AD7683BRMZ Datasheet, PDF (13/16 Pages) Analog Devices – 16-Bit, 100 kSPS, Single-Ended PulSAR ADC in MSOP/QFN

(NOTE 1)
REF
C_REF
2.2µF TO 10µF
(NOTE 2)
100nF
2.7V TO 5.25V
AD7683
0V TO V_REF
(NOTE 3)
33Ω
2.7nF
(NOTE 4)
REF
+IN
AD7683
–IN
GND
VDD
DCLOCK
D_OUT
CS
3-WIRE INTERFACE
NOTES
1. SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.
2. C_REF IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
3. SEE DRIVER AMPLIFIER CHOICE SECTION.
4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
Figure 22. Typical Application Diagram
TYPICAL CONNECTION DIAGRAM
Figure 22 shows an example of the recommended application
pass filter that reduces undesirable aliasing effects and limits
the noise.
diagram for the AD7683.
When the source impedance of the driving circuit is low, the
ANALOG INPUT
Figure 23 shows an equivalent circuit of the input structure of
the AD7683. The two diodes, D1 and D2, provide ESD protec-
tion for the analog inputs, +IN and −IN. Care must be taken to
AD7683 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
ensure that the analog input signal never exceeds the supply rails
Although the AD7683 is easy to drive, the driver amplifier
by more than 0.3 V because this causes these diodes to become
needs to meet the following requirements:
forward-biased and start conducting current. However, these
diodes can handle a forward-biased current of 130 mA maximum.
For instance, these conditions can eventually occur when the
input buffer (U1) supplies are different from VDD. In such a
case, use an input buffer with a short-circuit current limitation
to protect the part.
VDD
• The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7683. Note that the AD7683
has a noise figure 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 AD7683
+IN
OR –IN
GND
D1
C_PIN
D2
R_IN
C_IN
analog input circuit, 1-pole, low-pass filter made by RIN
and CIN or by the external filter, if one is used.
• For ac applications, the driver needs to have a THD
performance suitable to that of the AD7683. Figure 15
Figure 23. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the differen-
tial 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 impedance of the
analog input, +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
600 Ω and is a lumped component consisting of some serial
shows the THD vs. frequency that the driver should
exceed.
• For multichannel multiplexed applications, the driver
amplifier and the AD7683 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 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.
Table 10. Recommended Driver Amplifiers
Amplifier
Typical Application
resistors and the on resistance of the switches. CIN is typically
30 pF and is mainly the ADC sampling capacitor. During the
ADA4841
OP184
Very low noise and low power
Low power, low noise, and low frequency
conversion phase, when the switches are opened, the input
impedance is limited to CPIN. RIN and CIN make a 1-pole, low-
AD8605, AD8615 5 V single-supply, low power
AD8519
Low power and low frequency
AD8031
High frequency and low power
Rev. A | Page 13 of 16