Chop Enabled (Sinc Filter) - Analog Devices AD7194 Manual

Simultaneous 50 Hz and 60 Hz rejection is obtained when
FS[9:0] is set to 480 (master clock = 4.92 MHz), as shown in
Figure 41. The output data rate is 10 Hz when zero latency is
disabled and 3.3 Hz when zero latency is enabled. The sinc
filter has rejection of 100 dB minimum at 50 Hz ± 1 Hz and
60 Hz ± 1 Hz.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0
Figure 41. Sinc
Simultaneous 50 Hz/60 Hz rejection is also achieved using the
REJ60 bit in the mode register. When FS[9:0] is programmed to
96 and the REJ60 bit is set to 1, notches are placed at both 50 Hz
and 60 Hz for a stable 4.92 MHz master clock. Figure 42 shows
the frequency response of the sinc
Assuming a stable clock, the rejection at 50 Hz/60 Hz (±1 Hz)
is in excess of 67 dB minimum.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
0 25
Figure 42. Sinc
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30 60 90 120
FREQUENCY (Hz)
3
Filter Response (FS[9:0] = 480)
3
filter with this configuration.
50 75 100 125
FREQUENCY (Hz)
3
Filter Response (FS[9:0] = 96, REJ60 = 1)
CHOP ENABLED (SINC
With chop enabled, the ADC offset and offset drift are minimized.
The analog input pins are continuously swapped. With the
3
analog input pins connected in one direction, the settling time
of the sinc filter is allowed and a conversion is recorded. The
analog input pins are then inverted, and another settled conver-
sion is obtained. Subsequent conversions are averaged to
minimize the offset. This continuous swapping of the analog
input pins and the averaging of subsequent conversions means
that the offset drift is also minimized. With chop enabled, the
resolution increases by 0.5 bits.
CHOP
Output Data Rate and Settling Time (Sinc
Enabled)
15 0
4
For the sinc
f = f
ADC CLK
where:
f is the output data rate.
ADC
f is the master clock (4.92 MHz nominal).
CLK
FS[9:0] is the decimal equivalent of Bit FS9 to Bit FS0 in the
mode register.
The value of FS[9:0] can be varied from 1 to 1023. This results
in an output data rate of 1.17 Hz to 1200 Hz. The settling time is
equal to
t = 2/f
SETTLE
Table 32 gives some examples of FS[9:0] values and the corres-
ponding output data rates and settling times.
Table 32. Examples of Output Data Rates and the
Corresponding Settling Time
FS[9:0] Output Data Rate (Hz)
96 12.5
80 15
150
Rev. 0 | Page 43 of 56
4
FILTER)
ADC
3 4
MODULATOR SINC /SINC
Figure 43. Chop Enabled
filter, the output data rate is equal to
/(4 × 1024 × FS[9:0])
ADC
Settling Time (ms)
160
133
AD7194
POST FILTER
4
Chop