AD9874ABSTZ Datasheet, PDF (23/40 Pages) Analog Devices – IF Digitizing Subsystem

AD9874
The mixer’s differential LO port is driven by the LO buffer
stage shown in Figure 6, 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 negligible
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
⌺-⌬ ADC (i.e., f_{IF2_ADC} = f_CLK/8). The two inductors provide a
dc 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 ⌺-⌬ 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 ⌺-⌬ modulator, and so
must be tuned to the f_CLK/8 center frequency of the modulator.
The inductors should be chosen such that their impedance at
f_CLK/8 is about 140 Ω (i.e., L = 180/f_CLK). An accuracy of 20%
is considered to be adequate. For example, at f_CLK = 18 MHz,
L = 10 µH is a good choice. Once the inductors have been
selected, the required tank capacitance may be calculated using
the relation f_CLK/8 = 1/{2 ϫ ␲ ϫ (2L ϫ C)^1/2}.
For example, at f_CLK = 18 MHz and L = 10 µH, a capacitance of
250 pF is needed. However, in order to accommodate an induc-
tor tolerance of Ϯ10%, 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 capaci-
tances as high as 30 pF, the total capacitance will be less than
the minimum 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 that if f_CLK
is increased by a factor of 1.44 MHz to 26 MHz so that f_CLK/8
becomes 3.25 MHz, reducing L and C by approximately the
same factor (i.e., L = 6.9 µH and C = 120 pF) still satisfies the
requirements stated above.
The selection of the inductors is an important consideration in
realizing the full linearity performance of the AD9874. This is
true 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_CLK of 18 MHz varies between Toko’s FSLM
series and Coilcraft’s 1812CS series inductors. The graph also
shows the extrapolated point of intersection used to determine
the IIP3 performance. Note that the Coilcraft inductor provides
a 7 dB to 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 perfor-
mance similar to that of the 1812CS series. It is worth noting
that the difference in IMD performance between these two
inductor families with an f_CLK of 26 MHz is insignificant.
0
f_IN = 109.65MHz
–20
P_IN
–40
–60
TOKO INDUCTOR
–80
P_IMD = 2.64 ؋ P_IN + 4.6
–100
–120
COILCRAFT
P_IMD = 2.92 ؋ P_IN + 6.9
–140
–54
–48
–42
–36
–30
–24
–18
Figure 10. IMD Performance between Different Inductors
with LNA and Mixer at Full Bias and f_CLK of 18 MHz
Both the LNA and mixer have four programmable bias settings so
that current consumption can be minimized for a given application.
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 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.
13
–20
CLIP POINT
12
–18
11
–16
10
–14
NOISE FIGURE
9
–12
8
–10
1_0 1_1 1_2 1_3 2_0 2_1 2_2 2_3 3_0 3_1 3_2 3_3
LNA_MIXER BIAS SETTING
Figure 11a. LNA/Mixer Noise Figure and
Conversion Gain vs. Bias Setting
5
9.50
0
LNA_MIXER CURRENT
8.25
–5
7.00
–10
5.75
IIP3
–15
4.50
–20
3.25
–25
1_0 1_1
1_2 1_3 2_0 2_1 2_2 2_3 3_0
LNA_MIXER BIAS SETTING
3_1 3_2
2.00
3_3
Figure 11b. LNA/Mixer IIP3 and Current
Consumption vs. Bias Setting
REV. A
–23–