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參數(shù)資料
| 型號(hào)： | AD6121ARS |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 通信及網(wǎng)絡(luò) |
| 英文描述： | CDMA 3 V Receiver IF Subsystem with Integrated Voltage Regulator |
| 中文描述： | SPECIALTY TELECOM CIRCUIT, DSO28 |
| 封裝： | SSOP-28 |
| 文件頁(yè)數(shù)： | 12/16頁(yè) |
| 文件大?。?/td> | 258K |
| 代理商： | AD6121ARS |
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AD6121
–12–
REV. B
Power-Down Control
The AD6121 can operate with the IF amplifier and I and Q
demodulator stages both powered up, the IF amplifiers powered
up alone or both the IF amplifiers and demodulator powered
down. The AD6121 cannot operate with the demodulator pow-
ered up and the IF amplifiers powered down. The control for
these different modes is performed via the PD1 and PD2 pins.
Table I shows the decoding of the logic inputs.
Table I. AD6121 Operating Modes
PD1
PD2
IF Amplifiers
Demodulator
0
0
1
1
0
1
0
1
ON
ON
INVALID STATE
OFF
ON
OFF
INVALID STATE
OFF
Low Dropout Regulator
The AD6121 incorporates an integrated low dropout regulator.
The regulator accepts inputs from 2.9 V to 4.2 V and supplies
2.7 V at LDOC. The 2.7 V signal can be used to provide the dc
voltages required for the DVCC and IFVCC dc supplies. In order
to configure the low dropout regulator, an external pass transistor
is required. A pnp transistor with a minimum h
FE
of 100 and a
maximum h
FE
of 300 and a V
CE
(Sat.) of
–
0.4 V is required. In
order to use the low dropout regulator, configure the transistor
as shown in Figure 32. The 10 nF capacitor in Figure 32 is used
for decoupling the 2.7 V dc signal.
In addition to the low dropout regulator, there is a bandgap
voltage reference which produces a 1.23 V reference voltage at
pin REFOUT. This reference voltage will be present whenever a
voltage is applied to IFVCC and DVCC. This 1.23 V reference
voltage can then be used to provide the gain reference voltage
for the receive ADCs in the baseband converter.
AD6121
LDOC
LDOB
LDOE
REFOUT
1.23V
PASS
TRANSISTOR
2.7V
2.7V TO 4.2V
10nF
Figure 32.
It is possible to bypass the low dropout regulator on the AD6121
and use an external regulator instead. In order to bypass the
integrated low dropout regulator, connect pins LDOE, LDOB
and LDOC together and then connect them all to the external
regulator voltage. This configuration is shown in Figure 33.
Even when the low dropout regulator is bypassed, the 1.23V
reference voltage at pin REFOUT is still present.
Configuring the Low Dropout Regulator
AD6121
LDOC
LDOB
LDOE
REFOUT
1.23V
FROM EXTERNAL
VOLTAGE
REGULATOR
Figure 33. Configuration for Bypassing the Low Dropout
Regulator
ROOFING FILTER
When the IF amplifiers and I and Q demodulator of the AD6121
are both powered up, the parasitic impedances seen at the out-
put of the IF amplifiers and inputs of the I and Q demodulator
are high enough to create a low-pass filter, which may attenuate
the IF signal. Consequently, the parasitic capacitance must be
cancelled by using an external inductor to form a parallel reso-
nant circuit. The external inductor that is required and the
internal parasitic capacitors form what is known as the roofing
filter, with the resonant frequency given by Equation 1 (see IF
Amplifiers and Gain Control section of this data sheet).
The roofing filter may be composed of a shunt inductor between
the IF amplifiers differential output pins. Because the demodu-
lator is powered up when the output of the IF amplifiers are fed
into the I and Q demodulator, the output of the IF amplifiers
are not open collector. As a result, pull up inductors are not
required. This configuration is shown in Figure 34. The 10 nF
capacitors are used for ac coupling.
L
AD6121
DEMIPP
DEMIPN
IFOPN
IFOPP
2C
PAR
2C
PAR
10nF
10nF
Figure 34. Roofing Filter Configuration
In order to confirm whether the roofing filter has been correctly
designed, sweep the IF frequency and view the output of the I
and Q demodulator on a spectrum analyzer. The output level
of the I or Q signal should be approximately flat from dc to
16 MHz, after which the low-pass filters at the I and Q output
will attenuate the signal. With the correct roofing filter induc-
tor, the I and Q output signal will be higher than for any other
roofing filter inductor value.
It should be noted that the roofing filter is only required when
cascading the output from the IF amplifiers to the input of the I
and Q demodulator. If we are looking at the output of the IF
amplifiers, no roofing filter is required. Because the IF amplifi-
ers
’
outputs are open collector when the I and Q demodulator is
powered down, pull-up inductors will be required in order to set
the dc voltage (see the section of this data sheet entitled IF
Amplifiers and Gain Control) and to resonate the parasitic
capacitors that are present under these conditions.
LEVEL DIAGRAM
Figure 35 is provided in order to better understand the different
voltage and power levels you can expect to see at different points
on the AD6121. It represents the levels that would be seen on
Rev. B of the AD6121 Customer Evaluation Board. When try-
ing to make these measurements, a high impedance (10 M
)
active FET probe (for example, the TEK P6204 from Tektronix)
should be used to minimize the effects of loading the circuits
with the probe.
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