LT5518 데이터 시트보기 (PDF) - Linear Technology
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LT5518 Datasheet PDF : 16 Pages
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LT5518
APPLICATIO S I FOR ATIO
Some DACs use an output common mode voltage of 3.3V.
In that case, the interface circuit drawn in Figure 4 may be
used. The performance is very similar to the performance
of the DAC interface drawn in Figure 3, since the source
and load impedances of the lowpass ladder filter are both
200Ω differential and the current drive is the same. There
are some small differences:
• The baseband drive capability cannot be improved using
an extra supply voltage, since the compliance range of
the DACs in Figure 4 is typically 3.3V – 0.5V to 3.3V +
0.5V, so its range has already been fully used.
• GDC and f–3dB are a little different, since R3A (and R3B)
is 4.99k instead of 5.6k to accommodate the proper
DC level shift.
LO Section
The internal LO input amplifier performs single-ended to
differential conversion of the LO input signal. Figure 5
shows the equivalent circuit schematic of the LO input.
The internal, differential LO signal is split into in-phase
and quadrature (90° phase shifted) signals that drive LO
buffer sections. These buffers drive the double balanced I
and Q mixers. The phase relationship between the LO input
and the internal in-phase LO and quadrature LO signals
is fixed, and is independent of start-up conditions. The
phase shifters are designed to deliver accurate quadrature
signals for an LO frequency near 2GHz. For frequencies
VCC
20pF
LO
INPUT
5518 F05
ZIN ≈ 57Ω
Figure 5. Equivalent Circuit Schematic of the LO Input
significantly below 1.8GHz or above 2.4GHz, the quadra-
ture accuracy will diminish, causing the image rejection
to degrade. The LO pin input impedance is about 50Ω,
and the recommended LO input power is 0dBm. For lower
LO input power, the gain, OIP2, OIP3 and dynamic range
will degrade, especially below –5dBm and at TA = 85°C.
For high LO input power (e.g. 5dBm), the LO feedthrough
will increase, without improvement in linearity or gain.
Harmonics present on the LO signal can degrade the image
rejection, because they introduce a small excess phase shift
in the internal phase splitter. For the second (at 4GHz) and
third harmonics (at 6GHz) at –20dBc level, the introduced
signal at the image frequency is about –55dBc or lower,
corresponding to an excess phase shift much less than 1
degree. For the second and third harmonics at –10dBc,
still the introduced signal at the image frequency is about
–46dBc. Higher harmonics than the third will have less
impact. The LO return loss typically will be better than
14dB over the 1.7GHz to 2.4GHz range. Table 1 shows
the LO port input impedance vs frequency.
RF = 5.5dBm, MAX
5V
VCC
C
BALUN
3.3V
0mA TO
20mA
DAC
0mA TO
20mA
10
C4A
3.3nF
LOMI
L1A
L2A
3.3VDC
BBPI
200Ω
R4A
3.01k
R3A 2.1VDC
4.99k
1.8pF
1.3k
C1
C2
C3
GND
L1B
L2B
R3B
R4B
4.99k
1.8pF
1.3k
3.01k
BBMI
200Ω
3.3VDC
2.1VDC
VREF = 500mV
CM
C4B
GND
3.3nF
Figure 4. LT5518 5th Order Filtered Baseband Interface with 3.3VCM DAC (Only I-Channel is Shown).
LT5518
FROM
Q
LOPI
5518 F04
5518f
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