AD9865BCP Analog Devices Inc, AD9865BCP Datasheet - Page 34

• Manufacturer Part Number: AD9865BCP
• Description: IC FRONT-END MIXED-SGNL 64-LFCSP
• Manufacturer: Analog Devices Inc

Specifications of AD9865BCP

• Rohs Status: RoHS non-compliant
• Rf Type: HPNA, VDSL
• Features: 10-bit ADC(s), 10-bit DAC(s)
• Package / Case: 64-VFQFN, CSP Exposed Pad

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)
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
–10
–15
–20
–25
–30
–35
–5
5
0
0
Figure 70. LPF’s Normalized Wideband Gain Response
0.5
1.0
FREQUENCY
Bit
(0)
(7:0)
1.5
2.0
Description
Enable Rx LPF
Target value
−3 dB
−3dB
of 55 MHz.
2.5
)
−3 dB
3.0
,
Rev. A | Page 34 of 48
(7)
The −3 dB cut-off 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.
The following formula
RxPGA gain setting of 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.
Figure 71. LPF’s Normalized Pass-Band Gain and Group Delay Responses
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
–1.75
–2.00
–2.25
–2.50
–2.75
–3.00
0.25
f
0
−3dB_0dB
–12
–15
–18
0
–3
–6
–9
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
NORMALIZED GROUP DELAY
NORMALIZED GAIN RESPONSE
3
0
0
0.1
= (128/target) × (f
5
(f
0.2
−3 dB
10
= 32 MHz (@ 0 dB and f
NORMALIZED FREQUENCY
0.3
−3 dB
1
15
INPUT FREQUENCY (MHz)
can be used to estimate f
0.4
range of 15 MHz to 35 MHz and f
20
0.5
−3 dB
ADC
25
/80) ×(f
, is programmable by writing
0.6
−3 dB
30
ADC
0.7
, varies as a function of
= 80 MSPS)
ADC
35
0.8
/30 + 23.83) f
40
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
−3 dB
0.9
−3 dB
45
ADC
for a
1.0
using this
of 40 MSPS
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
ADC
50
, and
(8)