LT1886CS8 Просмотр технического описания (PDF) - Linear Technology
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LT1886CS8 Datasheet PDF : 16 Pages
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LT1886
APPLICATIO S I FOR ATIO
Figures 3 and 4 can be combined as shown in Figure 5. The
gain is unity at low frequencies, 1 + RF/RG at mid-band and
for stability, a gain of 10 or greater at high frequencies.
Vi
RC
CC
(OPTIONAL)
+
–
RF
Vo = 1 + RF
Vi
RG
Vo (RC || RG) ≤ RF/9
1
< 15MHz
2πRCCC
RG
1886 F03
Figure 3. Compensation for Noninverting Gains
+
Vi
–
RF
RG
CC
Vo = 1 (LOW FREQUENCIES)
Vi
VO
= 1 + RF (HIGH FREQUENCIES)
RG
RG ≤ RF/9
1
< 15MHz
2πRGCC
1886 F04
Figure 4. Alternate Noninverting Compensation
Vi
RC
CC
RG
CBIG
+
–
RF
Vo
Vo = 1 AT LOW FREQUENCIES
Vi
= 1 + RF AT MEDIUM FREQUENCIES
RG
= 1 + RF
AT HIGH FREQUENCIES
(RC || RG)
1886 F05
Figure 5. Combination Compensation
Output Loading
The LT1886 output stage is very wide bandwidth and able
to source and sink large currents. Reactive loading, even
isolated with a back-termination resistor, can cause ring-
ing at frequencies of hundreds of MHz. For this reason, any
design should be evaluated over a wide range of output
conditions. To reduce the effects of reactive loading, an
optional snubber network consisting of a series RC across
the load can provide a resistive load at high frequency.
Another option is to filter the drive to the load. If a back-
termination resistor is used, a capacitor to ground at the
load can eliminate ringing.
Line Driving Back-Termination
The standard method of cable or line back-termination is
shown in Figure 6. The cable/line is terminated in its
characteristic impedance (50Ω, 75Ω, 100Ω, 135Ω, etc.).
A back-termination resistor also equal to to the
chararacteristic impedance should be used for maximum
pulse fidelity of outgoing signals, and to terminate the line
for incoming signals in a full-duplex application. There are
three main drawbacks to this approach. First, the power
dissipated in the load and back-termination resistors is
equal so half of the power delivered by the amplifier is
wasted in the termination resistor. Second, the signal is
halved so the gain of the amplifer must be doubled to have
the same overall gain to the load. The increase in gain
increases noise and decreases bandwidth (which can also
increase distortion). Third, the output swing of the ampli-
fier is doubled which can limit the power it can deliver to
the load for a given power supply voltage.
+
Vi
–
RF
RG
CABLE OR LINE WITH
CHARACTERISTIC IMPEDANCE RL
RBT
VO
RL
RBT = RL
Vo = 1
Vi
2
(1 + RF/RG)
1886 F06
Figure 6. Standard Cable/Line Back-Termination
An alternate method of back-termination is shown in
Figure 7. Positive feedback increases the effective back-
termination resistance so RBT can be reduced by a factor
of n. To analyze this circuit, first ground the input. As RBT␣ =
RL/n, and assuming RP2>>RL we require that:
Va = Vo (1 – 1/n) to increase the effective value of
RBT by n.
Vp = Vo (1 – 1/n)/(1 + RF/RG)
Vo = Vp (1 + RP2/RP1)
Eliminating Vp, we get the following:
(1 + RP2/RP1) = (1 + RF/RG)/(1 – 1/n)
11
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