Datasheet
AD7796/AD7797
Rev. A | Page 16 of 24
ADC CIRCUIT INFORMATION
OVERVIEW
The AD7796/AD7797 are low power ADCs that incorporate a
Σ-Δ modulator, in-amp, and an on-chip digital filter intended
for measuring wide dynamic range, low frequency signals, such
as those in pressure transducers and weigh scales.
Each part has one differential input that is buffered.
Figure 10
shows the basic connections required to operate the part.
AD7796/AD7797
AIN(+)
AIN(–)
INTERNAL
CLOCK
AV
DD
DV
DD
REFIN(–)
REFIN(+)
DOUT/RDY
DIN
SCLK
MUX
GND
GND
CLK
Σ-Δ
ADC
V
DD
V
DD
×128
TEMP
SENSOR
SERIAL
INTERFACE
AND
CONTROL
LOGIC
CS
IN+
IN–
OUT–
OUT+
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Figure 10. Basic Connection Diagram
The output rate of the AD7796/AD7797 (f
ADC
) is user-
programmable. The allowable update rates, along with the
corresponding settling times, are listed in
Table 1 4. Normal
mode rejection is the major function of the digital filter.
Simultaneous 50 Hz and 60 Hz rejection is optimized when the
update rate equals 16.7 Hz or less because notches are placed at
both 50 Hz and 60 Hz with these update rates (see
Figure 12).
The AD7796/AD7797 use slightly different filter types,
depending on the output update rate used to optimize the
rejection of quantization noise and device noise. When the
update rate is 4.17 Hz to 12.5 Hz, a Sinc
3
filter and an averaging
filter are used. When the update rate is 16.7 Hz to 33.2 Hz,
a modified Sinc
3
filter is used. This filter gives simultaneous
50 Hz/60 Hz rejection when the update rate equals 16.7 Hz.
A Sinc
4
filter is used when the update rate is from 50 Hz to
123 Hz.
Figure 11 to Figure 13 show the frequency response of
the different filter types for some of the update rates.
0
–20
–40
–60
–80
–100
0110080604020
FREQUENCY (Hz)
(dB)
20
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Figure 11. Filter Profile with Update Rate = 4.17 Hz
0
–20
–40
–60
–80
–100
0218016014012010080604020
FREQUENCY (Hz)
(dB)
00
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Figure 12. Filter Profile with Update Rate = 16.7 Hz
0
–20
–40
–60
–80
–100
0 1000900800700600500400300200100
FREQUENCY (Hz)
(dB)
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Figure 13. Filter Profile with Update Rate = 50 Hz