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LT1886 Fiches technique(PDF) 10 Page - Linear Technology

No de pièce LT1886
Description  Dual 700MHz, 200mA Operational Amplifier
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

LT1886 Fiches technique(HTML) 10 Page - Linear Technology

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10
LT1886
APPLICATIO S I FOR ATIO
The dissipation for the amplifiers is:
PD = (63.5mA)(12V) – (4V/√2)2/(50) = 0.6W
The total package power dissipation is 0.6W. When a 2500
sq. mm PC board with 2oz copper on top and bottom is
used, the thermal resistance is 80
°C/W. The junction
temperature TJ is:
TJ = (0.6W)(80°C/W) + 85°C = 133°C
The maximum junction temperature for the LT1886 is
150
°C so the heat sinking capability of the board is
adequate for the application.
If the copper area on the PC board is reduced to 180 sq.
mm the thermal resistance increases to 122
°C/W and the
junction temperature becomes:
TJ = (0.6W)(122°C/W) + 85°C = 158°C
which is above the maximum junction temperature indi-
cating that the heat sinking capability of the board is
inadequate and should be increased.
Capacitive Loading
The LT1886 is stable with a 1000pF capacitive load. The
photo of the small-signal response with 1000pF load in a
gain of 10 shows 50% overshoot. The photo of the large-
signal response with a 1000pF load shows that the output
slew rate is not limited by the short-circuit current. The
Typical Performance Curve of Frequency Response vs
Capacitive Load shows the peaking for various capacitive
loads.
This stability is useful in the case of directly driving a
coaxial cable or twisted pair that is inadvertently
unterminated. For best pulse fidelity, however, a termina-
tion resistor of value equal to the characteristic impedance
of the cable or twisted pair (i.e., 50
Ω/75Ω/100Ω/135Ω)
should be placed in series with the output. The other end
of the cable or twisted pair should be terminated with the
same value resistor to ground.
Compensation
The LT1886 is stable in a gain 10 or higher for any supply
and resistive load. It is easily compensated for lower gains
with a single resistor or a resistor plus a capacitor.
Figure 2 shows that for inverting gains, a resistor from the
inverting node to AC ground guarantees stability if the
parallel combination of RC and RG is less than or equal to
RF/9. For lowest distortion and DC output offset, a series
capacitor, CC, can be used to reduce the noise gain at lower
frequencies. The break frequency produced by RC and CC
should be less than 15MHz to minimize peaking. The
Typical Curve of Frequency Response vs Supply Voltage,
AV = –1 shows less than 1dB of peaking for a break
frequency of 12.8MHz.
+
6V
–6V
909
100
100
1886 F01
1K
50
–4V
4V
f = 1MHz
Figure 1. Thermal Calculation Example
RG
RC
Vo
Vi
CC
(OPTIONAL)
1886 F02
RF
=
–RF
RG
Vo
Vi
< 15MHz
1
2
πRCCC
(RC || RG) ≤ RF/9
Figure 2. Compensation for Inverting Gains
Figure 3 shows compensation in the noninverting configu-
ration. The RC, CC network acts similarly to the inverting
case. The input impedance is not reduced because the
network is bootstrapped. This network can also be placed
between the inverting input and an AC ground.
Another compensation scheme for noninverting circuits is
shown in Figure 4. The circuit is unity gain at low frequency
and a gain of 1 + RF/RG at high frequency. The DC output
offset is reduced by a factor of ten. The techniques of


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