Datasheet

Data Sheet AD7190
Rev. C | Page 27 of 40
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0 25 50 75 100 125 150
FILTER GAIN (dB)
FREQUENCY (Hz)
07640-014
Figure 20. Sinc
3
Filter Response (50 Hz Output Data Rate)
The sinc
4
filter provides 50 Hz (±1 Hz) rejection in excess of
120 dB, assuming a stable master clock, while the sinc
3
filter
gives a rejection of 100 dB. The stop-band attenuation is
typically 53 dB for the sinc
4
filter but equal to 40 dB for the
sinc
3
filter.
The 3 dB frequency for the sinc
4
filter is equal to
f
3dB
= 0.23 × f
ADC
and for the sinc
3
filter is equal to
f
3dB
= 0.272 × f
ADC
Chop Enabled
With chop enabled, the ADC offset and offset drift are
minimized. When chop is enabled, the analog input pins are
continuously swapped. Therefore, with the analog input pins
connected in one direction, the settling time of the sinc filter is
allowed to elapse until a valid conversion is available. The analog
input pins are then inverted and another valid conversion is
obtained. Subsequent conversions are then averaged so that the
offset is minimized. This continuous swapping of the analog
input pins and the averaging of subsequent conversions means
that the offset drift is also minimized.
Chopping affects the output data rate and settling time of the
ADC. For the sinc
4
filter, the output data rate is equal to
f
ADC
= f
CLK
/(4 × 1024 × FS[9:0])
For sinc
3
filter, the output data rate is equal to
f
ADC
= f
CLK
/(3 × 1024 × FS[9:0])
where:
f
ADC
is the output data rate.
f
CLK
= master clock (4.92 MHz nominal).
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 1.173 Hz to 1200 Hz for the sinc
4
filter
and 1.56 Hz to 1600 Hz for the sinc
3
filter. The settling time for
the sinc
3
or sinc
4
filter is equal to
t
SETTLE
= 2/f
ADC
Therefore, with chop enabled, the settling time is reduced for a
given output data rate compared to the chop disabled mode.
However, for a given FS[9:0] value, the output data rate is less
with chop enabled compared with the chop disabled mode.
For either the sinc
3
or the sinc
4
filter, the cutoff frequency f
3dB
is
equal to
f
3dB
= 0.24 × f
ADC
Figure 21 and Figure 22 show the filter response for the sinc
4
and sinc
3
filters, respectively, when chop is enabled. As shown
in the plots, the stop-band attenuation is less compared with the
chop disabled modes.
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0 25 50 75 100 125 150
FILTER GAIN (dB)
FREQUENCY (Hz)
07640-015
Figure 21. Sinc
4
Filter Response (Output Data Rate = 12.5 Hz, Chop Enabled)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0 25 50 75 100 125 150
FILTER GAIN (dB)
FREQUENCY (Hz)
07640-016
Figure 22. Sinc
3
Filter Response (Output Data Rate = 16.6 Hz, Chop Enabled)