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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
Agilent DSO6054A Bandwidth (-3 dB)
DC-500 MHz 		 DC vertical gain accuracy
±2.0% of full scale
Tektronix DPO4054 Analog Bandwidth (-3 dB)
500 MHz 		DC Gain Accuracy
±1.5% with offset set to 0 V
LeCroy
WaveRunner 6050A 
Bandwidth
500 MHz 		 DC Accuracy
±1.5% of full scale, ≥ 10 mV/div
Yokogawa DL9140 Bandwidth
500 MHz 		 DC Accuracy
±(1.5% of 8 div + offset voltage accuracy)
Fluke 199C Bandwidth
200 MHz 		Accuracy
±(1.5% of reading + 0.04 * range/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.

Agilent 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.