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FIGURE 5. Driving Capacitive Loads with ROUT for
Improved Stability
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor ROUT. Figure 5 shows the use
of a series output resistor, ROUT as it might be applied when
driving an analog to digital converter. The charts "Suggested
RO vs. Cap Load" in the Typical Performance Section give a
recommended value for mitigating capacitive loads. The val-
ues suggested in the charts are selected for .5dB or less of
peaking in the frequency response. This gives a good com-
promise between settling time and bandwidth. For applica-
tions where maximum frequency response is needed and
some peaking is tolerable, the value of RO can be reduced
slightly from the recommended values. There will be ampli-
tude lost in the series resistor unless the gain is adjusted to
compensate; this effect is most noticeable with heavy resis-
tive loads.
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 opera-
tion. Do not use wire wound resistors.
The LMH6609 operates best with a feedback resistor of ap-
proximately 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 di-
rectly 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 perfor-
mance if care is taken to balance the impedances seen by the
two inputs. The parallel combination of the gain setting RG
and feedback RF resistors should be equal to RSEQ, the re-
sistance of the source driving the op amp in parallel with any
terminating Resistor (See Figure 1). Combining this with the
non inverting gain equation gives the following parameters:
RF = AVRSEQ
RG = RF/(AV−1)
For Inverting gains the bias current cancellation is accom-
plished by placing a resistor RB on the non-inverting input
equal in value to the resistance seen by the inverting input
(See Figure 2). RB = RF || (RG + RS)
The additional noise contribution of RB 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 tempera-
ture 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:
PMAX = (150º - TAMB)/θJA
Where TAMB = 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 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 perfor-
mance with both PAL and NTSC composite video signals.
The LMH6609 is specified for PAL signals. NTSC perfor-
mance is typically marginally better due to the lower frequen-
cy 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 reflec-
tions from the transmission line and effectively masks trans-
mission 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 6)
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FIGURE 6. Typical Video Application
ESD PROTECTION
The LMH6609 is protected against electrostatic discharge
(ESD) on all pins. The LMH6609 will survive 2000V Human
Body model or 200V Machine model events.
Under closed loop operation the ESD diodes have no effect
on circuit performance. There are occasions, however, when
the ESD diodes may be evident. For instance, if the amplifier
13 www.national.com
LMH6609