Datasheet AD7724 (Analog Devices) - 8

AD7724 TERMINOLOGY (IDEAL FIR FILTER USED WITH AD7724
rms sum of all of the nonfundamental signals and harmonics up
[FIGURE 1])
to half the Output Data Rate (fO/2), excluding dc. Signal-to-
Integral Nonlinearity
(Noise + Distortion) is dependent on the number of quantiza- This is the maximum deviation of any code from a straight line tion levels used in the digitization process; the more levels, the passing through the endpoints of the transfer function. The smaller the quantization noise. The theoretical Signal-to-(Noise endpoints of the transfer function are zero scale (not to be con- + Distortion) ratio for a sine wave input is given by fused with bipolar zero), a point 0.5 LSB below the first code Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB transition (100 . 00 to 100 . 01 in bipolar mode and 000 . 00 to 000 . 01 in unipolar mode) and full scale, a where N is the number of bits. point 0.5 LSB above the last code transition (011 . 10 to
Total Harmonic Distortion
011 . 11 in bipolar mode and 111 . 10 to 111 . 11 in THD is the ratio of the rms sum of harmonics to the rms value unipolar mode). The error is expressed in LSBs. of the fundamental. THD is defined as
Common-Mode Rejection Ratio
The ability of a device to reject the effect of a voltage applied to (V 2 +V 2 +V 2 +V 2 +V 2) THD = 20 log 2 3 4 5 6 both input terminals simultaneously—often through variation of V1 a ground level—is specified as a common–mode rejection ratio. CMRR is the ratio of gain for the differential signal to the gain where V1 is the rms amplitude of the fundamental and V2, V3, for the common-mode signal. V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonic.
Unipolar Offset Error
Unipolar offset error is the deviation of the first code transition
Spurious Free Dynamic Range
from the ideal VIN(+) voltage which is (VIN(–) + 0.5 LSB) Spurious free dynamic range is the difference, in dB, between when operating in the unipolar mode. the peak spurious or harmonic component in the ADC output spectrum (up to fO/2 and excluding dc) and the rms value of the
Bipolar Offset Error
fundamental. Normally, the value of this specification will be This is the deviation of the midscale transition (111 . 11 determined by the largest harmonic in the output spectrum of to 000 . 00) from the ideal VIN(+) voltage which is (VIN(–) the FFT. For input signals whose second harmonics occur in –0.5 LSB) when operating in the bipolar mode. the stop band region of the digital filter, a spur in the noise floor
Gain Error
limits the SFDR. The first code transition should occur at an analog value 1/2 LSB
Intermodulation Distortion
above minus full scale. The last code transition should occur for With inputs consisting of sine waves at two frequencies, fa and an analog value 1 1/2 LSB below the nominal full-scale. Gain fb, any active device with nonlinearities will create distortion error is the deviation of the actual difference between first and products at sum and difference frequencies of mfa ± nfb where last code transitions and the ideal difference between first and m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are last code transitions. those for which neither m nor n are equal to zero. For example,
Signal-to-(Noise + Distortion)
the second order terms include (fa + fb) and (fa – fb), while the Signal-to-(Noise + Distortion) is the measured signal-to-noise third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and plus distortion ratio at the output of the ADC. The signal is the (fa – 2fb). rms magnitude of the fundamental. Noise plus distortion is the REV. B –7–