AD7821
TERMINOLOGY
INTERMODULATION DISTORTION
LEAST SIGNIFICANT BIT (LSB)
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities will create distortion
products, of order (m+n), at sum and difference frequencies of
mfa+nfb, where m, n = 0, 1, 2, 3…. Intermodulation terms are
those for which m or n is not equal to zero. For example, the
second order terms include (fa + fb) and (fa – fb), and the third
order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
(fa – 2fb). For the AD7821 intermodulation distortion is calcu-
lated separately for both the second and third order terms.
An ADC with 8-bit resolution can resolve one part in 28 (1/256
of full scale). For the AD7821 operating in either the unipolar
or bipolar input range with 5 V full scale, one LSB is 19.53 mV.
TOTAL UNADJUSTED ERROR
This is a comprehensive specification which includes relative
accuracy, offset error, and full-scale error.
SLEW RATE
Slew rate is the maximum allowable rate of change of input
signal such that the digital sample values are not in error.
SIGNAL-TO-NOISE RATIO (SNR)
Signal-to-noise ratio is measured signal-to-noise at the output of
the ADC. The signal is the rms magnitude of the fundamental.
Noise is the rms sum of all nonfundamental signals (excluding
dc) up to half the sampling frequency. SNR is dependent on the
number of quantization levels used in the digitization process.
The theoretical SNR for a sine wave input is given by:
TOTAL HARMONIC DISTORTION (THD)
Total harmonic distortion is the ratio of the square root of the
sum of the squares of the rms value of the harmonics to the rms
value of the fundamental. For the AD7821, total harmonic dis-
tortion is defined as
SNR = 6.02N + 1.76 dB
where N is the number of bits in the ADC. Thus, for an ideal
8-bit ADC, SNR = 50 dB.
(
)
V22 +V32 +V42 +V52 +V62
(
)
20 log
dB
V
1
PEAK HARMONIC OR SPURIOUS NOISE
where V1 is the rms amplitude of the fundamental and V2, V3, V4,
V5, and V6 are the rms amplitudes of the individual harmonics.
Peak harmonic or spurious noise is the rms value of the largest
nonfundamental frequency (excluding dc) up to half the sam-
pling frequency to the rms value of the fundamental.
REV. B
–5–
ADI [ ADI ]
