Low-Pass Filter - Analog Devices AD9865 Manual

AD9865

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) Bit
0x07 (0)
0x08 (7:0)
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
FREQUENCY
Figure 70. LPF's Normalized Wideband Gain Response
of 55 MHz.
−3 dB
Description
Enable Rx LPF
Target value
,
−3 dB
) (7)
−3dB
1.5 2.0 2.5 3.0
Rev. A | Page 34 of 48
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 0.2 0.3 0.4 0.5
NORMALIZED FREQUENCY
Figure 71. LPF's Normalized Pass-Band Gain and Group Delay Responses
The −3 dB cut-off frequency, f
−3 dB
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
–12
–15
–18
0 5 10 15 20
INPUT FREQUENCY (MHz)
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
(f = 32 MHz (@ 0 dB and f
−3 dB
1
The following formula can be used to estimate f
RxPGA gain setting of 0 dB:
f = (128/target) × (f
−3dB_0dB ADC
Figure 73 compares the measured and calculated f
formula.
1
Empirically derived for a f range of 15 MHz to 35 MHz and f
−3 dB
to 80 MSPS with an RxPGA = 0 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
0.6 0.7 0.8 0.9 1.0
, is programmable by writing
, and
ADC
, varies as a function of
−3 dB
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
25 30 35 50
40 45
= 80 MSPS)
ADC
for a
−3 dB
/80) ×(f /30 + 23.83) f (8)
ADC
using this
−3 dB
of 40 MSPS
ADC