Improving Rds(on) Measurement Accuracy
The advantages of 12 bit technology in an oscilloscope are not always clearly understood. The enhanced resolution is relatively straight forward: an 8 bit A/D converter has 2^8 (=256) discrete levels, whereas a 12 bit A/D converter has 2^12 (4096) discrete levels. Quite a difference!What does this really mean when using an oscilloscope ?
- Finer vertical resolution
- More accurate measurement capability
- Better overall ENOB (effective number of bits) number.
Example – switched mode power supply Rds(on)
To demonstrate the benefits of a 12 bit architecture, we can consider the RDS(on ) of a switched mode power supply i.e. the conduction resistance of the switching FET or IGBT. The overall accuracy depends on accurate current and voltage measurements. This requires fine granularity in the oscilloscope vertical resolution.The switching FET or IGBT will typically have 460V across it in its open state. This will then drop to near zero in its closed state. The resistance of this closed state needs to be measured in real-world conditions.The oscilloscope vertical sensitivity will typically be set to 100V/div. An 8 bit oscilloscope will have a vertical voltage resolution of 800 / 2^8 = 3.125V whereas a 12 bit oscilloscope has a vertical resolution of 800 / 2^12 = 195mV – significantly finer measurement granularity.
Less bits means insufficient resolution
It is clear that the Rds(on) calculated at a voltage resolution of 3.125V does not provide sufficient accuracy. A resolution of 195mV is required – i.e. 12 bits.Figure 1 and Figure 2 show a power supply test where the Rds(on) is calculated by using a 12-bit oscilloscope (in this case a Teledyne LeCroy HDO4000) and a 8-bit Oscilloscope (in this case a Teledyne LeCroy WaveSurfer)In both cases, the Voltage Vds is acquired on channel 1 (top yellow trace) using a differential probe. Channel 2 (red trace) is the Current Ids. The Yellow traces on the bottom of each figure, respectively F1 and Math, display the Rds(on) waveform, calculated by using the basic math function (C1/C2). Clearly an 8-bit oscilloscope is inadequate to make this critical measurement with the required accuracy.
Input channel overload
8 bit oscilloscope users try to achieve accuracy by increasing the vertical sensitivity, ie, change from 100V/div to 20V/div or even 10V/div or less.This does improve the vertical measurement granularity, however, it also overloads the input channel amplifier. In the example above, a 460V input voltage is now being fed into an oscilloscope an input setting of 400V, 160V or less.Input amplifier overload can manifest itself in multiple ways, all of which equate to a loss of measurement accuracy for example:
- erroneous (incorrect) waveform offset
- waveform voltage skewing,
- edge distortion
- other amplifier recovery artifacts.
Figures 3 and 4 show an overload of Channel 1 and consequently an invalid measurement of the Rds(on) caused by increasing the vertical sensitivity.
Conclusion – More Accuracy requires more bits!
Accurate measurements require sufficient vertical granularity which in turn means enough bits in the ADC to achieve an appropriate level of resolution without overloading the input channel amplifier. In the case above it is clear that an 8 bit oscilloscope cannot deliver the required accuracy to measure the RDS(on) and a 12 bit model is required.