Datasheet

AD7946
Rev. A | Page 13 of 24
TYPICAL CONNECTION DIAGRAM
Figure 25 shows an example of the recommended connection diagram for the AD7946 when multiple supplies are available.
AD7946
REF
GND
VDD
IN–
IN+
VIO
SDI
SCK
SDO
CNV
3- OR 4-WIRE INTERFACE
(NOTE 5)
100nF
100nF
5V
10µF
(NOTE 2)
V+
V+
V–
1.8V TO VDD
REF
0V TO REF
33Ω
2.7nF
04656-025
NOTES
1. SEE REFERENCE SECTION FOR REFERENCE SELECTION.
2
.
C
REF
IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
3
. SEE DRIVE
R
AMPLIFIER CHOICE SECTION.
4
. OPTION
A
L FILTER. SEE ANALOG INPUT SECTION.
5. SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.
(NOTE 1)
(ADA4841
OR NOTE 3)
(NOTE 4)
Figure 25. Typical Application Diagram with Multiple Supplies
ANALOG INPUT
Figure 26 shows an equivalent circuit of the input structure of
the AD7946.
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 never exceeds the supply rails by more
than 0.3 V because this causes these diodes to begin to forward-
bias and start conducting current. These diodes can handle a
maximum forward-biased current of 130 mA. 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.
C
IN
R
IN
D1
D2
C
PIN
IN+
OR IN–
GND
VDD
04656-026
Figure 26. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the
differential signal between IN+ and IN−. By using this
differential input, small signals common to both inputs are
rejected, as shown in
Figure 27, which represents the typical
CMRR over frequency. For instance, by using IN− to sense a
remote signal ground, ground potential differences between the
sensor and the local ADC ground are eliminated.
FREQUENCY (kHz)
CMRR (dB)
04656-027
50
40
60
70
101 100 1k 10k
VDD = 5V
Figure 27. Analog Input CMRR vs. Frequency
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor, C
PIN
, and the network formed by the series connec-
tion of R
IN
and C
IN
. C
PIN
is primarily the pin capacitance. R
IN
is
typically 600 Ω and is a lumped component made up 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, which reduces undesirable aliasing effects and
limits the noise.
When the source impedance of the driving circuit is low, the
AD7946 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