What is the amplitude accuracy of my oscilloscope?
When oscilloscope manufacturers specify vertical accuracy, they typically only mention bandwidth and DC accuracy. The bandwidth specification addresses the AC accuracy. Tighter AC accuracy specifications are very uncommon.
Occasionally you might see a vertical accuracy specification that is not explicitly DC or AC. Your next question should then be, "Over what range of frequencies?"
Perhaps the most important oscilloscope specification is bandwidth. A scope's bandwidth is the range of frequencies over which the scope's response is almost constant, to within 3 dB (±29.3%). Bandwidth is usually the only specification a manufacturer provides that addresses AC amplitude accuracy.
Let's take a look at the relevant specifications of five oscilloscope models from five different manufacturers. These specs were taken directly from the scopes' data sheets or manuals. All published specs related to vertical accuracy, with the exception of offset accuracy, appear in the table below.
| Oscilloscope | Bandwidth specification | Other amplitude accuracy specification |
|---|---|---|
| Keysight DSOX3104T |
Bandwidth (-3dB) 1 GHz |
DC vertical gain accuracy Specifications are valid after a 30-minute warm-up period and ± 10 °C from firmware calibration temperature |
| Tektronix MDO3104 |
Bandwidth (-3dB) 1 GHz |
DC gain accuracy ±1.5% for 5 mV/div and above, derated at 0.10%/°C above 30 °C ±2.0% for 2 mV/div, derated at 0.10%/°C above 30 °C ±2.5% for 1 mV/div, derated at 0.10%/°C above 30 °C ±3.0% for variable gain, derated 0.10%/°C above 30 °C |
| LeCroy WaveSurfer 3054 |
Bandwidth (-3dB) 500 MHz |
DC Gain Accuracy ±(1.5%) Full Scale, Offset at 0V, > 5mV/div; ±(2.5%) < 5 mV/div |
| Rohde & Schwarz RTM2104 |
Bandwidth (-3dB) 1 GHz |
DC gain accuracy input sensitivity > 5 mV/div ±1.5 % input sensitivity ≤ 5 mV/div ±2 % offset and position = 0 maximum operating temperature change of ±5 °C after self-alignment |
| Fluke 190-504 |
Bandwidth (-3dB) 500 MHz |
Accuracy ± (2.1 % of reading + 0.04 x range/div) @ 5 mV/div to 100 V/div |
You will notice that none of the scopes in the above table have an explicit AC accuracy specification. The Fluke model has a generic "Accuracy" specification, but no frequency range is given. Given the definition of bandwidth, we expect this 200 MHz scope's response to be down by 3 dB at (or a little above) 200 MHz. 3 dB is 29.3%, but the accuracy specification is only 1.5%. So it is not clear whether the accuracy is 29.3% or 1.5% at 200 MHz. Unless the bandwidth specification is extremely conservative, the accuracy specification conflicts with it. One might conclude that the accuracy specification only applies at or near DC.
Keysight tests its scopes to a higher standard than 3 dB before shipping them to customers. Presumably other scope manufacturers do the same. You should find that most scopes have a relatively flat frequency response that is generally much better than the guaranteed 3 dB across most of the passband. But due to various design and manufacturing challenges, it is generally not practical for a manufacturer to guarantee AC accuracy better than ±3 dB.