AD9865
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
0.25
0
LOW-PASS FILTER
NORMALIZED GAIN RESPONSE
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.
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
–1.75
–2.00
–2.25
–2.50
–2.75
–3.00
NORMALIZED GROUP DELAY
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
NORMALIZED FREQUENCY
Figure 71. LPF’s Normalized Pass-Band Gain and Group Delay Responses
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.
The −3 dB cut-off frequency, f−3 dB, is programmable by writing
an 8-bit word, referred to as the target, to Register 0x08. The
cutoff frequency is a function of the ADC sample rate, fADC, and
to a lesser extent, the RxPGA gain setting (in dB). Figure 72
shows how the frequency response, f−3 dB, varies as a function of
the RxPGA gain setting.
Table 20. SPI Registers for Rx Low-Pass Filter
Address (Hex)
Bit
Description
Enable Rx LPF
Target value
0x07
0x08
(0)
(7:0)
3
–6dB GAIN
0dB GAIN
0
–3
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
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−3 dB
,
–6
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:
–9
–12
–15
–18
Actual GDT = Normalized GDT/(2.45 × f−3dB
)
(7)
5
0
0
5
10
15
20
25
30
35
45
50
40
INPUT FREQUENCY (MHz)
–5
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
(f−3 dB = 32 MHz (@ 0 dB and fADC = 80 MSPS)
–10
–15
–20
–25
–30
–35
The following formula1 can be used to estimate f−3 dB for a
RxPGA gain setting of 0 dB:
f
−3dB_0dB = (128/target) × (fADC/80) ×(fADC/30 + 23.83) f
(8)
Figure 73 compares the measured and calculated f−3 dB using this
formula.
0
0.5
1.0
1.5
2.0
2.5
3.0
FREQUENCY
1 Empirically derived for a f−3 dB range of 15 MHz to 35 MHz and fADC of 40 MSPS
to 80 MSPS with an RxPGA = 0 dB.
Figure 70. LPF’s Normalized Wideband Gain Response
Rev. A | Page 34 of 48
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