REV. A

AD7724

–7–

TERMINOLOGY (IDEAL FIR FILTER USED WITH AD7724

[FIGURE 1])

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 zero scale (not to be con-

fused with bipolar zero), a point 0.5 LSB below the first code

transition (100 . . . 00 to 100 . . . 01 in bipolar mode and

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 and 111 . . . 10 to 111 . . . 11 in

unipolar mode). The error is expressed in LSBs.

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

from the ideal VIN(+) voltage which is (VIN(–) + 0.5 LSB)

when operating in the unipolar mode.

Bipolar Offset Error

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

to 000 . . . 00) from the ideal VIN(+) voltage which is (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 minus full scale. The last code 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.

Signal-to-(Noise + Distortion)

Signal-to-(Noise + Distortion) is the measured signal-to-noise

plus distortion ratio at the output of the ADC. The signal is the

rms magnitude of the fundamental. Noise plus distortion is the

rms sum of all of the nonfundamental signals and harmonics up

(Noise + Distortion) is dependent on the number of quantiza-

tion levels used in the digitization process; the more levels, the

smaller the quantization noise. The theoretical Signal-to-(Noise

+ Distortion) ratio for a sine wave input is given by

Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB

where N is the number of bits.

Total Harmonic Distortion

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

of the fundamental. THD is defined as

THD

= 20 log

(V

2

2

+V

3

2

+V

4

2

+V

5

2

+V

6

V

1

sixth harmonic.

Spurious Free Dynamic Range

Spurious free dynamic range is the difference, in dB, between

the peak spurious or harmonic component in the ADC output

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, a spur in the noise floor

limits the SFDR.

Intermodulation Distortion

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

fb, any active device with nonlinearities will create distortion

products at sum and difference frequencies of mfa

± nfb where

m, n = 0, 1, 2, 3, etc. 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).