It is very important to know the limitations of your instruments. I have been stupid enough to measure a DC voltage from my chip which had an output impedance close to a mega ohm with a multimeter whose input impedance was also close to a mega ohm! I kept seeing a lower than expected output voltage until I swapped the meter with a 10Gohm meter then I saw the correct value!
Multimeters
- Normal Digital Multimeters have an input resistance of 1Mohm or 10Mohm.
Usage Tips
- Doing a measurement with your DMM in the ACV position on your DC circuit will give a quick indication of any excess ripple on the supply when you don't have a scope at hand.
- You can almost always determine the leads of a bipolar transistor with an ohm meter. b-e and b-c junctions will measure like a diode with the b-c junction reading slightly lower than the b-e junction when forward biased.
- DMM can upset sensitive circuits from noise generated inside it. It is always good to test your instruments when in doubt by doing some sanity check measurements.
- When probing directly on a crystal of a uP, use 10kOhm or so resistor in series with the probe tip to prevent loading from stopping the oscillator.
- When measuring the voltage of a high impedance output always make sure your DMM input impedance is high enough so as not to load the output and change the reading.
Spectrum Analyzer
Usage Tips
- Making measurements near a spec-analyzer's noise floor will give 3dB errors.
Oscilloscopes
- Oscilloscope grounds are generally the common ground so you cannot connect 1 probe ground to something else and the other to something else. Use either diff probes or measure the 2 signals separately
Probes
- Oscilloscopes have an input capacitance of 13pF but that is quite different what the circuit would see. The circuit sees the probe capacitance. This shows the 10:1 attenuation on the probe.
- R2, R3 and C2 are tuned out to produce the best frequency response. See the article:Scope Probe Schematic
- The higher the attenuation on the scope probe the smaller is the probe input capacitance. For example a 1:1 probe will have an input capacitance of 50pF to 100pF while a 10:1 probe will have 9pF to 15pF of input capacitance.
- Also see this excellent article explaining the anatomy of probes and the type of probes and how they effect measurements and how to choose a probe here.
Usage Tips
- When probing directly on a crystal of a uP, use 10kOhm or so resistor in series with the probe tip to prevent loading from stopping the oscillator.
- It is easier to see what is happening on the ports using a scope when you trigger one channel against the CPU clock.