AD7691 Analog Devices, AD7691 Datasheet - Page 15

• Manufacturer Part Number: AD7691
• Manufacturer: Analog Devices

Specifications of AD7691

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

TYPICAL CONNECTION DIAGRAM
Figure 29 shows an example of the recommended connection diagram for the AD7691 when multiple supplies are available.
ANALOG INPUTS
Figure 30 shows an equivalent circuit of the input structure of
the AD7691.
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 if the input buffer (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
serial resistors and the on resistance of the switches. C
typically 30 pF and is mainly the ADC sampling capacitor.
IN
is typically 3 kΩ and is a lumped component composed of
OR IN–
GND
IN+
Figure 30. Equivalent Analog Input Circuit
IN
PIN
and C
C
, and the network formed by the series
PIN
IN
0 TO V
V
VDD
REF
. C
ADA4841-2
ADA4841-2
D1
D2
TO 0
PIN
REF
is primarily the pin capacitance.
1
2
3
4
5
SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.
C
SEE TABLE 9 FOR ADDITIONAL RECOMMENDED AMPLIFIERS.
OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
SEE THE DIGITAL INTERFACE SECTION FOR MOST CONVENIENT INTERFACE MODE.
3
3
REF
V+
V–
V+
V–
V+
IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
R
Figure 29. Typical Application Diagram with Multiple Supplies
IN
REF
4
4
2.7nF
2.7nF
15Ω
15Ω
1
C
10µF
IN
IN
2
is
IN+
IN–
Rev. B | Page 15 of 28
GND
REF
AD7691
VDD
During the conversion phase, where the switches are opened,
the input impedance is limited to C
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits noise.
When the source impedance of the driving circuit is low, the
AD7691 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 as shown in Figure 31.
VIO
SCK
SDO
CNV
SDI
Figure 31. THD vs. Analog Input Frequency and Source Resistance
–100
–105
–110
–115
–120
–125
–130
–80
–85
–90
–95
0
100nF
100nF
V
REF
10
250Ω
= VDD 5V
100Ω
3- OR 4-WIRE INTERFACE
20
5V
1.8V TO VDD
50Ω
30
FREQUENCY (kHz)
15Ω
40
33Ω
PIN
50
5
. R
IN
60
and C
70
IN
make a
80
AD7691
9
0