AD7787 Datasheet, PDF (13/20 Pages) Analog Devices – Low Power, 2-Channel 24-Bit Sigma-Delta ADC

AD7787
FILTER REGISTER (RS1, RS0 = 1, 0; POWER-ON/RESET = 0×04)
The filter register is an 8-bit register from which data can be read or to which data can be written. This register is used to set the output
word rate. Table 11 outlines the bit designations for the filter register. FR0 through FR7 indicate the bit locations, FR denoting the bits are
in the filter register. FR7 denotes the first bit of the data stream. The number in the parenthesis indicates the power-on/reset default status
of that bit.
FR7
FR6
FR5
0 (0)
0 (0)
CDIV1 (0)
FR4
CDIV0 (0)
FR3
FR2
0 (0)
FS2 (1)
FR1
FS1 (0)
FR0
FS0 (0)
Table 11. Filter Register Bit Designations
Bit
Location Bit Name Description
FR7 to
0
FR6
These bits must be programmed with a Logic 0 for correct operation.
FR5 to
FR4
CLKDIV1
to CDIV0
These bits are used to operate the AD7787 in the lower power modes. The clock is internally divided and the
power is reduced. In the low power modes, the update rates will scale with the clock frequency so that dividing
the clock by 2 causes the update rate to be reduced by a factor of 2 also.
00
Normal Mode
01
Clock Divided by 2
10
Clock Divided by 4
11
Clock Divided by 8
FR3
0
This bit must be programmed with a Logic 0 for correct operation.
FR2 to
FR0
FS2 to FS0
These bits set the output word rate of the ADC. The update rate influences the 50 Hz/60 Hz rejection and the
noise. Table 12 shows the allowable update rates when normal power mode is used. In the low power modes,
the update rate is scaled with the clock frequency. For example, if the internal clock is divided by a factor of 2,
the corresponding update rates are divided by 2 also.
Table 12. Update Rates
FS2 FS1 FS0
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
fADC (Hz)
120
100
33.3
20
16.6
16.7
13.3
9.5
f3dB (Hz)
28
24
8
4.7
4
4
3.2
2.3
RMS Noise (µV)
40
25
3.36
1.6
1.5
1.5
1.2
1.1
Rejection
25 dB @ 60 Hz
25 dB @ 50 Hz
80 dB @ 60 Hz
65 dB @ 50 Hz/60 Hz (Default Setting)
80 dB @ 50 Hz
67 dB @ 50 Hz/60 Hz
DATA REGISTER (RS1, RS0 = 1, 1; POWER-ON/RESET = 0×000000)
The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from
this register, the RDY bit/pin is set.
Rev. 0 | Page 13 of 20