Data Sheet AD7683
Rev. B | Page 13 of 16
04301-022
AD7683
REF
GND
VDD
–IN
+IN
DCLOCK
D
OUT
CS
3-WIRE INTERFACE
100nF
2.7V TO 5.25V
C
REF
2.2µF TO 10µF
(NOTE 2)
REF
0V TO V
REF
33Ω
2.7nF
(NOTE 3)
(NOTE 4)
(NOTE 1)
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 23. Typical Application Diagram
TYPICAL CONNECTION DIAGRAM
Figure 23 shows an example of the recommended application
diagram for the AD7683.
ANALOG INPUT
Figure 24 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
ensure that the analog input signal never exceeds the supply rails
by more than 0.3 V because this causes 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 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.
04301-023
C
IN
R
IN
D1
D2
C
PIN
+IN
OR –IN
GND
VDD
Figure 24. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the differen-
tial signal between +IN and −IN. By using this dierential 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 C
PIN
and the network formed by the series connection
of R
IN
and C
IN
. C
PIN
is primarily the pin capacitance. R
IN
is typically
600 Ω and is a lumped component consisting of some serial
resistors and the on resistance of the switches. C
IN
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 C
PIN
. R
IN
and C
IN
make a 1-pole, low-
pass filter that reduces undesirable aliasing effects and limits
the noise.
When the source impedance of the driving circuit is low, the
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
Although the AD7683 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 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
analog input circuit, 1-pole, low-pass filter made by R
IN
and C
IN
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 16 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 11. Recommended Driver Amplifiers
Amplifier Typical Application
ADA4841-1 Very low noise and low power
OP184 Low power, low noise, and low frequency
AD8605, AD8615 5 V single-supply, low power
AD8519 Low power and low frequency
AD8031 High frequency and low power
