LMH6629_1011 Datasheet, PDF(18/28 Page) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

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LMH6629_1011 Datasheet, PDF (18/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

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FIGURE 4. External Compensation

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This circuit operates by increasing the Noise Gain (NG) be-

yond the minimum stable gain of the LMH6629 while main-

taining a positive loop gain phase angle at 0dB. There are two

constraints shown in Figure 4; âConstraint 1â ensures that NG

has increased to at least 10 V/V when the loop gain ap-

proaches 0dB, and âConstraint 2â places an upper limit on the

feedback phase lead network frequency to make sure it is fully

effective in the frequency range when loop gain approaches

0dB. These two constraints allow one to estimate the âstarting

valueâ for Rc and Cc which may need to be fine tuned for

proper response.

Here is an example worked out for more clarification:

Assume that the objective is to use the SOT23-5 version of

the LMH6629 for a closed loop gain of +3.7 V/V using the

technique shown in Figure 4.

Selecting Rf = 249â¦ â Rg = 91â¦ â REQ= 66.6â¦.

For 50â¦ source termination (Rs= 50â¦), select RT= 50â¦ â

Rp = 25â¦.

Using âConstraint 1â (= 10V/V) allows one to compute Rc â

56â¦. Using âConstraint 2â (= 90 MHz) defines the appropriate

value of Ccâ 33 pF.

The frequency response plot shown in Figure 5 is the mea-

sured response with Rc and Cc values computed above and

shows a -3dB response of about 1GHz.

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FIGURE 5. SOT23-5 Package Low Closed Loop Gain

Operation with External Compensation

For the Figure 5 measured results, a compensation capacitor

(Cf') was used across Rf to compensate for the summing node

net capacitance due to the board and the SOT23â5

LMH6629. The RA and RB combination reduces the effective

capacitance of Cfâ by the ratio of 1+RB / RA, with the constraint

that RB << Rf, thereby allowing a practical capacitance value

(> 1pF) to be used. The LLP-8 package does not need this

compensation across Rf due to its lower parasitics.

With the COMP pin HI (LLP-8 package only) or with the

SOT23â5 package, this circuit achieves high slew rate and

takes advantage of the LMH6629âs superior low-noise char-

acteristics without sacrificing stability, while enabling lower

gain applications. It should be noted that the Rc, Cc combina-

tion does lower the input impedance and increases noise gain

at higher frequencies. With these values, the input impedance

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