AD7890 Analog Devices, AD7890 Datasheet - Page 9

• Manufacturer Part Number: AD7890
• Description: LC2MOS 8-Channel, 12-Bit Serial Data Acquisition System
• Manufacturer: Analog Devices

Specifications of AD7890

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 117kSPS
• Interface: Ser
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip 10V,Uni 2.5V,Uni 4.096V
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC

TERMINOLOGY
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 rms sum of all nonfundamental
signals up to half the sampling frequency (f
The ratio is dependent upon 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 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 AD7890, it is defined as
where:
V
V
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
fundamental. Normally, the value of this specification is
determined by the largest harmonic in the spectrum, but for
parts where the harmonics are buried in the noise floor, it is
determined by 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 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 AD7890 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 and third order terms are of different
significance. 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 fundamental expressed in dBs.
1
2
, V
is the rms amplitude of the fundamental and
Signal to (Noise + Distortion) = (6.02N + 1.76) dB
THD
3
, V
4
, V
(
dB
5,
and V
)
=
20
S
/2 and excluding dc) to the rms value of the
6
log
are the rms amplitudes of the second
V
2
2
+
V
3
2
+
V
V
1
4
2
+
S
/2), excluding dc.
V
5
2
+
V
6
2
Rev. C | Page 9 of 28
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 1 kHz
signal to any one of the other seven inputs and determining how
much that signal is attenuated in the channel of interest. The figure
given is the worst case across all eight channels.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
deviation from a straight line passing through the endpoints of
the ADC transfer function.
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB
change between any two adjacent codes in the ADC.
Positive Full-Scale Error (AD7890-10)
This is the deviation of the last code transition (01 . . . 110 to
01 . . . 111) from the ideal (4 × REF IN − 1 LSB) after the
bipolar zero error has been adjusted out.
Positive Full-Scale Error (AD7890-4)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (1.638 × REF IN − 1 LSB) after the
unipolar offset error has been adjusted out.
Positive Full-Scale Error (AD7890-2)
This is the deviation of the last code transition (11 . . . 110 to
11 . . . 111) from the ideal (REF IN − 1 LSB) after the unipolar
offset error has been adjusted out.
Bipolar Zero Error (AD7890-10)
This is the deviation of the midscale transition (all 0s to all 1s)
from the ideal 0 V (AGND).
Unipolar Offset Error (AD7890-2, AD7890-4)
This is the deviation of the first code transition (00 . . . 000 to
00 . . . 001) from the ideal 0 V (AGND).
Negative Full-Scale Error (AD7890-10)
This is the deviation of the first code transition (10 . . . 000 to
10 . . . 001) from the ideal (−4 × REF IN + 1 LSB) after bipolar
zero error has been adjusted out.
Track/Hold Acquisition Time
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 conversion (the point at which the
track/hold returns to track mode). It also applies to situations
where a change in the selected input channel takes place or
where there is a step input change on the input voltage applied
to the selected V
must wait for the duration of the track/hold acquisition time
after the end of conversion or after a channel change/step input
change to V
the part operates to specification.
IN
before starting another conversion, to ensure that
IN
input of the AD7890. It means that the user
AD7890