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參數資料
| 型號： | AD9866-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Broadband Modem Mixed Signal Front End |
| 中文描述： | 寬帶調制解調器混合信號前端 |
| 文件頁數： | 34/48頁 |
| 文件大小： | 1647K |
| 代理商： | AD9866-EB |
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AD9866
LOW-PASS FILTER
The low-pass filter (LPF) provides a third order response with a
cut-off frequency that is typically programmable over a 15 MHz
to 35 MHz span. Figure 68 shows that the first real pole is
implemented within the first CPGA gain stage, and the complex
pole pair is implemented in the second CPGA gain stage.
Capacitor arrays are used to vary the different R-C time
constants within these two stages in a manner that changes the
cut-off frequency while preserving the normalized frequency
response. Because absolute resistor and capacitor values are
process-dependent, a calibration routine lasting less than 100 μs
automatically occurs each time the target
cut-off frequency
register (Reg. 0x08) is updated, ensuring a repeatable cut-off
frequency from device to device.
Rev. 0 | Page 34 of 48
Although the default setting specifies that the LPF be active, it
can also be bypassed providing a nominal f
3 dB
of 55 MHz.
Table 20 shows the SPI registers pertaining to the LPF.
Table 20. SPI Registers for Rx Low-Pass Filter
Address (Hex)
Bit
0x07
(0)
0x08
(7:0)
Description
Enable Rx LPF
Target
value
The normalized wideband gain response is shown in Figure 70.
The normalized pass-band gain and group delay responses are
shown in Figure 71. The normalized cut-off frequency, f
3 dB
,
results in 3 dB attenuation. Also, the actual group delay time
(GDT) response can be calculated given a programmed cut-off
frequency using the following equation:
)
45
.
/(
3
dB
f
GDT
Normalized
GDT
Actual
=
Equation 7.
0
FREQUENCY
G
0
5
1.0
3.0
–35
–30
–25
–20
–15
–10
–5
0
2.5
2.0
1.5
0.5
Figure 70. LPF’s Normalized Wideband Gain Response
0
NORMALIZED FREQUENCY
G
0
0.5
1.0
0.9
0.3
0.4
0.8
0.2
0.1
N
T
1.30
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
0.25
–3.00
–2.75
–2.50
–2.25
–2.00
–1.75
–1.50
–1.25
–1.00
–0.75
–0.50
–0.25
0
0.6
0.7
NORMALIZED GROUP DELAY
NORMALIZED GAIN RESPONSE
Figure 71. LPF’s Normalized Pass-Band Gain and Group Delay Responses
The 3 dB cut-off frequency, f
3 dB
, is programmable by writing
an 8-bit word, referred to as the target,
to Reg. 0x08. The cut-off
frequency is a function of the ADC sample rate, f
ADC
, and to a
lesser extent RxPGA gain setting (in dB). Figure 72 shows how
the frequency response, f
3 dB
, varies as a function of the RxPGA
gain setting.
0
INPUT FREQUENCY (MHz)
F
0
10
50
30
25
5
3
–18
–12
–6
0
15
20
35
40
–15
–9
–3
45
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
( f
3 dB
= 32 MHz (@ 0 dB and f
ADC
= 80 MSPS)
The following formula
1
can be used to estimate f
3 dB
for a
RxPGA gain setting of 0 dB:
)
83
.
23
30
/
(
)
80
/
(
)
/
128
(
0
_
3
+
=
ADC
ADC
dB
dB
f
f
target
f
Equation 8.
Figure 73 compares the measured and calculated f
3 dB
using this
formula.
1
Empirically derived for a f
3 dB
range of 15 MHz to 35 MHz and f
ADC
of 40 MSPS
to 80 MSPS with an RxPGA = 0 dB.
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