AD7401A
Rev. C | Page 13 of 20
TERMINOLOGY
Differential Nonlinearity (DNL)
DNL is the difference between the measured and the ideal 1
LSB change between any two adjacent codes
in the ADC.
Integral Nonlinearity (INL)
INL is the maximum deviation from a straight line passing
through the endpoints of the ADC transfer function. The
endpoints of the transfer function are specified negative full
scale, −250 mV (V
IN
+ − V
IN
−), Code 7169 for the 16-bit level,
and specified positive full scale, +250 mV (V
IN
+ − V
IN
−), Code
58366 for the 16-bit level.
Offset Error
Offset error is the deviation of the midscale code (32768 for the
16-bit level) from the ideal V
IN
+ − V
IN
− (that is, 0 V).
Gain Error
The gain error includes both positive full-scale gain error and
negative full-scale gain error. Positive full-scale gain error is the
deviation of the specified positive full-scale code (58366 for the
16-bit level) from the ideal V
IN
+ − V
IN
− (+250 mV) after the
offset error is adjusted out. Negative full-scale gain error is the
deviation of the specified negative full-scale code (7169 for the
16-bit level) from the ideal V
IN
+ − V
IN
− (−250 mV) after the
offset error is adjusted out. Gain error includes reference error.
Signal-to-(Noise and Distortion) Ratio (SINAD)
SINAD is the measured ratio of signal-to-noise and 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 digitization
process; the more levels, the smaller the quantization noise. The
theoretical signal-to-(noise and distortion) ratio for an ideal
N-bit converter with a sine wave input is given by
Signal-to-(Noise and Distortion) = (6.02N + 1.76) dB
Therefore, for a 12-bit converter, this is 74 dB.
Effective Number of Bits (ENOB)
ENOB is defined by
ENOB = (SINAD − 1.76)/6.02 bits
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the
fundamental. For the AD7401A, it is defined as
1
6
54
32
V
VVVVV
THD
22222
log20(dB)
++++
=
where:
V
1
is the rms amplitude of the fundamental.
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, excluding dc) to the rms value of the
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but
for ADCs where the harmonics are buried in the noise floor,
it is a noise peak.
Common-Mode Rejection Ratio (CMRR)
CMRR is defined as the ratio of the power in the ADC output
at ±250 mV frequency, f, to the power of a 250 mV p-p sine
wave applied to the common-mode voltage of V
IN
+ and V
IN
−
of frequency, f
S
, as
CMRR (dB) = 10 .log(Pf/Pf
S
)
where:
Pf is the power at frequency, f, in the ADC output.
Pf
S
is the power at frequency, f
S
, in the ADC output.
Power Supply Rejection Ratio (PSRR)
Variations in power supply affect the full-scale transition but
not the converter’s linearity. PSRR is the maximum change in
the specified full-scale (±250 mV) transition point due to a
change in power supply voltage from the nominal value (see
Figure 6).
Isolation Transient Immunity
The isolation transient immunity specifies the rate of rise/fall of
a transient pulse applied across the isolation boundary beyond
which clock or data is corrupted. The AD7401A was tested
using a transient pulse frequency of 100 kHz.
