AD7476ART Analog Devices, AD7476ART Datasheet - Page 7

• Manufacturer Part Number: AD7476ART
• Description: 1 MSPS/ 12-/10-/8-Bit ADCs in 6-Lead SOT-23
• Manufacturer: Analog Devices

TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line passing through
the endpoints of the ADC transfer function. For the AD7476/
AD7477, the endpoints 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. For the AD7478,
the endpoints of the transfer function are zero scale, a point
1 LSB below the first code transition, and full scale, a point
1 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.
Offset Error
This is the deviation of the first code transition (00 . . . 000)
to (00 . . . 001) from the ideal (i.e., AGND + 0.5 LSB). For
the AD7478, this is the deviation of the first code transition
(00 . . . 000) to (00 . . . 001) from the ideal (i.e., AGND +
1 LSB).
Gain Error
For the AD7476/AD7477, this is the deviation of the last code
transition (111 . . . 110) to (111 . . . 111) from the ideal (i.e.,
V
For the AD7478, this is the deviation of the last code transi-
tion (111 . . . 110) to (111 . . . 111) from the ideal (i.e., V
– 1 LSB) after the offset error has been adjusted.
Track/Hold Acquisition Time
The track/hold amplifier returns into track mode after the end
of conversion. Track/Hold acquisition time is the time required
for the output of the track/hold amplifier to reach its final value,
within ± 0.5 LSB, after the end of conversion. See Serial Inter-
face Timing section for more detail.
Signal-to-(Noise + Distortion) Ratio
This is the measured ratio of signal-to-(noise + distortion) at
the output of the A/D converter. The signal is the rms amplitude
of the fundamental. Noise is the sum of all nonfundamental
signals up to half the sampling frequency (f
The ratio is dependent on the number of quantization levels in
the digitization process; the more levels, the smaller the quanti-
zation noise. The theoretical signal-to-(noise + distortion) ratio
for an ideal N-bit converter with a sine wave input is given by:
Thus for a 12-bit converter, this is 74 dB and for a 10-bit con-
verter it is 62 dB, for an 8-bit converter it is 50 dB.
REF
– 1.5 LSB) after the offset error has been adjusted out.
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
S
/2), excluding dc.
REF
Total Unadjusted Error
This is a comprehensive specification which includes gain error,
linearity error, and offset error.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7476/AD7477/
AD7478 it is defined as:
where V
V
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
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 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 is 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 AD7476/AD7477/AD7478 are tested using the CCIF standard
where two input frequencies are used, fa = 498.7 kHz and
fb = 508.7 kHz. 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.
4
, V
5
, and V
THD dB
1
is the rms amplitude of the fundamental and V
(
6
are the rms amplitudes of the second through the
)
S
/2 and excluding dc) to the rms value of the
=
AD7476/AD7477/AD7478
20
log
(
V
2
2
+
V
3
2
+
V
V
1
4
2
+
V
5
2
+
V
6
2
2
)
, V
3
,