LMH6629_1011 Datasheet, PDF(19/28 Page) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LMH6629_1011 Datasheet, PDF (19/28 Pages) National Semiconductor (TI) – Ultra-Low Noise, High-Speed Operational Amplifier with Shutdown

◁

reduces by 3dB at 490 MHz. The Noise Gain transfer function

âzeroâ is given by the equation below and it has a 3dB in-

crease at 32.8 MHz with these values:

Equation 1: External Compensation Noise Gain

Increase

(1)

CANCELLATION OF OFFSET ERRORS DUE TO INPUT

BIAS CURRENTS

The LMH6629 offers exceptional offset voltage accuracy. In

order to preserve the low offset voltage errors, care must be

taken to avoid voltage errors due to input bias currents. This

is important in both inverting and non inverting applications.

The non-inverting circuit is used here as an example. To can-

cel 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

(Rseq) as defined in Figure 6. Combining this constraint with

the non-inverting gain equation also seen in Figure 6 allows

both Rf and Rg to be determined explicitly from the following

equations:

Rf = AVRseq and Rg = Rf/(AV-1)

30068019

FIGURE 7. Inverting Amplifier Configuration

TOTAL INPUT NOISE vs. SOURCE RESISTANCE

To determine maximum signal-to-noise ratios from the

LMH6629, an understanding of the interaction between the

amplifierâs intrinsic noise sources and the noise arising from

its external resistors is necessary. Figure 8 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+ = inâ) source,

there is also thermal voltage noise (et = â(4KTR)) associated

with each of the external resistors.

30068018

FIGURE 6. Non-Inverting Amplifier Configuration

When driven from a 0â¦ source, such as the output of an op

amp, the non-inverting input of the LMH6629 should be iso-

lated with at least a 25â¦ series resistor.

As seen in Figure 7, bias current cancellation is accomplished

for the inverting configuration by placing a resistor (Rb) on the

non-inverting input equal in value to the resistance seen by

the inverting input (Rf || (Rg+Rs)). Rb should to be no less than

25â¦ for optimum LMH6629 performance. A shunt capacitor

(not shown) can minimize the additional noise of Rb.

30068020

FIGURE 8. Non-Inverting Amplifier Noise Model

Equation 2 provides the general form for total equivalent input

voltage noise density (eni).

Equation 2: General Noise Equation

(2)

Equation 3 is a simplification of Equation 2 that assumes Rf ||

Rg = Rseq for bias current cancellation:

Equation 3: Noise Equation with

Rf || Rg = Rseq

(3)

Figure 9 schematically shows eni alongside VIN (the portion of

VS source which reaches the non-inverting input of Figure 6)

www.national.com

▷