Making sense of ADC specs – Part 1

I’ve been thinking about what to write here for quite some time. There are so many things I want to discuss. I’ll start with discussing ADC specifications.

Like any other analog block, you can’t represent entire ADC performance with just one or two specifications. It’s essential for both ADC designers and system level engineers to understand all the specifications properly.

One can find some/most of these specifications in an ADC datasheet.
*********************************************
Related to resolution
Resolution
Effective resolution
Noise free resolution
Effective number of bits (ENOB)

Related to analog input
Voltage range
Common mode input range
Input impedance

Related to sampling dynamics
Throughput
Acquisition time
Transient response
Aperture delay
Aperture jitter
-3dB input bandwidth

DC specifications
Differential linearity error (DNL)
Integral linearity error (INL)
Gain error
Gain error drift
Offset error
Offset error drift
Zero error
Zero error drift

AC specifications
Dynamic range
Signal-to-Noise ratio (SNR)
Spurious-free dynamic range (SFDR)
Total harmonic distortion (THD)
Signal-to-(Noise+Distortion) – SINAD

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I’ll talk about “Resolution” today. This may be most strange thing for some one who is new to ADC. What does  it mean when it’s mentioned that resolution of a ADC is 12 bits.

a) Will the SNR of that ADC corresponds to 73.7dB (corresponding to 6N+1.6 dB) ?
Usually not. If you can calculate signal to quantization noise of that ADC, it will be 6N+1.6 dB. Usually other thermal noise from various blocks in ADC can degrade it’s SNR from 6N+1.6dB.

b) Will DNL of that ADC is less than 1LSB at 12 bit level ?
Again don’t have to be. So if you’re looking to make a 12 bit accurate system, you should look at DNL/INL specifications rather than resolution.

So what does it mean then ?  It just means that the ADC gives output which is 12 bit representation of the input.

There are couple of other resolution like specifications, which give more insight into accuracy of the ADC. “Noise free resolution” and “Effective resolution” measure ADC performance essentially at DC. All you’ve to know to calculate these specifications are  ADC input range and noise.

Effective resolution = log2 (FS input voltage/RMS noise)
Noise free resolution = log2 (FS input voltage/peak-peak noise)

Under identical conditions, effective resolution is going to be larger than noise free resolution. Noise free resolution has a physical significance. If you take a 20-bit ADC whose noise free resolution is 16 that means last four bits of the digital output keeps flickering.

“Effective number of bits” (ENOB) calculation takes both noise and spectral distortion at that particular sampling frequency.

ENOB = (Signal to noise and distortion ratio-1.76)/6.02

If  these  four specifications are mentioned for an ADC (which never happens), we can understand how many bits ADC output will have, how good is it’s DC performance and how it’s dynamic performance is at a particular sampling frequency.

I’ll discuss few other specifications in my next post. If you have any suggestions/comments, please add them comments section.

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