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AD7980 Datasheet(PDF) 12 Page - Analog Devices

Part No. AD7980
Description  16-bit, 1 MSPS PulSAR ADC in MSOP/QFN
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7980 Datasheet(HTML) 12 Page - Analog Devices

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AD7980
Preliminary Technical Data
Rev Pr C | Page 12 of 22
than 0.3 V because this causes these diodes to begin to forward-
bias and start conducting current. These diodes can handle a
forward-biased current of 130 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from VDD. In such a case, an input
buffer with a short-circuit, current limitation can be used to
protect the part.
CIN
RIN
D1
D2
CPIN
IN+
OR IN–
GND
VDD
Figure 14. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the true
differential signal between IN+ and IN−. By using these
differential inputs, signals common to both inputs are rejected.
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, CPIN, and the network formed by the series
connection of RIN and CIN. CPIN is primarily the pin capacitance.
RIN is typically 400 Ω and is a lumped component made up of
some serial resistors and the on resistance of the switches. CIN is
typically 30 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to CPIN. RIN and CIN make a 1-
pole, low-pass filter that reduces undesirable aliasing effects and
limits the noise.
When the source impedance of the driving circuit is low, the
AD7980 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
harmonic distortion (THD). The dc performances are less
sensitive to the input impedance. The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency.
DRIVER AMPLIFIER CHOICE
Although the AD7980 is easy to drive, the driver amplifier
needs to meet the following requirements:
The noise generated by the driver amplifier needs to be
kept as low as possible in order to preserve the SNR and
transition noise performance of the AD7980. The noise
coming from the driver is filtered by the AD7980 analog
input circuit 1-pole, low-pass filter made by RIN and CIN or
by the external filter, if one is used. Because the typical
noise of the AD7980 is 38 μV rms, the SNR degradation
due to the amplifier is
+
=
2
2
)
(
2
π
44
44
20log
N
3dB
LOSS
Ne
f
SNR
where:
f–3dB is the input bandwidth in MHz of the AD7980
(10MHz) or the cutoff frequency of the input filter, if one is
used.
N is the noise gain of the amplifier (for example, +1 in
buffer configuration).
eN is the equivalent input noise voltage of the op amp, in
nV/√Hz.
For ac applications, the driver should have a THD
performance commensurate with the AD7980.
For multichannel multiplexed applications, the driver
amplifier and the AD7980 analog input circuit must settle
for a full-scale step onto the capacitor array at a 16-bit level
(0.0015%, 15 ppm). In the amplifier’s data sheet, settling at
0.1% to 0.01% is more commonly specified. This could
differ significantly from the settling time at a 16-bit level
and should be verified prior to driver selection.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4941
Very low noise, low power single to
Differential
ADA4841
Very low noise, small and low power
AD8021
Very low noise and high frequency
AD8022
Low noise and high frequency
OP184
Low power, low noise, and low frequency
AD8655
5 V single-supply, low noise
AD8605, AD8615
5 V single-supply, low power


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