AD7690 Analog Devices, AD7690 Datasheet - Page 14

• Manufacturer Part Number: AD7690
• Manufacturer: Analog Devices

Specifications of AD7690

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 400kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

AD7690
ANALOG INPUTS
Figure 27 shows an equivalent circuit of the input structure of
the AD7690.
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 supply rails by more
than 0.3 V because this causes the diodes to become forward
biased 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 (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+ and IN−) can be modeled as a parallel combination
of the capacitor, C
connection of R
R
of serial resistors and the on resistance of the switches. C
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 C
pole, low-pass filter that reduces undesirable aliasing effects and
limits the noise.
IN
is typically 600 Ω and is a lumped component composed
OR IN–
90
85
80
75
70
65
60
55
50
45
40
GND
IN+
1
Figure 28. Analog Input CMRR vs. Frequency
Figure 27. Equivalent Analog Input Circuit
IN
PIN
and C
C
, and the network formed by the series
10
PIN
IN
VDD
. C
FREQUENCY (kHz)
D1
D2
PIN
is primarily the pin capacitance.
100
PIN
. R
IN
R
and C
IN
1000
V
REF
= VDD = 5V
IN
C
make a 1-
IN
10000
IN
is
Rev. B | Page 14 of 24
When the source impedance of the driving circuit is low, the
AD7690 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 AD7690 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 AD7690. The noise from the driver is
filtered by the AD7690 analog input circuit’s 1-pole, low-
pass filter made by R
if one is used. Because the typical noise of the AD7690 is
28 μV rms, the SNR degradation due to the amplifier is
where:
f
(9 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
of the op amps connected to IN+ and IN−, in nV/√Hz.
This approximation can be used when the resistances
around the amplifiers are small. If larger resistances are
used, their noise contributions should also be root
summed squared.
Figure 29. THD vs. Analog Input Frequency and Source Resistance
SNR
−3 dB
–100
–105
–110
–115
–120
–125
–130
N+
–80
–85
–90
–95
and e
LOSS
0
is the input bandwidth in megahertz of the AD7690
V
REF
N−
=
10
250Ω
= VDD 5V
are the equivalent input noise voltage densities
20
100Ω
log
20
50Ω
⎛
⎜
⎜
⎜
⎜ ⎜
⎝
30
IN
28
FREQUENCY (kHz)
15Ω
and C
2
+
40
33Ω
π
2
IN
f
−
50
or by the external filter,
3
dB
(
Ne
60
N
28
+
)
70
2
+
π
2
80
f
−
3
dB
9
0
(
Ne
N
−
)
2
⎞
⎟
⎟
⎟
⎟ ⎟
⎠