AD7453
Rev. B | Page 8 of 20
TERMINOLOGY
Signal-to-(Noise + Distortion) Ratio
The measured ratio of signal to (noise + distortion) at the
output of the ADC. The signal is the rms amplitude of the fun-
damental. Noise is the sum of all nonfundamental signals up to
half the sampling frequency (f
S
/2), excluding dc. The ratio is
dependent on the number of quantization levels in the digitiza-
tion process; the more levels, the smaller the quantization noise.
The theoretical signal-to-(noise + distortion) ratio for an ideal
N-bit converter with a sine wave input is given by
Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB
Thus, for a 12-bit converter, this is 74 dB.
Total Harmonic Distortion (THD)
Total harmonic distortion is the ratio of the rms sum of
harmonics to the fundamental. For the AD7453, it is defined as
1
2
6
2
5
2
4
2
3
2
2
VVVVV
THD
++++
= log20)dB(
where V
1
is the rms amplitude of the fundamental and V
2
, V
3
,
V
4
, V
5
, and V
6
are the rms amplitudes of the second to the sixth
harmonics.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
rms value of the next largest component in the ADC output
spectrum (up to f
S
/2 and excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is deter-
mined by the largest harmonic in the spectrum, but for ADCs
where the harmonics are buried in the noise floor, it is a noise
peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
fb, any active device with nonlinearities creates distortion
products at the sum and difference frequencies of mfa ± nfb
where m, n = 0, 1, 2, 3, and so on. 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).
The AD7453 is tested using the CCIF standard where two input
frequencies near the top end of the input bandwidth are used.
In this case, the second-order terms are usually distanced in
frequency from the original sine waves while the third-order
terms are usually at a frequency close to the input frequencies.
As a result, the second- and third-order terms are specified
separately. The calculation of the intermodulation distortion is
as per the THD specification where it is the ratio of the rms
sum of the individual distortion products to the rms amplitude
of the sum of the fundamentals expressed in dB.
Aperture Delay
The amount of time from the leading edge of the sampling
clock until the ADC actually takes the sample.
Aperture Jitter
The sample-to-sample variation in the effective point in time at
which the actual sample is taken.
Full Power Bandwidth
The full power bandwidth of an ADC is the input frequency at
which the amplitude of the reconstructed fundamental is
reduced by 0.1 dB or 3 dB for a full-scale input.
Integral Nonlinearity (INL)
The maximum deviation from a straight line passing through
the endpoints of the ADC transfer function.
Differential Nonlinearity (DNL)
The difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Offset Error
The deviation of the first code transition (000...000 to 000...001)
from the ideal (i.e., AGND + 1 LSB)
Gain Error
This is the deviation of the last code transition (111...110 to
111...111) from the ideal (i.e., VREF – 1 LSB), after the offset
error has been adjusted out.
Track-and-Hold Acquisition Time
The minimum time required for the track and hold amplifier to
remain in track mode for its output to reach and settle to within
0.5 LSB of the applied input signal.
Power Supply Rejection Ratio (PSRR)
The ratio of the power in the ADC output at full-scale fre-
quency, f, to the power of a 100 mV p-p sine wave applied to the
ADC V
DD
supply of frequency f
S
. The frequency of this input
varies from 1 kHz to 1 MHz.
PSRR(dB) = 10log(Pf/Pf
S
)
Pf is the power at frequency f in the ADC output; Pfs is the
power at frequency f
S
in the ADC output.
