Datasheet

AD7766
Rev. C | Page 16 of 24
0
–20
–40
–60
–80
–100
–120
–140
–160
0 128k112k96k80k64k48k32k16k
AMPLITUDE (dB)
FREQUENCY (Hz)
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Figure 32. AD7766 Digital Filter Frequency Response
0
–20
–40
–60
–80
–100
–120
–140
–160
0 64k56k48k40k32k24k16k8k
AMPLITUDE (dB)
FREQUENCY (Hz)
06449-217
Figure 33. AD7766-1 Digital Filter Frequency Response
0
–20
–40
–60
–80
–100
–120
–140
–160
0 32k28k24k20k16k12k8k4k
AMPLITUDE (dB)
FREQUENCY (Hz)
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Figure 34. AD7766-2 Digital Filter Frequency Response
ANALOG INPUT STRUCTURE
The AD7766/AD7766-1/AD7766-2 are configured as a differential
input structure. A true differential signal is sampled between the
analog inputs V
IN+
and V
IN−
, Pin 4 and Pin 5, respectively. Using
differential inputs provides rejection of signals that are common
to both the V
IN+
and V
IN−
pins.
Figure 35 shows the equivalent analog input circuit of the
AD7766/AD7766-1/AD7766-2. The two diodes on each of the
differential inputs provide ESD protection for the analog inputs.
06449-219
V
IN+
V
REF+
C1
D
D
GND
AGND
R
IN
C2
V
IN–
V
REF+
C1
D
D
GND
AGND
R
IN
C2
Figure 35. Equivalent Analog Input Structure
Take care to ensure that the analog input signal does not exceed
the reference supply voltage (V
REF+
) by more than 0.3 V, as specified
in the Absolute Maximum Ratings section. If the input voltage
exceeds this limit, the diodes become forward biased and start
to conduct current. The diodes can handle 130 mA maximum.
The impedance of the analog inputs can be modeled as a parallel
combination of C1 and the network formed by the series con-
nection of R
IN
, C1, and C2. The value of C1 is dominated by the
pin capacitance. R
IN
is typically 1.4 k, the lumped component
of serial resistors and the R
ON
of the switches. C2 is typically
22 pF, and its value is dominated by the sampling capacitor.
SUPPLY AND REFERENCE VOLTAGES
The AD7766/AD7766-1/AD7766-2 operate from a 2.5 V supply
applied to the DV
DD
and AV
DD
pins. The interface is specified to
operate between 1.7 V and 3.6 V. The AD7766/AD7766-1/
AD7766-2 operate from a reference input in the range of 2.2 V
to 2 × AV
DD
applied to the V
REF+
pin. The nominal reference
supply voltage is 5 V, but a 2.5 V supply can also be used. When
using a 5 V reference, the recommended reference devices are
the ADR445, ADR435, or ADR425; when using 2.5 V, the ADR441,
ADR431, or ADR421 are recommended. The voltage applied to
the reference input (V
REF+
) operates both as a reference supply
and as a power supply to the AD7766/AD7766-1/AD7766-2
devices. Therefore, when using a 5 V reference input, the full-scale
differential input range of the AD7766/AD7766-1/AD7766-2 is
10 V. See the Driving the AD7766/AD7766-1/AD7766-2 section
for details on the maximum input voltage.