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AD7982 Datasheet(PDF) 16 Page - Analog Devices

Part No. AD7982
Description  18-Bit, 1 MSPS PulSAR 7 mW ADC in MSOP/LFCSP
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7982 Datasheet(HTML) 16 Page - Analog Devices

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Data Sheet
AD7982
Rev. D | Page 15 of 25
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of the
AD7982.
The two diodes, D1 and D2, provide electrostatic discharge
(ESD) protection for the IN+ analog input and the IN− analog
input. Take care to ensure the analog input signal does not
exceed the reference input voltage (REF) by more than 0.3 V. If
the analog input signal exceeds the 0.3 V level, the diodes
become forward-biased and begin conducting current. These
diodes can handle a forward-biased current of 130 mA maximum.
However, if the supplies of the input buffer (for example, the
supplies of the ADA4807-1 in Figure 23) are different from those of
the REF, the analog input signal can eventually exceed the
supply rails by more than 0.3 V. In such a case (for example, an
input buffer with a short-circuit), the current limitation can protect
the device.
CPIN
REF
RIN
CIN
D1
D2
IN+ OR IN–
GND
Figure 24. 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.
90
85
80
75
70
65
60
1
10
100
1000
10000
FREQUENCY (kHz)
Figure 25. Analog Input CMRR vs. Frequency
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 composed of serial resistors
and the on resistance of the switches. CIN is typically 30 pF and
is mainly the ADC sampling capacitor.
During the sampling phase where the switches are closed, the input
impedance is limited to CPIN. RIN and CIN make a 1-pole, low-pass
filter that reduces undesirable aliasing effects and limits noise.
When the source impedance of the driving circuit is low, the
AD7982 can be driven directly. Large source impedances
significantly affect the ac performance, especially 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 AD7982 is easy to drive, the driver amplifier must
meet the following requirements:
The noise generated by the driver amplifier must be kept
as low as possible to preserve the SNR and transition noise
performance of the AD7982. The noise from the driver is
filtered by the analog input circuit of the AD7982 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
AD7982 is 40 μV rms, the SNR degradation due to the
amplifier is
2
2
)
(
2
π
40
40
log
20
N
3dB
LOSS
Ne
f
SNR
where:
f–3dB is the input bandwidth, in megahertz, of the AD7982
(10 MHz) 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 must have a THD perfor-
mance commensurate with the AD7982.
For multichannel, multiplexed applications, the driver
amplifier and the AD7982 analog input circuit must settle
for a full-scale step onto the capacitor array at an 18-bit level
(0.0004%, 4 ppm). In the data sheet of the amplifier, settling at
0.1% to 0.01% is more typically specified. Settling time can
differ significantly from the settling time at an 18-bit level
and must be verified prior to driver selection.
Table 9. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4941-1
Very low noise, low power, single to differential
ADA4940-1
Very low noise, low power, single to differential
ADA4807-2
Very low noise and low power
ADA4627-1
Precision, low noise and low input bias
ADA4522-2
Precision, zero drift, and electromagnetic
interference (EMI) enhanced
ADA4500-2
Precision, rail-to-rail input and output (RRIO), and
zero input crossover distortion


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