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AD7887 Fiches technique(PDF) 9 Page - Analog Devices

No de pièce AD7887
Description  2.7 V to 5.25 V, Micropower, 2-Channel, 125 kSPS, 12-Bit ADC in 8-Lead MSOP
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
Logo AD - Analog Devices

AD7887 Fiches technique(HTML) 9 Page - Analog Devices

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AD7887
Rev. D | Page 9 of 24
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 ½ LSB
below the first code transition, and full scale, a point ½ 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, that is, AGND + 0.5 LSB.
Offset Error Match
This is the difference in offset error between any two channels.
Gain Error
This is the deviation of the last code transition (111 . . . 110) to
(111 . . . 111) from the ideal (that is, VREF − 1.5 LSB) after the
offset error has been adjusted out.
Gain Error Match
This is the difference in gain error between any two channels.
Track/Hold Acquisition Time
The track/hold amplifier returns to 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 ±1/2 LSB, after the end of a 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 fun-
damental. Noise is the sum of all nonfundamental signals up to half
the sampling frequency (fS/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 12-bit converter, this is 74 dB.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
harmonics to the fundamental. For the AD7887, it is defined as
1
2
6
2
5
2
4
2
3
2
2
log
20
)
dB
(
V
V
V
V
V
V
THD
+
+
+
+
=
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 fS/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, the
largest harmonic could be a noise peak.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and fb,
any active device with nonlinearities creates distortion products
at sum and difference frequencies of mfa ± nfb where m, n = 0,
1, 2, 3, and so on. Intermodulation distortion terms are those for
which neither m nor n are equal to 0. For example, the second-
order terms include (fa + fb) and (fa − fb), and the third order
terms include (2fa + fb), (2fa − fb), (fa + 2fb) and (fa − 2fb).
The AD7887 is tested using the CCIF standard in which 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, and 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 decibels.
Channel-to-Channel Isolation
Channel-to-channel isolation is a measure of the level of
crosstalk between channels. It is measured by applying a full-
scale 25 kHz sine wave signal to the nonselected input channel
and determining how much that signal is attenuated in the
selected channel. The figure given is the worst case across both
channels for the AD7887.
Power Supply Rejection (PSR)
Variations in power supply affect the full-scale transition, but
not the converter’s linearity. PSR is the maximum change in the
full-scale transition point due to a change in power supply voltage
from the nominal value. See Figure 7.
PSRR is defined as the ratio of the power in the ADC output at
frequency f to the power of a full-scale sine wave applied to the
ADC of frequency fS:
PSRR (dB) = 10 log(Pf/Pfs)
where Pf is the power at frequency f in ADC output and Pfs is
the power at frequency fS in ADC full-scale input.


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