CLC5801IM datasheet(9/14 Pages) NSC

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Application Information

Introduction

The CLC5801 is a very wide gain-bandwidth, low noise volt-

age feedback operational amplifier which enables applica-

tions areas such as medical diagnostic ultrasound, magnetic

tape & disk storage and fiber-optics to achieve maximum

high-frequency signal-to-noise ratios. The following discus-

sion will describe the proper selection of external compo-

nents in order to achieve optimum device performance.

Bias Current Cancellation

In order to cancel the bias current errors of the non-inverting

configuration, the parallel combination of the gain-setting

g) and feedback (Rf) resistors should equal the equivalent

source resistance (R

seq) as defined in Figure 1. Combining

this constraint with the non-inverting gain equation also seen

Figure 1, allows both R

f and Rg to be determined explicitly

from the following equations: R

VRseq

and R

f/(AV−1). When driven from a 0Ω source, such as that from

the output of an op amp, the non-inverting input of the

CLC5801 should be isolated with at least a 25

Ω series resis-

tor.

As seen in

Figure 2, bias current cancellation is accom-

plished for the inverting configuration by placing a resistor

b) on the non-inverting input equal in value to the resis-

tance seen by the inverting input (R

f || (Rg +Rs)). Rb is

recommended to be no less than 25

Ω for best CLC5801 per-

formance. The additional noise contribution of R

b can be

minimized through the use of a shunt capacitor.

Total Input Noise vs. Source Resistance

In order to determine maximum signal-to-noise ratios from

the CLC5801, an understanding of the interaction between

the amplifier’s intrinsic noise sources and the noise arising

from its external resistors is necessary.

Figure 3 describes the noise model for the non-inverting am-

plifier configuration showing all noise sources. In addition to

the intrinsic input voltage noise (e

and current noise

n =in

+ =i

−) sources, there also exists thermal voltage

noise (

) associated with each of the external re-

sistors.

Equation (1) provides the general form for total

equivalent input voltage noise density (e

ni). Equation (2) is a

simplification of

Equation (1) that assumes R

f || Rg =Rseq for

bias current cancellation.

Figure 4 illustrates the equivalent

noise model using this assumption.

Figure 5 is a plot of e

against equivalent source resistance (R

seq) with all of the

contributing noise sources of

Equation (2) shown. This plot

gives the expected e

ni for a given Rseq which assumes Rf ||

g =Rseq for bias current cancellation. The total equivalent

output voltage noise (e

no)iseni xAV.

(1)

DS101307-28

FIGURE 1. Non-Inverting Amplifier Configuration

DS101307-29

FIGURE 2. Inverting Amplifier Configuration

DS101307-30

FIGURE 3. Non-Inverting Amplifier Noise Model

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