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AD9866
LOW-PASS FILTER
The low-pass filter (LPF) provides a third order response with a
cutoff frequency that is typically programmable over a 15 MHz
to 35 MHz span. Figure 68 shows that the first real pole is im-
plemented 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 con-
stants within these two stages in a manner that changes the
cutoff 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 cutoff frequency
register (Register 0x08) is updated, ensuring a repeatable cutoff
frequency from device to device.
Although the default setting specifies that the LPF be active, it
can also be bypassed providing a nominal f
Table 20 shows the SPI registers pertaining to the LPF.
Table 20. SPI Registers for Rx Low-Pass Filter
Address (Hex)
0x07
0x08
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 cutoff frequency, f
results in −3 dB attenuation. Also, the actual group delay time
(GDT) response can be calculated given a programmed cutoff
frequency using the following equation:
Actual GDT = Normalized GDT/(2.45 × f
5
0
–5
–10
–15
–20
–25
–30
–35
0
0.5
1.0
Figure 70. LPF's Normalized Wideband Gain Response
of 55 MHz.
−3 dB
Bit
Description
(0)
Enable Rx LPF
(7:0)
Target value
−3 dB
)
−3 dB
1.5
2.0
2.5
3.0
FREQUENCY
0.25
0
NORMALIZED GAIN RESPONSE
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
–1.75
–2.00
–2.25
NORMALIZED GROUP DELAY
–2.50
–2.75
–3.00
0
0.1
Figure 71. LPF's Normalized Pass-Band Gain and Group Delay Responses
The −3 dB cutoff frequency, f
an 8-bit word, referred to as the target, to Register 0x08. The
cutoff frequency is a function of the ADC sample rate, f
to a lesser extent, the RxPGA gain setting (in dB). Figure 72
shows how the frequency response, f
the RxPGA gain setting.
3
0
,
–3
–6
–9
(7)
–12
–15
–18
0
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
The following formula
RxPGA gain setting of 0 dB:
f
= (128/target) × (f
−3 dB_0 dB
Figure 73 compares the measured and calculated f
formula.
1
Empirically derived for a f
to 80 MSPS with an RxPGA = 0 dB.
Rev. A | Page 34 of 48
0.2
0.3
0.4
0.5
0.6
0.7
NORMALIZED FREQUENCY
, is programmable by writing
−3 dB
, varies as a function of
−3 dB
5
10
15
20
25
30
INPUT FREQUENCY (MHz)
( f
= 32 MHz (@ 0 dB and f
= 80 MSPS)
−3 dB
ADC
1
can be used to estimate f
/80) × (f
ADC
range of 15 MHz to 35 MHz and f
−3 dB
1.30
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.8
0.9
1.0
, and
ADC
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
35
40
45
50
for a
−3 dB
/30 + 23.83)
(8)
ADC
using this
−3 dB
of 40 MSPS
ADC
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