AD6122ACP 查看數據表（PDF） - Analog Devices
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AD6122ACP Datasheet PDF : 20 Pages
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AD6122
MODOPP R1
ZIN
R2
MODOPN R1
AD6122
IFINP
IFINN
RSHUNT >>R2
Figure 28. Pad Topology
1
L
=
20
log
R1
1+ 1
R1 R2/
2
(3)
ZIN = 2R1+ R2
(4)
where L is the transducer loss (or loss through the pad) in dB
and ZIN is the desired input resistance in ohms. Using these
equations, we can design the attenuator circuit to provide what-
ever amount of attenuation we require.
This circuit is very sensitive to parasitic capacitances. As a re-
sult, extra care should be taken to ensure minimum and equal
printed circuit board transmission lines. We should also try to
keep R2 small in order to minimize the effects of printed circuit
board parasitic capacitance on loading the output of the pad.
In conclusion, we have to develop a system-level ACPR budget
for our radio, and from that budget determine how much ACPR
performance we desire from the AD6122. We then need to imple-
ment the appropriate attenuation network to get that ACPR
performance.
LEVEL DIAGRAM
Figure 29 is provided to better understand the different voltage
levels you can expect to see at different points of the AD6122. It
represents the voltage and power levels expected for a maximum
input condition of 500 mV p-p at the I and Q modulator and
maximum gain in the IF amplifiers. When trying to make these
measurements, a high impedance (10 MΩ) active FET probe
(for example, the Tek P6204, from Tektronix) should be used to
minimize the effects of loading the circuit with the probe.
In order to produce these results, the attenuator is designed to
have a 1 kΩ input impedance and the output of the IF amplifiers
are loaded with 1 kΩ. The roofing filter is designed to resonate
the parasitic capacitance at the IF frequency.
MODULATORS
I
500mV p-p
DIFFERENTIAL
LO
،2
100mV p-p
DIFFERENTIAL
Q
500mV p-p
DIFFERENTIAL
–21dBm
(REFERRED TO 1k⍀)
252.1mV p-p DIFFERENTIAL
MODOP
VCC
–41dBm
(REFERRED TO 1k⍀)
25.21mV p-p
DIFFERENTIAL
IF AMPLIFIERS
IFIN
VGAIN = 2.5V
GAIN = +34dB
ZIN = 1k⍀
20dB
ATTENUATOR
Figure 29. Level Diagram
–7dBm
(REFERRED TO 1k⍀)
1.263V p-p DIFFERENTIAL
TRANSMIT
OUTPUT
1k⍀
ZOUT = 1k⍀
–14–
REV. B
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