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Data Sheet

AD7356

Rev. B | Page 11 of 20

Total HarmonicDistortion(THD)

THD is the ratio of the rms sum of harmonics to the

fundamental. For the AD7356, it is defined as

( )

THD

log

−

where:

V1 is the rms amplitudeof the fundamental.

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

through the sixth harmonics.

Spurious Free DynamicRange (SFDR)

SFDR is 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 valueof this

specification is determined by thelargest harmonicin the

spectrum, but for ADCswherethe harmonics are buriedin

the noise floor, it is a noise peak.

IntermodulationDistortion(IMD)

With inputs consisting of sine waves at two frequencies, fa

and fb, any active device with nonlinearities createsdistortion

products at sum and difference frequencies of mfa± nfb where

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

are thosefor which neitherm nor n is equal tozero.For example,

the second-ordertermsinclude (fa + fb)and (fa − fb), while the

third-ordertermsinclude (2fa + fb), (2fa − fb), (fa + 2fb), and

(fa − 2fb).

The AD7356 is tested using the CCIF standard where twoinput

frequencies nearthe top end of the inputbandwidth areused.

In this case, the second-order termsare usually distanced in

frequency from theoriginal sine waves and the third-order

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

As a result, the second-and third-order termsare 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 rmsamplitude

of the sum of the fundamentals expressed in decibels.

Thermal Hysteresis

Thermal hysteresis is defined as the absolutemaximumchange

of referenceoutput voltage after the device is cycled through

temperature from either

T_HYS+ = +25°C to TMAX to +25°C

T_HYS– = +25°C to TMIN to +25°C

Thermal hysteresis is expressedin ppm using the following

equation:

)

(

)

(

)

(

)

ppm

(

−

HYS

REF

HYS

where:

VREF(25°C) is VREF at 25°C.

VREF(T_HYS) is the maximum change of VREF at T_HYS+

or T_HYS–.

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