Datasheet

LMH6609
www.ti.com SNOSA84F –AUGUST 2003–REVISED MARCH 2013
COMPONENT SELECTION AND FEEDBACK RESISTOR
Surface mount components are highly recommended for the LMH6609. Leaded components will introduce
unpredictable parasitic loading that will interfere with proper device operation. Do not use wire wound resistors.
The LMH6609 operates best with a feedback resistor of approximately 250Ω for all gains of +2 and greater and
for −1 and less. With lower gains in particular, large value feedback resistors will exaggerate the effects of
parasitic capacitances and may lead to ringing on the pulse response and frequency response peaking. Large
value resistors also add undesirable thermal noise. Feedback resistors that are much below 100Ω will load the
output stage, which will reduce voltage output swing, increase device power dissipation, increase distortion and
reduce current available for driving the load.
In the buffer configuration the output should be shorted directly to the inverting input. This feedback does not
load the output stage because the inverting input is a high impedance point and there is no gain set resistor to
ground.
OPTIMIZING DC ACCURACY
The LMH6609 offers excellent DC accuracy. The well-matched inputs of this amplifier allows even better
performance if care is taken to balance the impedances seen by the two inputs. The parallel combination of the
gain setting R
G
and feedback R
F
resistors should be equal to R
SEQ
, the resistance of the source driving the op
amp in parallel with any terminating Resistor (See Figure 32). Combining this with the non inverting gain equation
gives the following parameters:
R
F
= A
VRSEQ
R
G
= R
F
/(A
V
−1)
For Inverting gains the 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 (See Figure 33). R
B
= R
F
|| (R
G
+ R
S
)
The additional noise contribution of R
B
can be minimized by the use of a shunt capacitor (not shown).
POWER DISSIPATION
The LMH6609 has the ability to drive large currents into low impedance loads. Some combinations of ambient
temperature and device loading could result in device overheating. For most conditions peak power values are
not as important as RMS powers. To determine the maximum allowable power dissipation for the LMH6609 use
the following formula:
P
MAX
= (150º - T
AMB
)/θ
JA
(4)
Where T
AMB
= Ambient temperature (°C) and θJA = Thermal resistance, from junction to ambient, for a given
package (°C/W). For the SOIC package θJA is 148°C/W, for the SOT-23 it is 250°C/W. 150ºC is the absolute
maximum limit for the internal temperature of the device.
Either forced air cooling or a heat sink can greatly increase the power handling capability for the LMH6609.
VIDEO PERFORMANCE
The LMH6609 has been designed to provide good performance with both PAL and NTSC composite video
signals. The LMH6609 is specified for PAL signals. NTSC performance is typically marginally better due to the
lower frequency content of the signal. Performance degrades as the loading is increased, therefore best
performance will be obtained with back-terminated loads. The back termination reduces reflections from the
transmission line and effectively masks transmission line and other parasitic capacitances from the amplifier
output stage. This means that the device should be configured for a gain of 2 in order to have a net gain of 1
after the terminating resistor. (See Figure 37)
Copyright © 2003–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LMH6609