AD9869-EBZ Analog Devices Inc, AD9869-EBZ Datasheet - Page 25

• Manufacturer Part Number: AD9869-EBZ
• Description: BOARD EVAL FOR AD9869
• Manufacturer: Analog Devices Inc
• Type: ADC + DAC, Codec, Front End for RFr

Specifications of AD9869-EBZ

• Contents: Board
• For Use With/related Products: AD9869
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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. The first real pole is implemented within the
first CPGA gain stage (see Figure 24), and the complex pole pair
is implemented in the second CPGA gain stage. Capacitor
arrays are used to vary the different RC time constants 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 19 shows the SPI registers pertaining to the LPF.
Table 19. SPI Registers for Rx Low-Pass Filter
Address (Hex)
0x07
0x08
The normalized wideband gain response is shown in Figure 26.
The normalized pass-band gain and group delay responses are
shown in Figure 27. The −3 dB cutoff frequency, f
−3 dB attenuation. In addition, 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 26. LPF Normalized Wideband Gain Response
0.5
1.0
FREQUENCY
Bit
0
7:0
1.5
Description
Enable Rx LPF.
Target value.
2.0
−3 dB
−3dB
of 55 MHz.
2.5
)
−3 dB
, results in
B
3.0
Rev. 0 | Page 25 of 36
(4)
The f
as the target, to Register 0x08. The cutoff frequency is a function
of the ADC sample rate, f
gain setting (in dB). Figure 28 shows how f
function of the RxPGA gain setting.
The following formula
RxPGA gain setting of 0 dB:
Figure 29 compares the measured and calculated f
formula.
1
Empirically derived for an f
40 MSPS to 80 MSPS with an RxPGA = 0 dB.
–0.25
–0.50
–0.75
–1.00
–1.25
–1.50
–1.75
–2.00
–2.25
–2.50
–2.75
–3.00
Figure 27. LPF Normalized Pass-Band Gain and Group Delay Responses
0.25
f
−3 dB
−3dB_0dB
0
–12
–15
–18
0
Figure 28. Effects of RxPGA Gain on LPF Frequency Response
–3
–6
–9
NORMALIZED GROUP DELAY
NORMALIZED GAIN RESPONSE
3
0
0
is programmable by writing an 8-bit word, referred to
0.1
= (128/target) × (f
5
(f
0.2
−3 dB
10
= 32 MHz @ 0 dB and f
0.3
NORMALIZED FREQUENCY
1
−3 dB
15
INPUT FREQUENCY (MHz)
can be used to estimate f
0.4
ADC
range of 15 MHz to 35 MHz and an f
20
, and to a lesser extent, the RxPGA
0.5
ADC
25
/80) ×(f
0.6
30
ADC
0.7
= 80 MSPS)
ADC
35
−3 dB
/30 + 23.83) f
0.8
varies as a
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
40
−3 dB
0.9
−3 dB
45
AD9869
B
for a
1.0
using this
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
of
(5)