CLC418 datasheet(7/12 Pages) NSC

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following equation:

> 5 • f • V

peak

where Vpeak is the peak output sinusoidal voltage.

The slew rate of the CLC418 in inverting gains is always

higher than in non-inverting gains.

AC Design (linear phase/constant group delay)

The recommended value of Rf produces minimal peaking

and a reasonably linear phase response. To improve

phase linearity when |Av| < 6, increase Rf approximately

50% over its recommended value. Some adjustment of

Rf may be needed to achieve phase linearity for your

application. See the

AC Design (small signal band-

width) sub-section for other effects of changing Rf.

Propagation delay is approximately equal to group delay.

Group delay is related to phase by this equation:

where

φ(f) is the phase in degrees. Linear phase implies

constant group delay. The technique for achieving linear

phase also produces a constant group delay.

AC Design (peaking)

Peaking is sometimes observed with the recommended

Rf. If a small increase in Rf does not solve the problem,

then investigate the possible causes and remedies

listed below:

Capacitance across Rf

Do not place a capacitor across Rf

Use a resistor with low parasitic

capacitance for Rf

A capacitive load

Use a series resistor between the output

and a capacitive load (see the

Settling

Time versus CL plot)

Long traces and/or lead lengths between Rf

and the CLC418

Keep these traces as short as possible

For non-inverting and transimpedance gain configurations:

Extra capacitance between the inverting

pin and ground (Cg)

See the

Printed Circuit Board Layout

sub-section below for suggestions on

reducing Cg

Increase Rf if peaking is still observed

after reducing Cg

For inverting gain configurations:

Inadequate ground plane at the non-inverting

pin and/or long traces between non-inverting

pin and ground

Place a 50 to 200

Ω resistor between the

non-inverting pin and ground (see Rt in

Figure 2)

AC Design (crosstalk)

Crosstalk performance depends on the layout.

Three

layout techniques that can reduce crosstalk are:

Provide short symmetrical ground return paths for:

the inputs

the supply bypass capacitors

the load

Provide a short, grounded guard trace that:

goes underneath the package

is 0.1” (3mm) from the package pins

is on top and bottom of the printed circuit

board with connecting vias

Try different bypass capacitors to reduce high

frequency crosstalk

The CLC418’s evaluation board was used to produce the

Input-Referred Crosstalk plot.

Capacitive Loads

Capacitive loads, such as found in A/D converters,

require a series resistor (Rs) in the output to improve

settling performance. The

Settling Time vs. Capacitive

Load plot in the Typical Performance Characteristics

section provides the information for selecting this resistor.

Using a resistor in series with a reactive load will also

reduce the load’s effect on amplifier loop dynamics. For

instance, driving coaxial cables without an output series

resistor may cause peaking or oscillation.

Transmission Line Matching

One method for matching the characteristic impedance

of a transmission line is to place the appropriate

resistor at the input or output of the amplifier. Figure 6

shows the typical circuit configurations for matching

transmission lines.

Figure 6: Transmission Line Matching

In non-inverting gain applications, Rg is connected

directly to ground. The resistors R1, R2, R6, and R7 are

equal to the characteristic impedance, Zo, of the

transmission line or cable.

Use R3 to isolate the

amplifier from reactive loading caused by the transmis-

sion line, or by parasitics.

In inverting gain applications, R3 is connected directly to

ground. The resistors R4, R6, and R7 are equal to Zo. The

parallel combination of R5 and Rg is also equal to Zo.

The input and output matching resistors attenuate the

signal by a factor of 2, therefore additional gain is needed.

Use C6 to match the output transmission line over a greater

frequency range.

It compensates for the increase of

the op amps output impedance with frequency.

360

( ) = −

⋅

( )

≈ −

⋅

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∆

418 Fig6

V2 +-

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