AD7947 AD [Analog Devices], AD7947 Datasheet - Page 8

• Manufacturer Part Number: AD7947
• Description: 3mW, 100kSPS, 14-Bit ADC in 6-Lead SOT-23
• Manufacturer: AD [Analog Devices]

AD7940
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.
Offset Error
This is the deviation of the first code transition (00 . . . 000) to
(00 . . . 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., V
error has been adjusted out.
Track-and-Hold Acquisition Time
The track-and-hold amplifier returns to track mode at the end
of conversion. The 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 LSB, after the end of the conversion.
See the Serial Interface section for more details.
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
depends on the number of quantization levels in the digitization
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
Thus, for a 14-bit converter, this is 86.04 dB.
Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB
S
/2, excluding dc). The ratio
REF
− 1 LSB) after the offset
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Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of harmonics to the funda-
mental. For the AD7940, 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
damental. 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 prod-
ucts at sum and difference frequencies of mfa ± nfb where m, n =
0, 1, 2, 3. 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 AD7940 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 dBs.
4
, V
5
, and V
THD
1
is the rms amplitude of the fundamental and V
6
(dB)
are the rms amplitudes of the second through the
S
/2, excluding dc) to the rms value of the fun-
=
20
log
V
2
2
+
V
3
2
+
V
V
1
4
2
+
V
5
2
+
V
6
2
2
, V
3
,