LMH6629 datasheet(13/22 Pages) NSC

13 / 22 page

Application Section

INTRODUCTION

The LMH6629 is a very wide gain bandwidth, ultra low-noise

voltage feedback operational amplifier. The excellent noise

and bandwidth enables applications such as medical diag-

nostic ultrasound, magnetic tape & disk storage and fiberop-

tics to achieve maximum high frequency signal-to-noise

ratios. The following discussion will enable the proper selec-

tion of external components to achieve optimum system per-

formance.

The LMH6629 has some additional features to allow maxi-

mum performance. As shown in Figure 2 there are provisions

for low power shut down and two internal compensation set-

tings, which are further discussed below. Also provided is a

feedback (FB) pin which allows the placement of the feedback

resistor directly adjacent to the inverting input (IN-) pin. This

pin simplifies board layout and minimizes the possibility of

unwanted interaction between the feedback path and other

circuit elements.

30068061

FIGURE 2. 8-Pin LLP Pinout Diagram

The LLP-8 package requires the bottom-side Die Attach Pad-

dle (DAP) to be soldered to the circuit board for proper thermal

dissipation and to get the thermal resistance number speci-

fied. The DAP is tied to the V- potential within the LMH6629

package. Thus, the circuit board copper area devoted to DAP

heatsinking connection should be at the V- potential as well.

Please refer to the package drawing for the recommended

land pattern and recommended DAP connection dimensions.

30068052

FIGURE 3. 8-Pin LLP SDA088AD (Top View)

CONTROL PINS

The LMH6629 has two digital control pins; PD and COMP

pins. The PD pin, used for powerdown, floats high (on) when

not driven. When the PD pin is pulled low, the amplifier is dis-

abled and the amplifier output stage goes into a high

impedance state so the feedback and gain set resistors de-

termine the output impedance of the circuit. The other control

pin, the COMP pin, allows control of the internal compensa-

tion and defaults to the lower gain mode or logic 0.

COMPENSATION

Nearly all high-speed operational amplifiers are now internally

compensated. To use external compensation capacitors

would compromise stability and performance due to bond

wire and board parasitic reactances. The LMH6629 gives a

degree of flexibility that was lost with on chip compensation.

There are two compensation settings that can be controlled

by the COMP pin. The default setting is set through an internal

pull down resistor and places the COMP pin at the logic 0

state. In this configuration the on chip compensation is set to

the maximum and bandwidth is reduced to enable stability at

gains as low as 4V/V.

When this pin is driven to the logic 1 state the internal com-

pensation is decreased to allow higher bandwidth at higher

gains. In this state the minimum stable gain is 10V/V. Due to

the reduced compensation slew rate and large signal band-

width are significantly enhanced for the higher gains.

BIAS CURRENT CANCELLATION

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 (R

g) and feedback

f) resistors should equal the equivalent source resistance

seq) as defined in Figure 4. Combining this constraint with

the non-inverting gain equation also seen in Figure 4 allows

both R

f and Rg to be determined explicitly from the following

equations:

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

LMH6629 Datasheet(PDF) 13 Page - National Semiconductor (TI)

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