AD9864 Analog Devices, AD9864 Datasheet - Page 26

• Manufacturer Part Number: AD9864
• Description: IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9864

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 18MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9864
Table 13. SPI Registers Associated with CLK Synthesizer
Address
(Hex)
0x00
0x01
0x10
0x11
0x12
0x13
0x14
0x15
0x16
IF LNA/MIXER
The AD9864 contains a single-ended LNA followed by a Gil-
bert-type active mixer, shown in Figure 43 with the required
external components. The LNA uses negative shunt feedback to
set its input impedance at the IFIN pin, thus making it depend-
ent on the input frequency. It can be modeled as approximately
370 Ω||1.4 pF (±20%) below 100 MHz. Figure 44 and Figure 45
show the equivalent input impedance versus frequency charac-
teristics of the AD9864. The increase in shunt resistance versus
frequency can be attributed to the reduction in bandwidth, thus
the amount of negative feedback of the LNA. Note that the
input signal into IFIN should be ac-coupled via a 10 nF capaci-
tor since the LNA input is self-biasing.
IFIN
R
VDDI
F
R
R
Figure 43. Simplified Schematic of AD9864’s LNA/Mixer
BIAS
GAIN
Bit
Breakdown
(7:0)
(3:2)
(5:0)
(7:0)
(4:0)
(7:0)
(6)
(5)
(4:2)
(1:0)
(3:0)
(7:0)
CXVL
MXOP
L
MULTI-TANH
V–I STAGE
2.7V TO 3.6V
Width
8
2
6
8
5
8
1
1
3
1
4
8
C
50Ω
Default
Value
0xFF
0
00
0x38
0x00
0x3C
0
0
0
0
0x0
0x04
MXON
L
DC SERVO
LOOP
Name
STBY
CKOB
CKR (13:8)
CKF
CKINV
CKI
CKTM
CKR (7:0)
CKN (12:8)
CKN (7:0)
CKFA (13:8)
CKFA (7:0)
LO INPUT =
0.3V p-p TO
1.0V p-p
CXIF
CXVM
Rev. 0 | Page 26 of 44
Figure 45. The Shunt Capacitance vs. the Frequency of the AD9864’s IF1 Input
The mixer’s differential LO port is driven by the LO buffer
stage shown in Figure 43, which can be driven single-ended or
differential. Since it is self-biasing, the LO signal level can be
ac-coupled and range from 0.3 V p-p to 1.0 V p-p with negligi-
ble effect on performance. The mixer’s open-collector outputs,
MXOP and MXON, drive an external resonant tank consisting
of a differential LC network tuned to the IF of the band-pass
Σ-∆ ADC, i.e., f
bias path for the mixer core via a series resistor of 50 Ω, which
is included to dampen the common-mode response. The
mixer’s output must be ac-coupled to the input of the band-
pass Σ-∆ ADC, IF2P, and IF2N via two 100 pF capacitors to
ensure proper tuning of the LC center frequency.
The external differential LC tank forms the resonant element for
the first resonator of the band-pass Σ-∆ modulator, and so must
be tuned to the f
inductors should be chosen such that their impedance at f
about 140, i.e., L = 180/f
be adequate. For example, at f
choice. Once the inductors have been selected, the required tank
capacitance may be calculated using the relation
2.5
2.0
1.5
1.0
0.5
600
550
500
450
400
350
300
0
0
0
50
50
Figure 44. The Shunt Input Resistance vs.
the Frequency of the AD9864’s IF1 Input
IF2_ADC
CLK
/8 center frequency of the modulator. The
100
100
= f
CLK
CLK
FREQUENCY (MHz)
FREQUENCY (MHz)
. An accuracy of 20% is considered to
/8. The two inductors provide a dc
150
150
CLK
= 18 MHz, L = 10 µH is a good
200
200
250
250
300
300
350
350
CLK
/8 is