CLC408 datasheet(9/12 Pages) NSC

9 / 12 page

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We selected the component values as follows:

Rf1 = 3.0kΩ, for unity gain of the CLC408

Rm1 = Zo = 50Ω, the characteristic impedance

of the transmission line

Rf2 = Rg2 = 100Ω ≥ Rm1, the recommended

value for the CLC426 at Av = 2

These values give excellent isolation from the other input:

The CLC408 provides large output current drive, while

consuming little supply current, at the nominal bias

point. It also produces low distortion with large signal

swings and heavy loads. These features make the

CLC408 an excellent choice for driving transmission

lines. The CLC426 was used as the receiver because

it has good high frequency CMRR.

Precision, Low 1/f Noise Composite Amplifier

The circuit in Figure 7 has the DC precision and low-

frequency performance of U1, and the high-frequency

performance of U2. This means that the 1/f noise

performance is dominated by U1, not U2. Vin needs to

be a low impedance source to minimize the impact of

U2’s non-inverting bias current (IBN) and current noise

(ibn). R1 is an optional resistor that terminates the

source. The potentiometer R7 allows the gain at low

frequencies to be manually matched to the gain at high

frequencies.

Figure 7: Precision, Low-Noise Composite Amplifier

U1 needs to be an op amp with the following features:

voltage-feedback, low bandwidth (compared to U2),

low

offsets

and

low

1/f

noise.

National

Semiconductor’s OP-07 meets all of these requirements.

U2 is a high-frequency op amp that meets your high-

frequency requirements. This application circuit will

assume a current-feedback op amp (the CLC408) for

U2. This circuit also works well when U2 is a high-

frequency, voltage-feedback op amp (such as the

CLC425 or CLC428).

The transfer function is:

where AU1(jω) is the open-loop voltage gain of U1, and

ZU2(jω) is the open-loop transimpedance gain of U2.

The approximations hold when the bandwidth of U1 is

much less than the bandwidth of U2. Now the gain of

the composite amplifier can be selected:

Av must be within the stable gain range of U1.

Make R2, R6 and R7 small so that they produce little

thermal noise, but large enough to not overload the

output of U2. Minimize the input offset voltage by

making R2 = (R6 || R7):

, the value for gain flatness

The potentiometer should have a maximum value

about double the value calculated for R7. Use a

potentiometer with multiple turn capability, and low

parasitics. Replace R7 with a resistor when AC gain

and step response flatness are not a concern.

Set R5 to the recommended feedback resistor value for

the CLC408 at a gain of Av.

Select R3 and R4 so that the high-frequency gain is

correct, and so that any change in output impedance of

U1 has a minimal impact:

The selection of R3 and R4 affects the frequency where

U2 starts to dominate the performance of the composite

amplifier. This frequency is approximately:

R(R

||R

) –

Ω

38dB, f

5.0MHz

oA(B)

inB(A)

≈−

OP-07

CLC408

Vin

A(j )

Z(j )

A(j )

(j )

Z(j )









+⋅

















+⋅

⋅









≈+

≈

A(j )

U1 ω<<

=⋅ +









GBWP

≈⋅

⋅

RA R

≈

−

CLC408 Datasheet(PDF) 9 Page - National Semiconductor (TI)

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