Datasheet
AD7195
Rev. 0 | Page 40 of 44
CHOP ENABLED (SINC
3
FILTER)
With chop enabled, the ADC offset and offset drift are
minimized. The analog input pins are continuously swapped.
With the 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 invert and another settled
conversion 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. Using the sinc
3
filter with chop
enabled is suitable for output data rates up to 320 Hz.
08771-036
SINC
3
/SINC
4
MODULATOR
ADC
CHOP
Figure 45. Chop Enabled (Sinc
3
Chop Enabled)
Output Data Rate and Settling Time (Sinc
3
Chop
Enabled)
For the 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
is the 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 1.56 Hz to 1600 Hz. The settling time
is equal to
t
SETTLE
= 2/f
ADC
Table 34. Examples of Output Data Rates and the
Corresponding Settling Time (Chop Enabled, Sinc
3
Filter)
FS[9:0] Output Data Rate (Hz) Settling Time (ms)
96 16.7 120
80 20 100
When a channel change occurs, the modulator and filter are
reset. The complete settling time is required to generate the first
conversion after the channel change. Subsequent conversions on
this channel occur at 1/f
ADC
.
CHANNEL
CONVERSIONS
CHANNEL A
CH A CH A CH A CH BCH B CH B CH B
CHANNEL B
1/
f
ADC
CH B
0
8771-056
Figure 46. Channel Change (Sinc
3
Chop Enable)
If conversions are performed on a single channel and a step
change occurs, the ADC does not detect the change in analog
input; therefore, it continues to output conversions at the
programmed output data rate. However, it is at least two
conversions later before the output data accurately reflects
the analog input. If the step change occurs while the ADC is
processing a conversion, then the ADC takes three conversions
after the step change to generate a fully settled result.
1/
f
ADC
ANALOG
INPUT
ADC
OUTPUT
FULLY
SETTLED
08771-057
Figure 47. Asynchronous Step Change in Analog Input (Sinc
3
Chop Enabled)
The cutoff frequency f
3dB
is equal to
f
3dB
= 0.24 × f
ADC
50 Hz/60 Hz Rejection (Sinc
3
Chop Enabled)
When FS[9:0] is set to 96 and chopping is enabled, the filter
response shown in Figure 48 is obtained. The output data rate
is equal to 16.7 Hz for a 4.92 MHz master clock. The chopping
introduces notches at odd integer multiples of f
ADC
/2. The
notches due to the sinc filter in addtion to the notches intro-
duced by the chopping means that simultaneous 50 Hz and
60 Hz rejection is achieved for an output data rate of 16.7 Hz.
The rejection at 50 Hz/60 Hz ± 1 Hz is typically 53 dB,
assuming a stable master clock.
–120
–110
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
0 25 50 75 100 125 150
FREQUENCY (Hz)
FILTER GAIN (dB)
08771-058
Figure 48. Sinc
3
Filter Response (FS[9:0] = 96, Chop Enabled)