LMH6738 Datasheet, PDF (11/20 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Op Amp

LMH6738
www.ti.com
SNOSAC1E – APRIL 2004 – REVISED MARCH 2013
When using the LMH6738 as a low pass filter the value of R_F can be substantially reduced from the value
recommended in the R_F vs. Gain charts. The benefit of reducing R_F is increased gain at higher frequencies,
which improves attenuation in the stop band. Stability problems are avoided because in the stop band additional
device bandwidth is used to cancel the input signal rather than amplify it. The benefit of this change depends on
the particulars of the circuit design. With a high pass filter configuration reducing R_F will likely result in device
instability and is not recommended.
R_IN
75:
V_IN
R_G
550:
6.8 PF
C2
0.01 PF
C1
X1
++
--
R_F
550:
0.01 PF
C3
6.8 PF
R_OUT
75:
V_OUT
R_IN
R_G
51: 550:
X1
++
--
R_F
550:
R_OUT
51:
CL
10 pF
R_L
1 k:
C4
Figure 25. Typical Video Application
Figure 26. Decoupling Capacitive Loads
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the use of a series output resistor R_OUT. Figure 26 shows
the use of a series output resistor, R_OUT, to stabilize the amplifier output under capacitive loading. Capacitive
loads of 5 to 120 pF are the most critical, causing ringing, frequency response peaking and possible oscillation.
The charts “Suggested R_OUT vs. Cap Load” give a recommended value for selecting a series output resistor for
mitigating capacitive loads. The values suggested in the charts are selected for .5 dB or less of peaking in the
frequency response. This gives a good compromise between settling time and bandwidth. For applications where
maximum frequency response is needed and some peaking is tolerable, the value of R_OUT can be reduced
slightly from the recommended values.
An alternative approach is to place Rout inside the feedback loop as shown in Figure 27. This will preserve gain
accuracy, but will still limit maximum output voltage swing.
R_IN
R_G
51: 550:
X1
+
-
+
R_OUT
51:
-
CL
10 pF
R_F
550:
R_L
1 k:
Figure 27. Series Output Resistor Inside
Feedback Loop
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was not intentionally added. It comes about from
electrical interaction between conductors. Parasitic capacitance can be reduced but never entirely eliminated.
Most parasitic capacitances that cause problems are related to board layout or lack of termination on
transmission lines. Please see the section on Layout Considerations for hints on reducing problems due to
parasitic capacitances on board traces. Transmission lines should be terminated in their characteristic
impedance at both ends.
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