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參數資料
| 型號： | AD9866-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Broadband Modem Mixed Signal Front End |
| 中文描述： | 寬帶調制解調器混合信號前端 |
| 文件頁數： | 31/48頁 |
| 文件大小： | 1647K |
| 代理商： | AD9866-EB |
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AD9866
outputs left open
for optimum linearity performance. The
transformer
1
should be specified to handle the dc standing
current, I
BIAS
, drawn by the IAMP. Also, because I
BIAS
remains
signal independent, a series resistor (not shown) can be inserted
between AVDD and the transformer’s center-tap to reduce the
IAMP’s common-mode voltage, V
CM
, and reduce the power
dissipation on the IC. The V
CM
, bias should not exceed 5.0 V and
the power dissipated in the IAMP alone is as follows:
Rev. 0 | Page 31 of 48
CM
IAMP
P
V
I
G
N
+
=
)
(
2
Equation 2.
TxDAC
0
IOUTN–
IOUTN+
IOUTG–
IOUTG+
I
I
0 TO –7.5dB
0 TO –12dB
IAMP
R
R
R
SET
0.1
μ
F
R
L
AVDD
0.1
μ
F
I
BIAS
= 2
×
(N+G)
×
1
IOUT
PK
T:1
IOUT
PK
= (N+G)
×
1
P_OUT
PK
= (IOUT
PK
)
2
×
T
2
×
R
L
Figure 64. Current Mode Operation
A step-down transformer
1
with a turn ratio, T, can be used to
increase the output power, P_OUT, delivered to the load. This
causes the output load, R
L
, to be reflected back to the IAMP’s
differential output by T
2
resulting in a larger differential voltage
swing seen at the IAMP’s output. For example, the IAMP can
deliver 24 dBm of peak power to a 50 load, if a 1.41:1 step-
down transformer is used. This results in 5 V p-p voltage swings
appearing at IOUTN+ and IOUTN pins. Figure 42 shows how
the third order intercept point, OIP3, of the IAMP varies as a
function of common-mode voltage over a 2.5 MHz to 20.0 MHz
span with a 2-tone signal having a peak power of approximately
24 dBm with IOUT
PK
= 50 mA.
For applications requiring an IOUT
PK
exceeding 50 mA, set the
secondary’s path to deliver the additional current to the load.
IOUTG+ and IOUTN+ should be shorted as well as IOUTG
and IOUTN. If IOUT
PK
represents the peak current to be
delivered to the load, then the current gain in the secondary
path, G, can be set by the following equation:
4
5
12
/
=
PK
IOUT
G
Equation 3.
The linearity performance becomes limited by the secondary
mirror path’s distortion.
1
The B6080 and BX6090 transformers from Pulse Engineering are worthy of
consideration for current and voltage modes.
IAMP VOLTAGE MODE OPERATION
The voltage mode configuration is shown in Figure 65. This
configuration is suited for applications having a poorly defined
load that can vary over a considerable range. A low impedance
voltage driver can be realized with the addition of two external
RF bipolar npn transistors (Phillips PBR951) and resistors. In
this configuration, the current mirrors in the primary path
(IOUTN outputs) feed into scaling resistors, R, generating a
differential voltage into the bases of the npn transistors. These
transistors are configured as source followers with the secon-
dary path current mirrors appearing at IOUTG+ and IOUTG
providing a signal-dependent bias current. Note that the IOUTP
outputs
must
remain open for proper operation.
0
IOUTN–
IOUTN+
IOUTG–
IOUTG+
I
I
0 TO –7.5dB
0 TO –12dB
R
R
R
SET
0.1
μ
F
TO LOAD
AVDD
IOUT
PK
R
R
AVDD
R
S
0.1
μ
F
R
S
0.1
μ
F
DUAL NPN
PHILLIPS PBR951
IAMP
TxDAC
Figure 65. Voltage Mode Operation
The peak differential voltage signal developed across the npn’s
bases is as follows:
)
(
I
N
R
VOUT
PK
=
Equation 4.
where:
N
is the gain setting of the primary mirror.
I
is the standing current of the TxDAC defined in Equation 1.
The common-mode bias voltage seen at IOUTN+ and IOUTN
is approximately AVDD VOUT
PK
, while the common-mode
voltage seen at IOUTG+ and IOUTG is approximately the
npn’s V
BE
drop below this level (AVDD VOUT
PK
0.65). In
the voltage mode configuration, the total power dissipated
within the IAMP is as follows :
}
)
65
.
(
)
{(
2
G
VOUT
AVDD
N
VOUTPK
AVDD
I
P
+
PK
IAMP
=
Equation 5.
The emitters of the npn transistors are ac-coupled to the
transformer
1
via a 0.1 μF blocking capacitor and series resistor
of 1 to 2 . Note that protection diodes are not shown for
clarity purposes, but should be considered, if interfacing to a
power or phone line.
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