AD7982 Analog Devices, AD7982 Datasheet - Page 14

• Manufacturer Part Number: AD7982
• Manufacturer: Analog Devices

Specifications of AD7982

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOP

AD7982
ANALOG INPUTS
Figure 24 shows an equivalent circuit of the input structure of
the AD7982.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs, IN+ and IN−. Care must be taken to ensure that
the analog input signal does not exceed the reference input
voltage (REF) by more than 0.3 V. If the analog input signal
exceeds this level, the diodes become forward biased and start
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 ADA4841 in
Figure 23) are different from those of the REF, the analog input
signal may 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 be used to protect the part.
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 C
of R
400 Ω and is a lumped component composed of serial resistors
and the on resistance of the switches. C
is mainly the ADC sampling capacitor.
During the sampling phase, where the switches are closed, the
input impedance is limited to C
low-pass filter that reduces undesirable aliasing effects and
limits noise.
IN
and C
90
85
80
75
70
65
60
IN+ OR IN–
1
IN
GND
. C
Figure 25. Analog Input CMRR vs. Frequency
PIN
Figure 24. Equivalent Analog Input Circuit
PIN
and the network formed by the series connection
is primarily the pin capacitance. R
10
C
PIN
FREQUENCY (kHz)
REF
100
D1
D2
PIN
. R
IN
and C
IN
is typically 30 pF and
R
1000
IN
IN
make a 1-pole,
C
IN
IN
is typically
10000
Rev. A | Page 14 of 24
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:
•
•
•
Table 8. Recommended Driver Amplifiers
Amplifier
ADA4941
ADA4841
AD8021
AD8022
OP184
AD8655
AD8605,
where:
f
(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).
e
nV/√Hz.
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 AD7982 analog input circuit’s 1-pole, low-
pass filter made by R
one is used. Because the typical noise of the AD7982 is
40 μV rms, the SNR degradation due to the amplifier is
For ac applications, the driver should 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 commonly specified. This
may differ significantly from the settling time at an 18-bit
level and should be verified prior to driver selection.
–3dB
N
is the equivalent input noise voltage of the op amp, in
AD8615
is the input bandwidth, in megahertz, of the AD7982
SNR
LOSS
=
Typical Application
Very low noise, low power, single to differential
Very low noise, small, and low power
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single supply, low noise
5 V single supply, low power
20
log
IN
⎛
⎜
⎜
⎜
⎜
⎝
and C
40
2
+
IN
π
2
or by the external filter, if
40
f
−
3dB
(
Ne
N
)
2
⎞
⎟
⎟
⎟
⎟
⎠