+

-

RIN

51:

RF

550:

X1

-

+

ROUT

51:

RG

550:

CL

10 pF

RL

1 k:

+

-

6.8 PF

C2

0.01 PF

RIN

75:

RG

550:

RF

550:

C3

6.8 PF

C4

X1

-

+

ROUT

75:

C1

0.01 PF

VOUT

VIN

+

-

RIN

51:

RF

550:

X1

-

+

ROUT

51:

RG

550:

CL

10 pF

RL

1 k:

LMH6738

www.ti.com

SNOSAC1E – APRIL 2004 – REVISED MARCH 2013

When using the LMH6738 as a low pass filter the value of RF can be substantially reduced from the value

recommended in the RF vs. Gain charts. The benefit of reducing RF 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 RF will likely result in device

instability and is not recommended.

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 ROUT. Figure 26 shows

the use of a series output resistor, ROUT, 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 ROUT 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 ROUT 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.

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.

Copyright © 2004–2013, Texas Instruments Incorporated

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