CHOP ENABLED (SINC3 FILTER) With chop enabled, the" />
參數(shù)資料
型號: AD7195BCPZ-RL7
廠商: Analog Devices Inc
文件頁數(shù): 36/45頁
文件大?。?/td> 0K
描述: IC AFE 24BIT 4.8K 32LFSP
設(shè)計資源: Precision Weigh Scale Design Using AD7195 with Internal PGA and AC Excitation (CN0155)
標準包裝: 1,500
位數(shù): 24
通道數(shù): 4
電壓 - 電源,模擬: 4.75 V ~ 5.25 V
電壓 - 電源,數(shù)字: 2.7 V ~ 5.25 V
封裝/外殼: 32-WFQFN 裸露焊盤,CSP
供應(yīng)商設(shè)備封裝: 32-LFCSP-WQ(5x5)
包裝: 帶卷 (TR)
AD7195
Rev. 0 | Page 40 of 44
CHOP ENABLED (SINC3 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 sinc3 filter with chop
enabled is suitable for output data rates up to 320 Hz.
0
877
1-
0
36
SINC3/SINC4
MODULATOR
ADC
CHOP
Figure 45. Chop Enabled (Sinc3 Chop Enabled)
Output Data Rate and Settling Time (Sinc3 Chop
Enabled)
For the sinc3 filter, the output data rate is equal to
fADC = fCLK/(3 × 1024 × FS[9:0])
where:
fADC is the output data rate.
fCLK 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
tSETTLE = 2/fADC
Table 34. Examples of Output Data Rates and the
Corresponding Settling Time (Chop Enabled, Sinc3 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/fADC.
CHANNEL
CONVERSIONS
CHANNEL A
CH A
CH B
CHANNEL B
1/
fADC
CH B
0
87
71
-0
56
Figure 46. Channel Change (Sinc3 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/
fADC
ANALOG
INPUT
ADC
OUTPUT
FULLY
SETTLED
0
877
1-
0
57
Figure 47. Asynchronous Step Change in Analog Input (Sinc3 Chop Enabled)
The cutoff frequency f3dB is equal to
f3dB = 0.24 × fADC
50 Hz/60 Hz Rejection (Sinc3 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 fADC/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
25
50
75
100
125
150
FREQUENCY (Hz)
FIL
T
E
R
GA
IN
(
d
B
)
08
77
1-
0
58
Figure 48. Sinc3 Filter Response (FS[9:0] = 96, Chop Enabled)
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