Datasheet AD7720 (Analog Devices) - 8

AD7720 TERMINOLOGY (IDEAL FIR FILTER USED WITH AD7720
fundamental. Noise plus distortion is the rms sum of all of the
[FIGURE 1])
nonfundamental signals and harmonics to half the output word
Integral Nonlinearity
rate (fMCLK/128), excluding dc. Signal-to-(Noise + Distortion) is This is the maximum deviation of any code from a straight line dependent on the number of quantization levels used in the passing through the endpoints of the transfer function. The digitization process; the more levels, the smaller the quantiza- endpoints of the transfer function are zero scale (not to be con- tion noise. The theoretical Signal-to-(Noise + Distortion) ratio fused with bipolar zero), a point 0.5 LSB below the first code for a sine wave input is given by transition (100 . 00 to 100 . 01 in bipolar mode and Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB 000 . 00 to 000 . 01 in unipolar mode) and full scale, a point 0.5 LSB above the last code transition (011 . 10 to where N is the number of bits. 011 . 11 in bipolar mode and 111 . 10 to 111 . 11 in
Total Harmonic Distortion
unipolar mode). The error is expressed in LSBs. Total Harmonic Distortion (THD) is the ratio of the rms sum
Differential Nonlinearity
of harmonics to the rms value of the fundamental. For the This is the difference between the measured and the ideal 1 LSB AD7720, THD is defined as change between two adjacent codes in the ADC. (V 2 +V 2 +V 2 +V 2 +V 2)
Common-Mode Rejection Ratio
THD = 20 log 2 3 4 5 6 The ability of a device to reject the effect of a voltage applied to V1 both input terminals simultaneously—often through variation of where V1 is the rms amplitude of the fundamental and V2, V3, a ground level—is specified as a common-mode rejection ratio. V4, V5 and V6 are the rms amplitudes of the second through the CMRR is the ratio of gain for the differential signal to the gain sixth harmonic. for the common-mode signal.
Spurious Free Dynamic Range Unipolar Offset Error
Spurious free dynamic range is the difference, in dB, between Unipolar offset error is the deviation of the first code transition the peak spurious or harmonic component in the ADC output from the ideal VIN(+) voltage which is (VIN(–) + 0.5 LSB) spectrum (up to fMCLK/128 and excluding dc) and the rms value when operating in the unipolar mode. of the fundamental. Normally, the value of this specification will
Bipolar Offset Error
be determined by the largest harmonic in the output spectrum This is the deviation of the midscale transition (111 . 11 of the FFT. For input signals whose second harmonics occur in to 000 . 00) from the ideal VIN(+) voltage which is (VIN(–) the stop band region of the digital filter, a spur in the noise floor –0.5 LSB) when operating in the bipolar mode. limits the spurious free dynamic range.
Gain Error Intermodulation Distortion
The first code transition should occur at an analog value 1/2 With inputs consisting of sine waves at two frequencies, fa and LSB above minus full scale. The last code transition should fb, any active device with nonlinearities will create distortion occur for an analog value 3/2 LSB below the nominal full scale. products at sum and difference frequencies of mfa ± nfb where Gain error is the deviation of the actual difference between first m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are and last code transitions and the ideal difference between first those for which neither m or n are equal to zero. For example, and last code transitions. the second order terms include (fa + fb) and (fa – fb), while the
Signal-to-(Noise + Distortion)
third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and Signal-to-(Noise + Distortion) is measured signal-to-noise at the (fa – 2fb). output of the ADC. The signal is the rms magnitude of the REV. 0 –7–