AD7851
–8–
REV. B
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The end-
points of the transfer function are zero scale, a point 1/2 LSB
below the first code transition, and full scale, a point 1/2 LSB
above the last code transition.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Total Unadjusted Error
This is the deviation of the actual code from the ideal code tak-
ing all errors into account (gain, offset, integral nonlinearity, and
other errors) at any point along the transfer function.
Unipolar Offset Error
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB)
when operating in unipolar mode.
Positive Full-Scale Error
This applies to unipolar and bipolar modes and is the deviation of
the last code transition from the ideal AIN(+) voltage (AIN(–) +
full scale – 1.5 LSB) after the offset error has been adjusted out.
Negative Full-Scale Error
This applies to bipolar mode only and is the deviation of the
first code transition (10 . . . 000 to 10 . . . 001) from the ideal
AIN(+) voltage (AIN(–) – V
REF
/2 + 0.5 LSB).
Bipolar Zero Error
This is the deviation of the midscale transition (all 1s to all 0s)
from the ideal AIN(+) voltage (AIN(–) – 1/2 LSB).
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns into track mode at the end
of conversion. Track-and-hold acquisition time is the time
required for the output of the track-and-hold amplifier to reach
its final value, within ±1/2 LSB, after the end of conversion.
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal-to-(noise + distortion) at
the output of the ADC. The signal is the rms amplitude of the
fundamental. 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) = (6.02 N +1.76)dB
Thus, for a 14-bit converter, this is 86 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7851, it is defined as
THD
VVVVV
V
(d ) 20logB =
++++
()
2
2
3
2
4
2
5
2
6
2
1
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 through 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 parts
where the harmonics are buried in the noise floor, it will be a
noise peak.
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).
Testing is performed 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 dBs.
Power Supply Rejection Ratio (PSRR)
PSRR is defined as the ratio of the power in ADC output at fre-
quency f to the power of the full-scale sine wave applied to the
supply voltage (V
DD
). The units are in LSB, % of FS per % of
supply voltage, or expressed logarithmically, in dB (PSRR (dB)
= 10 log (Pf/Pfs)).
Full Power Bandwidth (FPBW)
FPBW is that frequency at which the amplitude of the recon-
structed fundamental (using FFTs and neglecting harmonics
and SNR) is reduced by 3 dB for a full-scale input.
