In the datasheet of ADE7953 it is mentioned that the ADC output swings between  -6500000*LSB(decimal) to 6500000*LSB(decimal). It totals up to 13,000,000 digital values but as the ADC output is 24-bit data-word which corresponds to 2^24 = 16,777,216 digital values.

So, where are the rest of 16,777,216 -13,000,000 = 3,777,216 digital values are being utilized?

Thank you

Regards

Satya Prakash Singh

• The ADC output is the output of the sinc filter. The performance of the ADE7953 is at a max adc input. The sigma-delta is run at about 80%(80% bitstream) of full-scale as performance declines above this level. So 13/16.77 is aprox 77%. This is our usable range. In a surge condition the adc input can be larger and you will get more codes but they may not meet our datasheet spec. Also the max codes are dependent of the reference voltage and gain errors in the ADC . This is why we calibrate to know inputs. The effective number of bits in the adc is around 12 the rest is noise.

adc snr = 72db = 3981.072   = aprox  2^12 = 4096 = 72.24db

Dave

• The effective number of bits in the adc is around 12 the rest is noise.

Dear Dave,

Could you please explain the above-quoted point a bit more? It would be very helpful.

Thanks and regards

Satya Prakash

• HI Satya,

www.analog.com/.../raq-issue-90.html

We spec an adc word of 24bits but the snr determines the number of bit useful for a single read at the sample rate. In this case SNR is 72db which approximates the upper12 bits.

Dave

• Dear Dave,

So, I can understand from the documentation you mentioned that with decreasing SNR ratio, the ENOB decreases, and the mentioned ENOB is at the worst SNR value.  Did I get it correctly?

Thanks and Regards

Satya Prakash

• Yes you are correct. At full scale you will have aprox ENOB of 12bit resolution at lower input levels your noise floor is the same. So if your input signal is down 20db  snr is now 72db -20db you will have ENOB = (72 - 20 – 1.76)/6.02 dB = 8bits

Dave