AD7760BCP datasheet(10/22 Pages) AD

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AD7760

Preliminary Technical Data

Rev. PrN | Page 10 of 22

TERMINOLOGY

Signal to (Noise + Distortion) Ratio

The measured ratio of signal to (noise + distortion) at the

output of the ADC. The signal is the rms amplitude of the

fundamental. Noise is the sum of all nonfundamental signals up

to half the sampling frequency (fS/2), excluding dc. The ratio is

dependent on the number of quantization levels in the

digitization process; the more levels, the smaller the

quantization noise. The theoretical signal to (noise + distortion)

ratio for an ideal N-bit converter with a sine wave input is given

(

) (

) dB

Distortion

Noise

Signal

Thus, for an 18-bit converter, this is 110.12dBs and for a 20-bit

converter, 122.16 dB.

Total Harmonic Distortion (THD)

The ratio of the rms sum of harmonics to the fundamental. For

the AD7760, it is defined as

()

THD

log

where:

V1 is the rms amplitude of the fundamental.

V2, V3, V4, V5, and V6 are the rms amplitudes of the second to the

sixth harmonics.

Peak Harmonic or Spurious Noise

The ratio of the rms value of the next largest component in the

ADC output spectrum (up to fS/2 and excluding dc) to the rms

value of the fundamental. Normally, the value of this

specification is determined by the largest harmonic in the

spectrum, but, for ADCs where the harmonics are buried in the

noise floor, it is a noise peak.

Non-Harmonic Spurious Free Dynamic Range (SFDR)

The ratio of the rms signal amplitude to the rms value of the

peak spurious spectral component excluding harmonics.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and

fb, any active device with nonlinearities creates distortion

products at sum and difference frequencies of mfa ± nfb, where

m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are

those for which neither m nor n are equal to zero. For example,

the second-order terms include (fa + fb) and (fa − fb), while the

third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb) and (fa

− 2fb).

The AD7760 is tested using the CCIF standard, where two input

frequencies near the top end of the input bandwidth are used.

In this case, the second-order terms are usually distanced in

frequency from the original sine waves, while the third-order

terms are usually at a frequency close to the input frequencies.

As a result, the second- and third-order terms are specified

separately. The calculation of the intermodulation distortion is

as per the THD specification, where it is the ratio of the rms

sum of the individual distortion products to the rms amplitude

of the sum of the fundamentals expressed in dB.

Integral Nonlinearity (INL)

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function.

Differential Nonlinearity (DNL)

The difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

The deviation of the first code transition (000...000 to 000...001)

from the ideal (that is, AGND + 1 LSB).

Gain Error

The deviation of the last code transition (111...110 to 111...111)

from the ideal (that is, VREF − 1 LSB), after the offset error has

been adjusted out.

Power Supply Rejection Ratio (PSRR)

The ratio of the power in the ADC output at full-scale

frequency, f, to the power of a 100 mV p-p sine wave applied to

the ADC VDD supply of frequency fs. The frequency of this

input varies from 1 kHz to 1 MHz.

( )

(

)

Pfs

PSRR

log

Pf is the power at frequency f in the ADC output; Pfs is the

power at frequency fs in the ADC output.

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