AD9866 Analog Devices, AD9866 Datasheet - Page 34

• Manufacturer Part Number: AD9866
• Manufacturer: Analog Devices

Specifications of AD9866

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Nibble
• Analog Input Type: Diff-Uni
• Ain Range: 6.3 V p-p,8 mV p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9866
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:
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
Actual GDT = Normalized GDT/(2.45 × f
0
–10
–15
–20
–25
–30
–35
0
–5
5
0
NORMALIZED GROUP DELAY
NORMALIZED GAIN RESPONSE
0
Figure 70. LPF’s Normalized Wideband Gain Response
0.1
0.2
0.5
0.3
NORMALIZED FREQUENCY
1.0
0.4
FREQUENCY
Bit
(0)
(7:0)
0.5
1.5
0.6
0.7
2.0
Description
Enable Rx LPF
Target value
−3 dB
0.8
−3 dB
of 55 MHz.
2.5
)
0.9
1.0
−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
3.0
,
Rev. B | Page 34 of 48
(7)
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.
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.
f
−3 dB_0 dB
–12
–15
–18
Figure 72. Effects of RxPGA Gain on LPF Frequency Response
–3
–9
–6
35
33
31
29
27
25
23
21
19
17
15
Figure 73. Measured and Calculated f
3
0
48
0
50 MSPS MEASURED
50 MSPS CALCULATED
= (128/target) × (f
64
( f
5
−3 dB
80
10
= 32 MHz (@ 0 dB and f
for f
TARGET-DECIMAL EQUIVALENT
−3 dB
96
ADC
1
15
INPUT FREQUENCY (MHz)
can be used to estimate f
= 50 MSPS and 80 MSPS
range of 15 MHz to 35 MHz and f
112
20
−3 dB
128
ADC
, is programmable by writing
25
/80) × (f
144
−3 dB
30
80 MSPS MEASURED
ADC
80 MSPS CALCULATED
160
−3 dB
, varies as a function of
= 80 MSPS)
ADC
35
vs. Target Value
176
/30 + 23.83)
40
–6dB GAIN
0dB GAIN
+6dB GAIN
+18dB GAIN
+30dB GAIN
+42dB GAIN
192
−3 dB
−3 dB
45
ADC
208
for a
using this
of 40 MSPS
ADC
224
50
, and
(8)