
To read the data at the SPI port, the simplest thing is to configure the communication with the sensor; otherwise, it won't send data at all. Then, check if the microcontroller is getting the interrupts and if it's reading the data the sensor gives; you can use a timer to add a timestamp to values and check the frequency they come.
(still WIP)
Oscilloscope probes aren't just pieces of wire with pointy end attached to them. Typical probe will in addition to the pointy stick and the alligator clip have the input attenuation circuitry and impedance matching circuitry inside.
Basically the oscilloscope input front-end has its own internal capacitance and its own internal resistance. In order to prevent signal distortion, the capacitance of the probe needs to match the capacitance of the scope. If it isn't well matched, you'll get overshooting or undershooting. Most scopes have on their front a probe compensation connector which provides test signal that can be used to tune the capacitance of the probe. Here's image of the effect:
The probe's resistance provides input attenuation and it works together with the input resistance of the oscilloscope. Usual 1x/10x probes will have a switch that inserts what's usually a \$9 \mbox{ } M \Omega\$ resistor in series with the signal. In the scope, there's usually a \$1 \mbox{ } M \Omega \$ resistor for input attenuation. In the 1x mode, you only have that resistor, while in the 10x mode, both resistors provide attenuation of \$10 \mbox{ } M \Omega\$.
In parallel with the probe's input resistance you have the compensation capacitor. When buying a probe, you should pay attention that the capacitor's value can match the value of scope's input capacitance.
Another important part of the probe is the tip capacitance. It's modeled as a capacitor in parallel with the signal. It's purpose is to slow down the rise time of the signal entering the probe, which is in general considered a negative effect. The tip capacitance probably won't be of too much importance for a scope such as the one you're considering, but for higher frequencies, it could cause problems when accurate measurement of rise time is needed.
Some probes have what's called high frequency compensation too. You probably don't want to pay for such a probe, but I'll just mention it for completeness. The high frequency compensation part will usually be in the BNC cable connector of the probe and consists of a series connection of a resistor and variable capacitor placed parallel to the signal path. It's used to fix problems with impedance increase with frequency due to cable inductance. Basically the impedance will decrease more or less linearly with the frequency until we reach the resonant frequency of the probe. After that, it will increase. The compensation system is used to move the resonant frequency point away from the frequency range we're interested in using the scope with.
Finally, there's a free book available from Tektronix (if you want to give them your e-mail address) which explains how probes work in great detail. It's called ABC of probes and is currently available from here.
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Best Answer
Depending on the oscilloscope you have (handheld vs. desktop with mains supply), the shield (reference) of your probe will be connected to the chassis ground of the scope. If you have a scope with mains supply, this will be protective earth (PE). If you connect a circuit that is also connected to PE to the reference at a place that is not Ground, current will flow through the shield of your probe and might damage the circuit or the scope.
Additionally, most Scopes are build with RF signals in mind, which require a reference signal close by for signal integrity. That's why you cannot treat them like a simple multimeter.
A handheld scope is easier to use similar to a multimeter, as it does not have a connection to PE. But even then, for high frequencies > 100kHz, you will most likely end up using it in a similar way and connecting the reference to a GND point on your PCB close to the point where you connect to your signal.
If you want to measure the difference between two points on your circuit, use two probes: Connect the reference of each to a GND point close to the probe point. Then on your scope, you can use the math function to view the difference between the two channels, which will give you the same result as if you had measured with a floating probe between the two probe points.