My wife, who’s not an engineer, recently heard me talk about SINAD and ENOB. After sometime she asked “who are SINAD and ENOB?” She was confused because I use these terms so loosely, a common misstep among us engineers. We use them so loosely, in fact, that sometimes they’re used in the wrong context. Let me explain what I mean:
When I hear ENOB, or effective number of bits, I need to know whether it’s pertaining to the analog to digital converter ( ADC), which is typically given in the product’s datasheet, or the system ENOB meaning that of the ADC, the amplifier, the passive components, the voltage reference (if any), the power supply and anything which generates noise.
In the case of delta sigma converters, ENOB is usually expressed as a function of output data rate and gain. Sometimes the order of the digital filter is also indicated and the results for ENOB are tabulated. So when the question is asked “what is the ENOB of your ADC?” it really should be followed by “at x frequency and at a specific gain”. Some recent delta sigma converters integrate EMI filters with the programmable gain amplifier (PGA) to reject inadvertent noise injection. These are low pass filters whose thermal noise is also accounted for. An example of such device is the ADS1220. For SAR ADC’s, ENOB is typically expressed as a function of input frequency and the voltage reference, just as spectral noise is measured versus frequency for op amps. ENOB is written as (SINAD-1.76)/6.02
SINAD is the ratio of the signal to noise plus any harmonics and is used to evaluate the dynamic performance of the ADC. SINAD is usually plotted over input frequency spectrum at a particular voltage from the reference and the supply.
Effective resolution on the other hand is from a DC perspective. It is written as ln(FSR/RMS noise)/ln2 , where FSR is the full scale range. To get the noise free resolution, use the peak-to-peak noise value instead of the root means square (RMS).
If you want a thorough computation of the noise, you should use the total noise in root sum square (RSS) fashion.
This is especially helpful when using high resolution converters like the ADS8881 with an amplifier to drive them. The result will give you a good indication whether the op amp you selected is good enough, noise wise, to maintain an effective resolution. After all, the last thing you want to do is to throw away bits you’ve already paid for!
