AD7684 Datasheet, PDF (13/16 Pages) Analog Devices – 16-Bit, 100 kSPS PulSAR Differential ADC in MSOP

AD7684
(NOTE 1)
REF
2.2µF TO 10µF
(NOTE 2)
100nF
2.7V TO 5.25V
0 TO V_REF
(NOTE 3)
V_REF TO 0
(NOTE 3)
33Ω
2.7nF
(NOTE 4)
33Ω
2.7nF
REF
+IN
AD7684
–IN
GND
VDD
DCLOCK
D_OUT
CS
3-WIRE INTERFACE
(NOTE 4)
NOTE 1: SEE REFERENCE SECTION FOR REFERENCE SELECTION.
NOTE 2: C_REF IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
NOTE 3: SEE DRIVER AMPLIFIER CHOICE SECTION.
NOTE 4: OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
NOTE 5: SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.
Figure 22. Typical Application Diagram
TYPICAL CONNECTION DIAGRAM
Figure 22 shows an example of the recommended application
diagram for the AD7684.
ANALOG INPUT
switches. CIN is typically 30 pF and is mainly the ADC sampling
capacitor. During the conversion phase, when 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.
Figure 23 shows an equivalent circuit of the input structure of
the AD7684. The two diodes, D1 and D2, provide ESD protec-
tion for the analog inputs, +IN and −IN. Care must be taken to
ensure that the analog input signal never exceeds the supply
rails by more than 0.3 V, because this will cause these diodes to
become forward-biased and start conducting current. However,
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.
VDD
+IN
OR –IN
GND
D1
C_PIN
D2
R_IN
C_IN
When the source impedance of the driving circuit is low, the
AD7684 can be driven directly. Large source impedances signi-
ficantly affect the ac performance, especially THD. The dc
performances are less sensitive to the input impedance.
DRIVER AMPLIFIER CHOICE
Although the AD7684 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 AD7684. Note that the
AD7684 has a noise much lower than most other 16-bit
ADCs and, therefore, can be driven by a noisier op amp
while preserving the same or better system performance.
The noise coming from the driver is filtered by the AD7684
analog input circuit 1-pole, low-pass filter made by RIN and
CIN or by the external filter, if one is used.
Figure 23. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the differ-
ential signal between +IN and −IN. By using this differential
input, small signals common to both inputs are rejected. For
instance, by using −IN to sense a remote signal ground, ground
potential differences between the sensor and the local ADC
ground are eliminated. During the acquisition phase, the impe-
dance of the analog input +IN can be modeled as a parallel
combination of the capacitor CPIN and the network formed by
the series connection of RIN and CIN. CPIN is primarily the pin
capacitance. RIN is typically 600 Ω and is a lumped component
made up of some serial resistors and the on-resistance of the
• For ac applications, the driver needs to have a THD
performance suitable to that of the AD7684. Figure 15
shows the THD vs. frequency that the driver should
exceed.
• For multichannel multiplexed applications, the driver
amplifier and the AD7684 analog input circuit must be able
to settle for a full-scale step of the capacitor array at a
16-bit level (0.0015%). 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.
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