CLC425AMC datasheet(5/9 Pages) NSC

5 / 9 page

Introduction

The CLC425 is a very wide gain-bandwidth, ultra-low

noise voltage feedback operational amplifier which en-

ables application areas such as medical diagnostic ultra-

sound, magnetic tape & disk storage and fiber-optics to

achieve maximum high-frequency signal-to-noise ratios.

The set of characteristic plots located in the "Typical

Performance" section illustrates many of the perfor-

mance trade-offs. The following discussion will enable

the proper selection of external components 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 (Rg) and feedback (Rf) resistors should equal

the equivalent source resistance (Rs

eq) as defined in

Figure 1. Combining this constraint with the non-invert-

ing gain equation also seen in Figure 1, allows both Rf

and Rg to be determined explicitly from the following

equations: Rf=AvRs

eq and Rg=Rf/(Av-1). When driven from

a 0

Ω source, such as that from the output of an op amp,

the non-inverting input of the CLC425 should be isolated

with at least a 25

Ω series resistor.

As seen in Figure 2, bias current cancellation is accom-

plished for the inverting configuration by placing a resis-

tor (Rb) on the non-inverting input equal in value to the

resistance seen by the inverting input (Rf||(Rg+Rs)). Rb is

recommended to be no less than 25

Ω for best CLC425

performance. The additional noise contribution of Rb can

be minimized through the use of a shunt capacitor.

16 4

Joules

=−

0.1

µF

0.1

µF

6.8

µF

6.8

µF

-Vcc

+Vcc

Vout

eq = Rs || RT

Av = 1 +

Vin

CLC425

in+

in-

√4kTRg

√4kTRf

√4kTRs

Total Input Noise vs. Source Resistance

In order to determine maximum signal-to-noise ratios

from the CLC425, 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

amplifier configuration showing all noise sources. In

addition to the intrinsic input voltage noise (en) and

current noise (in=in+=in-) sources, there also exists ther-

mal voltage noise (e

4TR

t =

) associated with each of

the external resistors. Equation 1 provides the general

form for total equivalent input voltage noise density (eni).

Equation 2 is a simplification of Equation 1 that assumes

Rf||Rg = Rs

for bias current cancellation. Figure 4

illustrates the equivalent noise model using this as-

sumption. Figure 5 is a plot of eni against equivalent

source resistance (Rs

eq) with all of the contributing volt-

age noise sources of Equation 2 shown. This plot gives

the expected eni for a given Rs

eq which assumes Rf||Rg =

eq for bias current cancellation. The total equivalent

output voltage noise (eno) is eni

∗A

Equation 1: General Noise Equation

kTR

kT R

() ++ ()

() + ()

+−

Figure 3: Non-inverting Amplifer Noise Model

Figure 1: Non-inverting Amplifier Configuration

Figure 4: Noise Model with R

f||Rg = Rseq

0.1

µF

0.1

µF

6.8

µF

6.8

µF

-Vcc

+Vcc

Vout

Vin

Av = -

CLC425

2Rs

√4kT2Rs

√2

Equation 2: Noise Equation with Rf||Rg = Rs

i R

() + ()

Figure 2: Inverting Amplifier Configuration

http://www.national.com

CLC425AMC データシート(PDF) 5 Page - National Semiconductor (TI)

CLC425AMC データシート(HTML) 5 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9

データシートダウンロード