AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 22

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

AD9874
The mixer’s differential LO port is driven by the LO buffer stage
shown in Figure 6 that 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 negligible effect on per-
formance. The mixer’s open-collector outputs, MXOP and MXON,
drive an external resonant tank consisting of a differential LC net-
work tuned to the IF of the band-pass Σ-∆ ADC (i.e., f
f
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
The inductors should be chosen such that their impedance at
f
considered to be adequate. For example, at f
µH is a good choice. Once the inductors have been selected, the
required tank capacitance may be calculated using the relation
f
For example, at f
250 pF is needed. However, in order to accommodate an induc-
tor tolerance of ± 10%, the tank capacitance must be adjustable
from 227 pF to 278 pF. Selecting an external capacitor of 180 pF
ensures that even with a 10% tolerance and stray capacitances
as high as 30 pF, the total capacitance will be less than the mini-
mum value needed by the tank. Extra capacitance is supplied by
the AD9874’s on-chip programmable capacitor array. Since the
programming range of the capacitor array is at least 160 pF, the
AD9874 has plenty of range to make up for the tolerances of low
cost external components. Note, if f
1.44 MHz to 26 MHz so that f
ing L and C by approximately the same factor (i.e., L = 6.9 µH
and C = 120 pF), the above stated requirements are satisfied.
The selection of the inductors is an important consideration in
realizing the full linearity performance of the AD9874. This is
especially the case when operating the LNA and mixer at maximum
bias and low clock frequency. Figure 10 shows how the two-tone
input-referred IMD versus the input level performance at an IF of
109 MHz and f
and Coilcraft’s 1812CS series inductors. The graph also shows
the extrapolated point of intersection used to determine the IIP3
performance. Note, the Coilcraft inductor provides a 7 dB–8 dB
improvement in performance and closely approximates the 3:1 slope
associated with a third order linearity compared to the 2.65:1
slope associated with the Toko inductor. The Coilcraft 1008CS
series showed similar performance to the 1812CS series. It is worth
noting that the difference in IMD performance between these
two inductor families with an f
CLK
CLK
CLK
/8 is about 140 Ω (i.e., L = 180/f
/8 = 1/{2
/8). The two inductors provide a dc bias path for the mixer
CLK
CLK
of 18 MHz varies between Toko’s FSLM series
= 18 MHz and L = 10 µH, a capacitance of
(2L
CLK
/8 center frequency of the modulator.
C)
1/2
CLK
CLK
}.
/8 becomes 3.25 MHz, reduc-
of 26 MHz is insignificant.
CLK
CLK
is increased by a factor of
). An accuracy of 20% is
CLK
= 18 MHz, L = 10
IF2_ADC
=
–22–
Both the LNA and mixer have four programmable bias settings
so that current consumption can be minimized for a given appli-
cation. Figures 11a, 11b, and 11c show how the LNA and mixer’s
noise figure (NF), linearity (IIP3), IF clip point, current consumption,
and frequency response are all affected for a given LNA/mixer
bias setting. The measurements were taken at an IF = 73.35 MHz
and LO = 71.1 MHz with supplies set to 3 V.
Figure 10. IMD Performance between Different Inductors
with LNA and Mixer at Full Bias and f
Figure 11b. LNA/Mixer IIP3 and Current Consumption
vs. Bias Setting
Figure 11a. LNA/Mixer Noise Figure and Conversion
Gain vs. Bias Setting
–10
–15
–20
–25
13
12
11
10
–5
9
8
5
0
–100
–120
–140
1_0
1_0
–20
–40
–60
–80
0
–54
1_1
1_1
NOISE FIGURE
1_2
1_2 1_3
LNA_MIXER CURRENT
–48
1_3 2_0
P
IMD
LNA_MIXER BIAS SETTING
LNA_MIXER BIAS SETTING
TOKO INDUCTOR
CLIP POINT
2_0 2_1 2_2
= 2.64
–42
P
IN
2_1 2_2 2_3
P
P
IN
IIP3
IMD
–36
+ 4.6
COILCRAFT
= 2.92
2_3 3_0
F
IN
= 109.65MHz
3_0
–30
P
IN
CLK
+ 6.9
3_1
3_1 3_2
of 18 MHz
–24
3_2
3_3
3_3
–20
–18
–16
–14
–12
–10
9.50
8.25
7.00
5.75
4.50
3.25
2.00
–18
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