AD7723 datasheet(12/23 Pages) AD

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AD7723

–12–

REV. 0

TERMINOLOGY

Signal-to-Noise Ratio (SNR)

SNR is the measured signal-to-noise ratio at the output of the

ADC. The signal is the rms magnitude of the fundamental.

Noise is the rms sum of all of the nonfundamental signals up to

half the output data rate (FO/2), excluding dc. The ADC is

evaluated by applying a low noise, low distortion sine wave

signal to the input pins. By generating a Fast Fourier Transform

(FFT) plot, the SNR data can then be obtained from the out-

put spectrum.

Total Harmonic Distortion (THD)

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

value of the fundamental. THD is defined as:

THD

= 20 log

V 2

2 +V

where V1 is the rms amplitude of the fundamental and V2, V3,

V4, V5 and V6 are the rms amplitudes of the second through

sixth harmonics. The THD is also derived from the FFT plot of

the ADC output spectrum.

Spurious Free Dynamic Range (SFDR)

Defined as the difference, in dB, between the peak spurious or

harmonic component in the ADC output spectrum (up to FO/2

and excluding dc) and the rms value of the fundamental.

Normally, the value of this specification will be determined by

the largest harmonic in the output spectrum of the FFT. For

input signals whose second harmonics occur in the stop band

region of the digital filter, the spur in the noise floor limits the

SFDR.

Passband Ripple

The frequency response variation of the AD7723 in the defined

passband frequency range.

Passband Frequency

The frequency up to which the frequency response variation is

within the passband ripple specification.

Cutoff Frequency

The frequency below which the AD7723’s frequency response

will not have more than 3 dB of attenuation.

Stopband Frequency

The frequency above which the AD7723’s frequency response

will be within its stopband attenuation.

Stopband Attenuation

The AD7723’s frequency response will not have less than 90 dB

of attenuation in the stated frequency band.

Integral Nonlinearity

This is the maximum deviation of any code from a straight line

passing through the endpoints of the transfer function. The

endpoints of the transfer function are minus full scale, a point

0.5 LSB below the first code transition (100 . . . 00 to 100 . . . 01

in bipolar mode, 000 . . . 00 to 000 . . . 01 in unipolar mode)

and plus full scale, a point 0.5 LSB above the last code transi-

tion (011 . . . 10 to 011 . . . 11 in bipolar mode, 111 . . . 10 to

111 . . . 11 in unipolar mode). The error is expressed in LSBs.

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB

change between two adjacent codes in the ADC.

Common-Mode Rejection Ratio

The ability of a device to reject the effect of a voltage applied to

both input terminals simultaneously—often through variation of

a ground level—is specified as a common–mode rejection ratio.

CMRR is the ratio of gain for the differential signal to the gain

for the common-mode signal.

Unipolar Offset Error

Unipolar offset error is the deviation of the first code transition

(10 . . . 000 to 10 . . . 001) from the ideal differential voltage

(VIN(+) – VIN(–)+ 0.5 LSB) when operating in the unipolar

mode.

Bipolar Offset Error

This is the deviation of the midscale transition code (111 . . . 11

to 000 . . . 00) from the ideal differential voltage (VIN(+) –

VIN(–) – 0.5 LSB) when operating in the bipolar mode.

Gain Error

The first code transition should occur at an analog value 1/2 LSB

above –full scale. The last transition should occur for an analog

value 1 1/2 LSB below the nominal full scale. Gain error is the

deviation of the actual difference between first and last code

transitions and the ideal difference between first and last code

transitions.

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