AD7725 datasheet(13/28 Pages) AD

13 / 28 page

REV. A

AD7725

–13–

TERMINOLOGY

Integral Nonlinearity (INL)

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 zero scale (not to be

confused with bipolar zero), 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 full scale, a

point 0.5 LSB above the last code transition (011 ...10 to

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

unipolar mode). The error is expressed in LSBs.

Differential Nonlinearity (DNL)

This is the difference between the measured and the ideal

1LSB change between two adjacent codes in the ADC.

Unipolar Offset Error

Unipolar offset error is the deviation of the first code transition

from the ideal V

IN(+) voltage, which is (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 V

IN(+) voltage,

which is (V

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

Gain Error

The first code transition should occur at an analog value

0.5 LSB above negative full scale. The last code transition

should occur for an analog value 1.5 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.

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 (f

O/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 Trans-

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

output 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

VVV

where V

1 is the rms amplitude of the fundamental and V2, V3,

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

sixth harmonics.

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

O/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.

Settling Time and Group Delay

The settling time of a digital filter is dependent on the amount

of decimation employed and the number of filter taps used in

the filter design and is calculated as follows:

settling time

data input rate

number of taps











 ×

The settling time for each filter stage should be calculated

separately and then added to get the total filter settling time.

Group delay is half the settling time.

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