LMH6321 datasheet(14/21 Pages) NSC

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BUFFERS

Buffers are often called voltage followers because they have

largely unity voltage gain, thus the name has generally come

to mean a device that supplies current gain but no voltage

gain. Buffers serve in applications requiring isolation of

source and load, i.e., high input impedance, low output

impedance (high output current drive). In addition, they offer

gain flatness and wide bandwidth.

Most operational amplifiers, that meet the other given re-

quirements in a particular application, can be configured as

buffers, though they are generally more complex and are, by

and large, not optimized for unity gain operation. The com-

mercial buffer is a cost effective substitute for an op amp.

Buffers serve several useful functions, either in tandem with

op amps or in standalone applications. As mentioned, their

primary function is to isolate a high impedance source from

a low impedance load, since a high Z source can’t supply the

needed current to the load. For example, in the case where

the signal source to an analog to digital converter is a

sensor, it is recommended that the sensor be isolated from

the A/D converter. The use of a buffer ensures a low output

impedance and delivery of a stable output to the converter.

In A/D converter applications buffers need to drive varying

and complex reactive loads.

Buffers come in two flavors: Open Loop and Closed Loop.

While sacrificing the precision of some DC characteristics,

and generally displaying poorer gain linearity, open loop

buffers offer lower cost and increased bandwidth, along with

less phase shift and propagation delay than do closed loop

buffers. The LMH6321 is of the open loop variety.

Figure 1 shows a simplified diagram of the LMH6321 topol-

ogy, revealing the open loop complementary follower design

approach. Figure 2 shows the LMH6321 in a typical applica-

tion, in this case, a 50

Ω coaxial cable driver.

SUPPLY BYPASSING

The method of supply bypassing is not critical for frequency

stability of the buffer, and, for light loads, capacitor values in

the neighborhood of 1 nF to 10 nF are adequate. However,

under fast slewing and large loads, large transient currents

are demanded of the power supplies, and when combined

with any significant wiring inductance, these currents can

produce voltage transients. For example, the LMH6321 can

slew typically at 1000 V/µs. Therefore, under a 50

Ω load

condition the load can demand current at a rate, di/dt, of

20 A/µs. This current flowing in an inductance of 50 nH

(approximately 1.5” of 22 gage wire) will produce a 1V

transient. Thus, it is recommended that solid tantalum ca-

pacitors of 5 µF to 10 µF, in parallel with a ceramic 0.1 µF

capacitor be added as close as possible to the device supply

pins.

20138627

FIGURE 1. Simplified Schematic

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