AD7705/AD7706
CLOCK REGISTER (RS2, RS1, RS0 = 0, 1, 0); POWER-ON/RESET STATUS: 05 HEXADECIMAL
The clock register is an 8-bit register from which data can be read or to which data can be written.
Table 18 outlines the bit designations for the clock register.
Table 18. Clock Register
ZERO (0)
ZERO (0)
ZERO (0)
CLKDIS (0)
CLKDIV (0)
CLK (1)
FS1 (0)
FS0 (1)
Table 19. Clock Register Description
Register
Description
ZERO
Zero. A zero must be written to these bits to ensure correct operation of the AD7705/AD7706. Failure to do so might result in
unspecified operation of the device.
CLKDIS
Master Clock Disable Bit. Logic 1 in this bit disables the master clock, preventing it from appearing at the MCLK OUT pin. When
disabled, the MCLK OUT pin is forced low. This feature allows the user the flexibility of either using the MCLK OUT as a clock
source for other devices in the system, or turning off the MCLK OUT as a power-saving feature. When using an external master
clock on the MCLK IN pin, the AD7705/AD7706 continue to have internal clocks and convert normally with the CLKDIS bit
active. When using a crystal oscillator or ceramic resonator across Pin MCLK IN and Pin MCLK OUT, the AD7705/AD7706 clocks
are stopped, and no conversions take place when the CLKDIS bit is active.
CLKDIV
CLK
Clock Divider Bit. With this bit at Logic 1, the clock frequency appearing at the MCLK IN pin is divided by 2 before being used
internally by the AD7705/AD7706. For example, when this bit is set to Logic 1, the user can operate with a 4.9152 MHz crystal
between Pin MCLK IN and Pin MCLK OUT, and internally the part operates with the specified 2.4576 MHz. With this bit at
Logic 0, the clock frequency appearing at the MCLK IN pin is the frequency used internally by the part.
Clock Bit. This bit should be set in accordance with the operating frequency of the AD7705/AD7706. If the device has a master
clock frequency of 2.4576 MHz (CLKDIV = 0) or 4.9152 MHz (CLKDIV = 1), this bit should be set to Logic 1. If the device has a
master clock frequency of 1 MHz (CLKDIV = 0) or 2 MHz (CLKDIV = 1), this bit should be set to Logic 0. This bit sets up the
appropriate scaling currents for a given operating frequency and, together with FS1 and FS0, chooses the output update rate
for the device. If this bit is not set correctly for the master clock frequency of the device, the AD7705/AD7706 might not
operate to specification.
FS1, FS0
Filter Selection Bits. Along with the CLK bit, FS1 and FS0 determine the output update rate, the filter’s first notch, and the −3 dB
frequency, as outlined in Table 20. The on-chip digital filter provides a sinc3 (or (sinx/x)3) filter response. In association with the
gain selection, it also determines the output noise of the device. Changing the filter notch frequency, as well as the selected gain,
impacts resolution. Table 5 through Table 8 show the effects of filter notch frequency and gain on the output noise and effective
resolution of the part. The output data rate, or effective conversion time, for the device is equal to the frequency selected for
the first notch of the filter. For example, if the first notch of the filter is selected at 50 Hz, a new word is available at a 50 Hz output
rate, or every 20 ms. If the first notch is at 500 Hz, a new word is available every 2 ms. A calibration should be initiated when
any of these bits are changed. The settling time of the filter to a full-scale step input is worst case 4 ꢀ 1/(output data rate). For
example, with the filter-first notch at 50 Hz, the settling time of the filter to a full-scale step input is 80 ms maximum. If the first
notch is at 500 Hz, the settling time is 8 ms maximum. This settling time can be reduced to 3 ꢀ 1/(output data rate) by
synchronizing the step input change with a reset of the digital filter. In other words, if the step input takes place with the
FSYNC bit high, the settling time is 3 ꢀ 1/(output data rate) from the time when the FSYNC bit returns low. The −3 dB
frequency is determined by the programmed first notch frequency according to the relationship:
filter − 3 dB frequency = 0.262 × filter - first notch frequency
Table 20. Output Update Rates
CLK1
FS1
FS0
0
1
0
1
0
1
0
1
Output Update Rate
20 Hz
25 Hz
100 Hz
200 Hz
50 Hz
60 Hz
250 Hz
500 Hz
−3 dB Filter Cutoff
5.24 Hz
6.55 Hz
26.2 Hz
52.4 Hz
13.1 Hz
15.7 Hz
65.5 Hz
131 Hz
0
0
0
0
1
1
1
1
0
0
1
1
0
0
1
1
1 Assumes correct clock frequency on MCLK IN pin with the CLKDIV bit set appropriately.
Rev. C | Page 19 of 44
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