參數(shù)資料
型號(hào)： LTI969CMS
英文描述： Voltage-Feedback Operational Amplifier
中文描述： 電壓反饋運(yùn)算放大器
文件頁(yè)數(shù)： 16/20頁(yè)
文件大?。?/td> 285K
代理商： LTI969CMS
16
LT1969
Figure 10. Receiver Configuration
R
BT
–V
a
V
a
–V
L
V
L
R
BT
1969 F10
1:n
R
L
+
–
V
BIAS
V
RX
+
–
LT1813
+
–
LT1813
R
F
R
F
R
G
R
D
R
G
R
D
R
L
n
2
= REFLECTED IMPEDANCE
R
L
2n
2
R
L
2n
2
= ATTENUATION OF V
a
+ R
BT
R
L
2n
2
R
L
2n
2
R
G
R
D
=
SET
+ R
BT
is also 10, but the power consumption has been reduced
to 350mW, a savings of 36% over the previous design.
Note that the reduction of the back-termination resistor
has allowed use of a 1:1 transformer ratio.
Table 2 compares the two approaches. It may seem that
the low power design is a clear choice, but there are further
system issues to consider. In addition to driving the line,
the amplifiers provide back-termination for signals that
are received simultaneously from the line. In order to
reject the drive signal, a receiver circuit is used such as
shown in Figure 10. Taking advantage of the differential
nature of the signals, the receiver can subtract out the
drive signal and amplify the received signal. This method
works well for standard back-termination. If the back-
termination resistors are reduced by positive feedback, a
portion of the received signal also appears at the amplifier
outputs. The result is that the received signal is attenuated
by the same amount as the reduction in the back-termina-
tion resistor. Taking into account the different transformer
turns ratios, the received signal of the low power design
will be one third of the standard design received signal.
The reduced signal has system implications for the sensi-
tivity of the receiver. The power reduction may, or may not,
be an acceptable system tradeoff for a given design.
Controlling the Quiescent Current
The quiescent current of the LT1969 is controlled via two
control pins, CTRL1 and CTRL2. The pins can be used to
either turn off the amplifiers, reducing the quiescent cur-
rent on
±
2.5V supplies to less than 500
μ
A per amplifier, or
to control the quiescent current in normal operation.
Figure 11 shows how the control pins are used in conjunc-
tion with external resistors to program the supply current.
In normal operation, each control pin is biased to approxi-
mately 1V above V
–
and by varying the resistor values, the
current from each control pin can be adjusted. It is this
current that sets the supply current of both amplifiers. If
one of the resistors is open, i.e. R2, the supply current of
the amplifiers will be set by CTRL1 and R1. Figure 12
shows supply current vs resistor value.
APPLICATIOU
W
U
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