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
• Introduction of a (low frequency) time constant dur-
ing startup. For TDMA-like systems the time constant
introduced by C4A and C4B can cause some delay
during start-up. The associated time constant is ap-
proximately given by TD = RIN, CM • (C4A + C4B). In
this example it will result in a delay of about TD = 105
• 6.6n = 693ns.
The maximum sinusoidal single sideband RF output power
is about 5.5dBm for a full 0mA to 20mA DAC swing.
This maximum RF output level is usually limited by the
compliance voltage range of the DAC (VCOMPL) which is
assumed here to be 1.25V. When the DAC output voltage
swing is larger than this compliance voltage, the baseband
signal will distort and linearity requirements usually will
not be met. The following situations can cause the DAC’s
compliance voltage limit to be exceeded:
1. Too high DAC load impedance. If the DC impedance to
ground is higher than VCOMPL/IMAX = 1.25/0.02 = 62.5Ω,
the compliance voltage is exceeded for a full DAC swing. In
Figure 3, two 100Ω resistors in parallel are used, resulting
in a DC impedance to ground of 50Ω.
2. Too much DC offset. In some DACs, an additional DC
offset current can be set. For example, if the maximum
offset current is set to IMAX/8 = 2.5mA, then the maxi-
mum DC DAC load impedance to ground is reduced to
VCOMPL/IMAX • (1 + 1/8) = 1.25/0.0225 = 55Ω.
3. DC shift caused by R3A, R3B, R4A and R4B if used. The
DC shift network consisting of R3A, R3B, R4A and R4B
will increase the voltage on the DAC output by dumping
an extra current into resistors R1A, R1B, R2A and R2B.
This current is about (VCC – VDAC)/(R3A + R4A) = (5
– 0.5)/(3.01k + 5.63k) = 0.52mA. Maximum impedance
to ground will then be VCOMPL/(IMAX + ILS) = 1.25/0.02052
= 60.9Ω.
4. Reflection of out-of-band baseband signal power. DAC
output signal components higher than the cut-off frequency
of the lowpass filter will not see R2A and R2B as load
resistors and therefore will see only R1A, R1B and the
filter components as a load. Therefore, it is important to
start the lowpass filter with a capacitor (C1), in order to
shunt the DAC higher frequency components and thereby,
limit the required extra voltage headroom.
The LT5518’s output 1dB compression point is about
8.5dBm, and with the interface network described above,
a common DAC cannot drive the part into compression.
However, it is possible to increase the driving capability
by using a negative supply voltage. For example, if a –1V
supply is available, resistors R1A, R1B, R2A and R2B
can be made 200Ω each and connected with one side to
the –1V supply instead of ground. Typically, the voltage
compliance range of the DAC is –1V to 1.25V, so the DAC’s
output voltage will stay within this range. Almost 6dB extra
voltage swing is available, thus enabling the DAC to drive
the LT5518 beyond its 1dB compression point. Resistors
R3A, R3B, R4A, R4B and the lowpass filter components
must be adjusted for this case.
RF = 5.5dBm, MAX
5V
VCC
C
BALUN
LOMI
LT5518
FROM
Q
C4A
LOPI
3.3nF
L1A
L2A
0.53VDC
BBPI
200Ω
0mA TO 20mA
DAC
0mA TO 20mA
R1A
100Ω
C1
R1B
100Ω
C2
L1B
C3
L2B
R2A R4A
100Ω 3.01k
R3A 2.1VDC
5.63k
1.8pF
1.3k
R2B
100Ω R4B
R3B
5.63k
1.8pF
1.3k
3.01k
BBMI
200Ω
0.53VDC
2.1VDC
GND
C4B
3.3nF
VREF = 500mV
CM
GND
Figure 3. LT5518 5th Order Filtered Baseband Interface with Common DAC (Only I-Channel is Shown)
5518 F03
5518f
9
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