AD7680 Datasheet, PDF (5/11 Pages) Analog Devices – 3mW, 100kSPS,16-Bit ADC in 6 Lead SOT-23

PRELIMINARY TECHNICAL DATA
AD7680
TERMINOLOGY
Integral Nonlinearity
This is the maximum deviation from a straight line pass-
ing through the endpoints of the ADC transfer function.
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.
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 + 1LSB
Gain Error
This is the deviation of the last code transition (111 . . .
110) to (111 . . . 111) from the ideal (i.e., V_REF – 1
LSB) after the offset error has been adjusted out.
Track/Hold Acquisition Time
The track/hold amplifier returns into track mode at 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 interface timing section for
more details.
Signal to (Noise + Distortion) Ratio
This is the measured ratio of signal to (noise + distor-
tion) 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_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 + 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 16-bit converter, this is 98 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms
sum of harmonics to the fundamental. For the AD7860, it
is defined as:
THD (dB ) = 20 log
V
2
2
+
V
2
3
+V
2
4
+
V
2
5
+
V
2
6
V₁
where V₁ is the rms amplitude of the fundamental and V₂,
V₃, V₄, V₅ and V₆ 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 determined 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 are
equal to zero. For example, the second order terms in-
clude (fa + fb) and (fa – fb), while the third order terms
include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
The AD7680 is tested using the CCIF standard where two
input frequencies nearthe 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 dis-
tortion products to the rms amplitude of the sum of the
fundamentals expressed in dBs.
REV. PrE
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