LMH6551MA datasheet(13/19 Pages) NSC

13 / 19 page

Application Section (Continued)

The LMH6551 requires supply bypassing capacitors as

shown in Figure 4 and Figure 5. The 0.01 µF and 0.1 µF

capacitors should be leadless SMT ceramic capacitors and

should be no more than 3 mm from the supply pins. The

SMT capacitors should be connected directly to a ground

plane. Thin traces or small vias will reduce the effectiveness

of bypass capacitors. Also shown in both figures is a capaci-

tor from the V

CM pin to ground. The VCM pin is a high

impedance input to a buffer which sets the output common

mode voltage. Any noise on this input is transferred directly

to the output. Output common mode noise will result in loss

of dynamic range, degraded CMRR, degraded Balance and

higher distortion. The V

CM pin should be bypassed even if

the pin in not used. There is an internal resistive divider on

chip to set the output common mode voltage to the mid point

of the supply pins. The impedance looking into this pin is

approximately 25k

Ω. If a different output common mode

voltage is desired drive this pin with a clean, accurate volt-

age reference.

20133201

FIGURE 4. Split Supply Bypassing Capacitors

20133212

FIGURE 5. Single Supply Bypassing Capacitors

LMH6551MA Datasheet(PDF) 13 Page - National Semiconductor (TI)