CLC1001ISO8 datasheet(13/17 Pages) EXAR

13 / 17 page

Data Sheet

13/17

Rev 1G

Application Information

Basic Operation

Figures 1 and 2 illustrate typical circuit configurations for

non-inverting, inverting, and unity gain topologies for dual

supply applications. They show the recommended bypass

capacitor values and overall closed loop gain equations.

0.1μF

6.8μF

Output

G = 1 + (Rf/Rg)

Input

+Vs

-Vs

0.1μF

6.8μF

Figure 1. Typical Non-Inverting Gain Circuit

Figure 2. Typical Inverting Gain Circuit

Achieving Low Noise in an Application

Making full use of the low noise of the CLC1001 requires

careful consideration of resistor values. The feedback and

gain set resistors (Rf and Rg) and the non-inverting source

impedance (Rsource) all contribute noise to the circuit and

can easily dominate the overall noise if their values are

too high. The datasheet is specified with an Rg of 22.1Ω,

at which point the noise from Rf and Rg is about equal to

the noise from the CLC1001. Lower value resistors could

be used at the expense of more distortion. Figure 3 shows

total input voltage noise (amp+resistors) versus Rf and

Rg. As the value of Rf increases, the total input referred

noise also increases.

0.5

0.75

1.25

1.5

1.75

2.25

2.5

2.75

100

1000

Rf (Ohms)

G = +11

G = +21

G = +41

Figure 3: Input Referred Voltage Noise vs. Rf and Rg

The noise caused by a resistor is modeled with either a

voltage source in series with the resistance:

4kTR

Or a current source in parallel with it:

R =

4kT

Op amp noise is modeled with three noise sources, en, in

and ii. These three sources are analogous to the DC input

voltage and current errors Vos, Ibn and Ibi.

The noise models must be analyzed in-circuit to determine

the effect on the op amp output noise.

Since noise is statistical in nature rather than a continuous

signal, the set of noise sources in circuit add in an RMS

(root mean square) fashion rather than in a linear fashion.

For uncorrelated noise sources, this means you add the

squares of the noise voltages. A typical non-inverting

application (see figure 1) results in the following noise at

the output of the op amp:

0.1μF

6.8μF

Output

G = - (Rf/Rg)

For optimum input offset

voltage set R1 = Rf || Rg

Input

+Vs

-Vs

0.1μF

6.8μF

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