參數(shù)資料
型號： DDC101
英文描述： 20-BIT ANALOG-TO-DIGITAL CONVERTER
中文描述： 20位模擬數(shù)字轉換器
文件頁數(shù)： 20/27頁
文件大?。?/td> 346K
代理商： DDC101
DDC101
20
Multiple Integration Frequency Response
If the DDC101 is operated in the multiple integrations per
conversion mode of operation, an additional sin(x)/x type
low pass filter is created. The filter creates an initial null
frequency at the conversion frequency, f
of the DDC101
and at multiples of f
. The –3dB point for this filter is also
at f
/2.26. The conversion time, T
, is the sum of the
integration times for multiple integrations that are averaged
together by the DDC101. T
= LN/f
. f
= l/T
.
If multiple integrations per conversion are used, this filter
will be the dominant low frequency filter of the DDC101.
Table VIII shows examples of the conversion time and
frequency for different parameter selections. Figure 18 shows
an example of the frequency response due to Multiple
Integrations. In the case of Figure 18, the integration time is
500
μ
s (N = 1000 clock periods) and L = 64 integrations per
conversion.
FIGURE 19. Product of Frequency Response of Basic Inte-
gration and Oversampling: 1ms Integration
Time, 256 Oversamples.
FIGURE 17. Oversampling Frequency Response for M = 256
(f
CLK
= 2MHz).
INTEGRATION
TIME
CONVERSION
TIME
–3dB
L
FREQUENCY
f
CONV
500Hz
125Hz
62.5Hz
15.6Hz
3.91Hz
50.0Hz
12.5Hz
6.25Hz
1.56Hz
0.39Hz
1ms
1ms
1ms
1ms
1ms
10ms
10ms
10ms
10ms
10ms
2
8
16
64
256
2
8
16
64
256
2ms
8ms
16ms
64ms
256ms
20ms
80ms
160ms
640ms
2560ms
221Hz
55Hz
27.5Hz
6.9Hz
1.73Hz
22.1Hz
5.5Hz
2.75Hz
0.69Hz
0.173Hz
TABLE VIII. Multiple Integration Time Examples.
System Noise implications
The noise at the digital output of the DDC101 consists of
system noise that is included in the analog input signal and
noise from the DDC101.
DDC101 Noise
—The noise of the DDC101 includes low
frequency and broadband noise. The low frequency noise is
reduced by the integrating function and the CDS function of
the DDC101. This is reflected in the basic integration
frequency response and in the multiple integration frequency
response. The broadband electronic noise is reduced prima-
rily by the oversampling function of the DDC101
Signal Noise
—The noise of the input signal is filtered and
reduced in a manner similar to the DDC101 noise reduction
through the integrating and oversampling functions of the
DDC101.
Figures 19 and 20 show the frequency response of the
DDC101 for the product of the basic integration and
oversampling frequency response for two different values of
M. In both examples, the integration time is 1ms, the only
difference is in the number of oversamples, M; for Figure
19, M = 256 oversamples was used; for Figure 20, M = 32
oversamples was used. The first null frequency is f
MEAS
and
subsequent nulls are at multiples of f
. The first example
with the larger number of oversamples (M = 256) clearly
reduces high frequency noise more than the second example
with M = 32.
For M = 256, f
is 7.8kHz, f
is 1.16kHz, and the –3dB
frequency is 507Hz. For M = 32, f
is 62.4kHz, f
MEAS
is
1.02kHz and the –3dB frequency is 453Hz.
FIGURE 18. A Multiple Integration Frequency Response
Example.
0
–5
–10
–15
–20
–25
–30
–35
–40
G
Frequency (Hz)
1k
10k
100k
1M
f
OS
0
–5
–10
–15
–20
–25
–30
–35
–40
1
10
100
1k
Frequency (Hz)
N = 1000
L = 64
f
CONV
= 31Hz
f
CONV
0
–5
–10
–15
–20
–25
–30
–35
–40
100
1k
10k
100k
G
Frequency (Hz)
N = 2000
M = 256
K = 16
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