LMH6715MA datasheet(10/12 Pages) NSC

10 / 12 page

Application Introduction (Continued)

has a typical short term settling time to 0.05% of 12ns for a

2V step. Also, the amplifier is virtually free of any long term

thermal tail effects at low gains.

When measuring settling time, a solid ground plane should

be used in order to reduce ground inductance which can

cause common-ground-impedance coupling. Power supply

and ground trace parasitic capacitances and the load ca-

pacitance will also affect settling time.

Placing a series resistor (R

s) at the output pin is recom-

mended for optimal settling time performance when driving a

capacitive load. The Typical Performance plot labeled “R

and Settling Time vs. Capacitive Load” provides a means for

selecting a value of R

s for a given capacitive load.

DC & NOISE PERFORMANCE

A current-feedback amplifier’s input stage does not have

equal nor correlated bias currents, therefore they cannot be

canceled and each contributes to the total DC offset voltage

at the output by the following equation:

The input resistance is the resistance looking from the non-

inverting input back toward the source. For inverting DC-

offset calculations, the source resistance seen by the input

resistor R

g must be included in the output offset calculation

as a part of the non-inverting gain equation. Application note

OA-7 gives several circuits for DC offset correction. The

noise currents for the inverting and non-inverting inputs are

graphed in the Typical Performance plot labeled “Equivalent

Input Noise”. A more complete discussion of amplifier input-

referred noise and external resistor noise contribution can be

found in OA-12.

DIFFERENTIAL GAIN & PHASE

The LMH6715 can drive multiple video loads with very low

differential gain and phase errors. The Typical Performance

plots labeled “Differential Gain vs. Frequency” and “Differen-

tial Phase vs. Frequency” show performance for loads from

1 to 4. The Electrical Characteristics table also specifies

performance for one 150

Ω load at 4.43MHz. For NTSC

video, the performance specifications also apply. Application

note OA-24 “Measuring and Improving Differential Gain &

Differential Phase for Video”, describes in detail the tech-

niques used to measure differential gain and phase.

I/O VOLTAGE & OUTPUT CURRENT

The usable common-mode input voltage range (CMIR) of

the LMH6715 specified in the Electrical Characteristics table

of the data sheet shows a range of ±2.2 volts. Exceeding

this range will cause the input stage to saturate and clip the

output signal.

The output voltage range is determined by the load resistor

and the choice of power supplies. With ±5 volts the class A/B

output driver will typically drive ±3.9V into a load resistance

of 100

Ω. Increasing the supply voltages will change the

common-mode input and output voltage swings while at the

same time increase the internal junction temperature.

Applications Circuits

SINGLE-TO-DIFFERENTIAL LINE DRIVER

The LMH6715’s well matched AC channel-response allows a

single-ended input to be transformed to highly matched

push-pull driver. From a 1V single-ended input the circuit of

Figure 3 produces 1V differential signal between the two

outputs. For larger signals the input voltage divider (R

1 =

2) is necessary to limit the input voltage on channel 2.

DIFFERENTIAL LINE RECEIVER

Figure 4 and Figure 5 show two different implementations of

an instrumentation amplifier which convert differential sig-

nals to single-ended. Figure 5 allows CMRR adjustment

through R

20042945

FIGURE 3. Single-to-Differential Line Driver

20042946

FIGURE 4. Differential Line Receiver

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