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參數(shù)資料
| 型號： | AD6122 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | CDMA 3 V Receiver IF Subsystem with Integrated Voltage Regulator(低功耗接收中頻子系統(tǒng)) |
| 中文描述： | CDMA的3 V接收機中頻子系統(tǒng)集成穩(wěn)壓器（低功耗接收中頻子系統(tǒng)） |
| 文件頁數(shù)： | 11/19頁 |
| 文件大?。?/td> | 262K |
| 代理商： | AD6122 |
REV. 0
AD6122
–11–
The AD6122’s overall gain, expressed in decibels, is linear in
dB with respect to the automatic gain control (AGC) voltage,
VGAIN (Pin 28). Either Pin 26 (REFOUT), or an external
reference voltage connected to REFIN (Pin 27), may be used to
set the voltage range for VGAIN. When the internal 1.23 V
reference, REFOUT (Pin 26), is connected to REFIN (Pin 27),
VGAIN will control the entire AGC range when it is typically
set between 0.5 V and 2.5 V. Minimum gain occurs at mini-
mum voltage on VGAIN and maximum gain occurs at maxi-
mum voltage on VGAIN. The maximum and minimum gain
will not change with a change in voltage at REFIN. Rather, the
slope of the gain curve will change as a result of a change in the
required range for VGAIN. Figure 24 shows the piecewise linear
approximation of the gain curve for the AD6122.
MINIMUM
GAIN
MAXIMUM
GAIN
G
VGAIN – V
Figure 24. Piecewise Linear Approximation for the
AD6122 Gain Curve
Because the minimum and maximum gain from the AD6122
are constant, we can determine the VGAIN range for a given
REFIN voltage by using Equations 1 and 2.
MinimumGain
VGAIN
REFIN
=
×
0 4
(1)
MaximumGain
VGAIN
REFIN
=
×
2
(2)
where
Maximum Gain
for the AD6122 is +34 dB or 50.1 V/V,
and
Minimum Gain
is –63 dB or 0.708 mV/V,
VGAIN
is the
gain control voltage applied to Pin 28 of the AD6122 and
RE-
FIN
is the reference input voltage, Pin 27. Consequently, for any
REFIN we choose, we can calculate the VGAIN range by rear-
ranging and solving Equations 1 and 2 for VGAIN.
Power-Down Control
The AD6122 can be operated with the IF amplifiers and quadra-
ture modulator both powered up, both powered down or with
the IF amplifiers powered up and the modulator powered down.
The AD6122 cannot operate with only the modulator powered
up. The control is provided via two control pins, PD1 (Pin 1)
and PD2 (Pin 2). Table I shows the operating modes of the
AD6122.
Table I. Operating Modes
PD1
PD2
IF Amp
Modulator
0
0
1
1
0
1
0
1
ON
ON
INVALID STATE
OFF
ON
OFF
INVALID STATE
OFF
Low Dropout Regulator
The AD6122 incorporates an integrated low dropout regulator.
The regulator accepts inputs from 2.9 V to 4.2 V and supplies a
constant 2.7 V reference output at Pin 5 (LDOC). The 2.7 V
signal can be used to provide the dc voltages required for the
DVCC (Pin 10), TXVCC (Pin 13) and IFVCC (Pin 25) dc
supplies. In order to configure the low dropout regulator, an
external pass transistor is required. A pnp bipolar junction tran-
sistor with a minimum h
FE
of 100 and a maximum h
FE
of 300
and a VCE
SAT
of –0.4 V is required. In order to use the low
dropout regulator, configure the transistor as shown in Figure
25. The 18 pF capacitor in Figure 25 is used for decoupling the
2.7 V dc signal.
In addition to the low dropout regulator, a band-gap voltage
reference produces a 1.23 V reference voltage at Pin 26
(REFOUT). This reference voltage will be present whenever a
2.7 V dc signal is present on pin LDOC (Pin 5). This 1.23 V
reference voltage can then be used to provide the gain reference
signal required for Pin 27 (REFIN) and the reference voltage
for the transmit DACs in a baseband converter.
AD6122
LDOE
LDOC
LDOB
18pF
2.9V – 4.2V
2.7V
REFOUT
1.23V
PASS
TRANSISTOR
Figure 25. Configuring the Low Dropout Regulator
It is possible to bypass the low dropout regulator on the AD6122
and use an external regulator instead. In order to bypass the
integrated low dropout regulator, connect pins LDOE (Pin 3),
LDOB (Pin 4) and LDOC (Pin 5) together and then connect
them all to the 2.7 V external regulator voltage. This configura-
tion is shown in Figure 26. Even when the low dropout regula-
tor is bypassed, the 1.23 V reference voltage at pin REFOUT
(Pin 26) is still present.
FROM EXTERNAL
VOLTAGE REGULATOR
AD6122
LDOE
LDOC
LDOB
REFOUT
1.23V
Figure 26. Configuration for Bypassing the Low Dropout
Regulator
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