CLC5801IM datasheet(10/14 Pages) NSC

10 / 14 page

Application Information (Continued)

(2)

As seen in

Figure 5,e

ni is dominated by the intrinsic voltage

noise (e

n) of the amplifier for equivalent source resistance

below 121

Ω. Between 121Ω and 5.11kΩ,e

ni is dominated by

the thermal noise (

) of the external resistors.

Above 5.11k

Ω,e

ni is dominated by the amplifier’s current

noise (

seq). The point at which the CLC5801’s volt-

age noise and current noise contribute equally occurs for

seq = 786Ω (

). As an example, configured with a

gain of +20V/V giving a −3dB of 90MHz and driven from an

seq =25Ω, the CLC5801 produces a total equivalent input

noise voltage (

) of 26µV

rms.

If bias current cancellation is not a requirement, then R

f || Rg

does not need to equal R

seq. In this case, according to Equa-

tion (1),R

f || Rg should be as low as possible in order to mini-

mize noise. Results similar to

Equation (1) are obtained for

the inverting configuration of

Figure 2 if R

seq is replaced by

b and Rg is replaced by Rg +Rs. With these substitutions,

Equation (1) will yield e

ni referred to the non-inverting input.

Referring e

ni to the inverting input is easily accomplished by

multiplying e

ni by the ratio of non-inverting to inverting gains.

Inverting Gains Less Than 10V/V

The lag compensation of

Figure 6 will achieve stability for

lower gains. Placing the network between the two input ter-

minals does not affect the closed-loop nor noise gain, but is

best used for the inverting configuration because of its affect

on the non-inverting input impedance.

Single-Supply Operation

The CLC5801 can be operated with single power supply as

shown in

Figure 7. Both the input and output are capacitively

coupled to set the DC operating point.

Low Noise Transimpedance Amplifier

Figure 8 shows a transimpedance amplifier used to amplify

the small signal from a Photodiode. Using a low noise ampli-

fier such as the CLC5801 and proper design, ensures that

the amplifier noise contribution is minimal. Here R

b can be

used to compensate for the input bias current of the

CLC5801. Generally, R

b is selected to be equal to Rf to can-

cel the effect of I

b flowing in each of the Op Amp input

terminals.

Figure 9 shows the equivalent noise analysis schematic for

this circuit. The complete expression for the amplifier stage

output rms noise is shown in

Equation (3).

(3)

DS101307-32

FIGURE 4. Noise Model with R

f || Rg =Rseq

DS101307-34

FIGURE 5. Voltage Noise Density vs. Source

Resistance

DS101307-35

FIGURE 6. External Lag Compensation

DS101307-36

FIGURE 7. Single Supply Operation

DS101307-37

FIGURE 8. Transimpedance Amplifier Configuration

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