CLC408 datasheet(6/12 Pages) NSC

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Figure 3: Transimpedance Gain

DC Design (level shifting)

Figure 4 shows a DC level shifting circuit for inverting

gain configurations. Vref produces a DC output level shift

which is independent of the DC output

produced by Vin.

Figure 4: Level Shifting Circuit

DC Design (DC offsets)

The DC offset model shown in Fig. 5 is used to calculate

the output offset voltage. The equation for output offset

voltage is:

The current offset terms, IBN and IBI, do not track

each other. The specifications are stated in terms of

magnitude only. Therefore, the terms Vos, IBN, and IBI

can have either polarity.

Matching the equivalent

resistance seen at both input pins does not reduce the

output offset voltage.

Figure 5: DC Offset Model

DC Design (output loading)

RL, Rf, and Rg load the op amp output. The equivalent

load seen by the output in Figure 5 is:

RL(eq) =

RL || (Rf + Req2), non-inverting gain

RL || Rf, inverting and transimpedance gain

The equivalent output load (RL(eq)) needs to be large

enough so that the output current can produce the

required output voltage swing.

AC Design (small signal bandwidth)

The CLC408 current-feedback amplifier bandwidth is a

function of the feedback resistor (Rf), not of the DC voltage

gain (AV). The bandwidth is approximately proportional

As a rule, if Rf doubles, the bandwidth is cut in half.

Other AC

specifications

will

also

degraded.

Decreasing Rf from the recommended value increases

peaking, and

for very small values of Rf oscillation

will occur.

AC Design (minimum slew rate)

Slew rate influences the bandwidth of large signal

sinusoids. To determine an approximate value of slew

rate necessary to support a large sinusoid, use the

following equation:

> 5 • f • V

peak

where Vpeak is the peak output sinusoidal voltage.

The slew rate of the CLC408 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

reasonably

linear

phase

response.

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 bandwidth) 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:

CLC408

408 Fi 3

0.1

µF

6.8

µF

VCC

0.1

µF

6.8

µF

VEE

Iin

Vin

CLC408

Vref

Rref

−⋅

ref

oos

eq1

eq2

=−

⋅

()⋅+











 +⋅

()

Req1

Req2

CLC408

IBI

IBN

Vos

{

φφ

360

() =−

⋅

()

≈−

⋅

()

∆

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