LT5558 데이터 시트보기 (PDF) - Linear Technology
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LT5558 Datasheet PDF : 16 Pages
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LT5558
APPLICATIONS INFORMATION
10
0 –40°C
85°C
–10
–20
–30
85°C
–40
PRF
25°C
–40°C
LOFT
VCC = 5V
EN = HIGH
fLO = 900MHz,
fBBI = 2MHz, 0°
fBBQ = 2MHz, 90°
fRF = fBB + fLO
PLO = 0dBm
–50
IR
–60
–40°C
–70
25°C
–80
–90
0
1
2
3
4
5
I AND Q BASEBAND VOLTAGE (VP-P,DIFF)
5558 F14
Figure 14. LO Feedthrough and Image Rejection vs
Baseband Drive Voltage After Calibration at 25°C
The common-mode voltage on the LT5558 is sampled
using resistors R7, R8, R17 and R18 and shifted up to
about 2.5V using resistor R9. Op amp U4 compensates
for the gain loss in the resistor networks and provides a
low-ohmic drive to steer the common-mode input pins
of U2 and U3. Resistors R20 and R21 improve op amp
U4’s stability while driving the large supply decoupling
capacitors C3 and C4. This corrected common-mode
voltage is applied to the common-mode input pins of U2
and U3 (pins 3). This results in a positive feedback loop
for the common mode voltage with a loop gain of about
-10dB. This technique ensures that the current compliance
on the baseband input pins of the LT5558 is not exceeded
under supply voltage or temperature extremes, and internal
diode voltage shifts or combinations of these. The core
current of the LT5558 is thus maintained at its designed
level for optimum performance. The recommended com-
mon-mode voltage applied to the inputs of the LTC1565
is about 2V. Resistor tolerances are recommended 1%
accuracy or better. The total current consumption is about
160mA and the noise floor at 20MHz offset is –147dBm/Hz
with 3.7dBm RF output power. For a 2VPP, DIFF baseband
input swing, the output power at fLO + fBB is 1.6dBm
and the third harmonic at fLO – 3fBB is –48.6dBm. For a
2.6VPP, DIFF input, the output power at fLO + fBB is 3.8dBm
and the third harmonic at fLO – 3fBB is –40.5dBm.
R20
249Ω
VCC
R5
3.57k
R6
R9
3.57k 88.7k
R22
22.1k
R22
22.1k
RF = 3dBm MAX
R24
3.32k
4– 5
3 +U4
2
LT1797
1
R16
3.57k
R15
3.57k
C1, C2
2 × 0.1µF
1
BB
+IN
SOURCE 2 –IN
U2
8
+OUT
–OUT 7
2.5VDC
C3
0.1µF
LTC1565-31
3 GND
V+ 6
4 V–
SHDN 5
R1
499Ω
2.5VDC
R2
499Ω
2.1VDC
R3
3.01k
R4
3.01k
2.1VDC
8, 13 11 1
14 VCC RF EN 7
BBPI
BBPQ
R7
R17
49.9k
49.9k
U1
LT5558
R8
R18
49.9k
49.9k
16 BBMI BBMQ 5
GND
LO
2, 4, 6, 9, 10
3
12, 15, 17
2.1VDC
R13
3.01k
R14
3.01k
2.1VDC
R11
499Ω
2.5VDC
R12
499Ω
4.5V to 5.25V
R21
249Ω
8
U3
+OUT
7 –OUT
1
+IN
BB
–IN 2 SOURCE
LTC1565-31
6 V+
GND 3
5 SHDN
V– 4
2.5VDC
C4
0.1µF
5558 F16
Figure 15. Baseband Interface Schematic of the LTC1565 with the LT5558 for RFID applications.
5558fa
14
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