參數(shù)資料
型號(hào)： DDC101
英文描述： 20-BIT ANALOG-TO-DIGITAL CONVERTER
中文描述： 20位模擬數(shù)字轉(zhuǎn)換器
文件頁(yè)數(shù)： 19/27頁(yè)
文件大?。?/td> 346K
代理商： DDC101
19
DDC101
Basic Integration Frequency Response
The sin(x)/x basic integration characteristic is controlled by
the digital filter’s measurement time (T
). The measure-
ment frequency, f
is l/T
. The input frequency re-
sponse of the DDC101 is down –3dB at f
/2.26 with a
null at f
MEAS
. Subsequent nulls are at harmonics 2f
,
3f
, 4f
, etc. as shown in the frequency response curve
below. This characteristic is often used to eliminate known
interference by setting f
or a harmonic to exactly the
frequency of the interference. Table VI illustrates the fre-
quency characteristics of the DDC101 integration function
for various measurement times. As an example, for N =
2272, K = 16, and M = 256: T
MEAS
= (N-M-K)/f
= (2272-
256-16)/2MHz = 1ms and f
INT
= 2272/2MHz
= 1.14ms; f
CONV
= l/T
INT
= 880Hz.
MEASUREMENT TIME
100
μ
s
1ms
10ms
16.66ms
20ms
–3dB FREQUENCY
f
MEAS
10kHz
1kHz
100Hz
60Hz
50Hz
4.42kHz
442Hz
44.2Hz
26.5Hz
22.1Hz
TABLE VI. Basic Integration Frequency Response Examples.
FIGURE 16. Basic Integration Frequency Response.
0.1f
MEAS
f
MEAS
Frequency
10f
MEAS
G
0
–10
–20
–30
–40
–50
–20dB/decade
Slope
Nyquist
(f
CONV
/2)
f
CONV
Oversampling Frequency Response
The M oversamples of the initial and the final data points
create an oversampling sin(x)/x type of low pass filter
response. The oversampling function reduces broadband
noise of the input signal and the DDC101. Broadband noise
is reduced approximately in proportion to the square root of
the number of oversamples, M. As an example, a conversion
with 128 oversamples will have approximately 1/2 the noise
of a conversion with 32 oversamples (
√
32/128 =
√
1/4 =
1/2) The oversampling low pass filter response creates a null
at f
= 1/T
OS
. The oversample time, T
, is M/f
. For M =
256 and f
CLK
= 2MHz, f
OS
is approximately 7.8kHz. Subse-
quent nulls are at harmonics 2f
, 3f
, 4f
, etc. The –3dB
point is at f
OS
/2.26. Table VII illustrates the DDC101
oversampling frequency characteristics with approximate
values for f
OS
and the –3dB frequency. An oversampling
frequency response graph is shown below in Figure 17. This
figure shows the frequency response for M = 256 oversamples
with an f
of 2MHz . The slope of the attenuation curve
decreases at approximately 20dB/decade.
OVERSAMPLES (M)
–3dB FREQUENCY
f
OS
256
128
64
16
3.5kHz
6.9kHz
13.9kHz
55kHz
7.8kHz
15.6kHz
31.2kHz
125kHz
TABLE VII. Oversample Frequency Response Examples.
Normalized DDC101 Frequency Response
The normalized frequency response, H(f), of the DDC101 that is applied to the input signal consists of the product of the three
frequency response components:
sin
π
f N
M
K
)
/f
CLK
π
f N
M
K
)
/f
CLK
Msin
π
f/f
CLK
(
Oversampling
Where:
f
f
CLK
N
is the signal frequency
is the system clock frequency, typically 2MHz
is the total number of clock periods in each integration time, T
INT
= N/f
CLK
, T
INT
is the DDC101 CDAC's
integration time
is the number of oversamples in one oversampled data point
is the number of clocks used in the acquisition time
is the digital filters measurement time, T
MEAS
, (T
MEAS
= T
INT
–(M+K)/f
CLK
)
is the oversample time, T
OS
is the total conversion time for multiple integrations, T
CONV
The DDC101's transfer response has a linear phase characteristic as indicated by the exponential term.
M
K
(N-M-K)/f
CLK
M/f
CLK
LN/f
CLK
H f
( )
=
(
)
sin
π
fM/f
CLK
(
)
)
sin
π
fLN/f
CLK
(
)
)
e
j
π
f LN
K
1
Linear Phase
Lsin
π
fN/f
CLK
(
)
/f
CLK
Basic Integration
Multiple Integrations
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