Datasheet

-
+
25:
V
IN
R
ISO
V
OUT
F =
1
2 S R
ISO
C
LOAD
LMH6654, LMH6655
www.ti.com SNOS956D –JUNE 2001–REVISED MARCH 2013
POWER DISSIPATION
The package power dissipation should be taken into account when operating at high ambient temperature and/or
high power dissipative conditions. In determining maximum operable temperature of the device, make sure the
total power dissipation of the device is considered; this power dissipated in the device with a load connected to
the output as well as the nominal dissipation of the op amp.
DRIVING CAPACITIVE LOADS
Capacitive loads decrease the phase margin of all op amps. The output impedance of a feedback amplifier
becomes inductive at high frequencies, creating a resonant circuit when the load is capacitive. This can lead to
overshoot, ringing and oscillation. To eliminate oscillation or reduce ringing, an isolation resistor can be placed as
shown in Figure 44 below. At frequencies above
(1)
the load impedance of the Amplifier approaches R
ISO
. The desired performance depends on the value of the
isolation resistor. The isolation resistance vs. capacitance load graph in the typical performance characteristics
provides the means for selection of the value of R
S
that provides ≤ 3 dB peaking in closed loop A
V
= 1 response.
In general, the bigger the isolation resistor, the more damped the pulse response becomes. For initial evaluation,
a 50Ω isolation resistor is recommended.
Figure 44. Isolation Resistor Placement
COMPONENTS SELECTION AND FEEDBACK RESISTOR
It is important in high-speed applications to keep all component leads short since wires are inductive at high
frequency. For discrete components, choose carbon composition axially leaded resistors and micro type
capacitors. Surface mount components are preferred over discrete components for minimum inductive effect.
Never use wire wound type resistors in high frequency applications.
Large values of feedback resistors can couple with parasitic capacitance and cause undesired effects such as
ringing or oscillation in high-speed amplifiers. Keep resistors as low as possible consistent with output loading
consideration. For a gain of 2 and higher, 402Ω feedback resistor used for the typical performance plots gives
optimal performance. For unity gain follower, a 25Ω feedback resistor is recommended rather than a direct short.
This effectively reduces the Q of what would otherwise be a parasitic inductance (the feedback wire) into the
parasitic capacitance at the inverting input.
BIAS CURRENT CANCELLATION
In order to cancel the bias current errors of the non-inverting configuration, the parallel combination of the gain
setting R
g
and feedback R
f
resistors should equal the equivalent source resistance R
seq
as defined in Figure 45.
Combining this constraint with the non-inverting gain equation, allows both R
f
and R
g
to be determined explicitly
from the following equations:
R
f
= A
V
R
seq
and R
g
= R
f
/(A
V
−1) (2)
For inverting configuration, bias current cancellation is accomplished by placing a resistor R
b
on the non-inverting
input equal in value to the resistance seen by the inverting input (R
f
//(R
g
+R
s
). The additional noise contribution of
R
b
can be minimized through the use of a shunt capacitor.
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