LMH6715QML

SNOSAQ3B – NOVEMBER 2010 – REVISED MAY 2013

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MATCHING PERFORMANCE

With proper board layout, the AC performance match between the two LMH6715's amplifiers can be tightly

controlled as shown in Typical Performance plot labeled “Small-Signal Channel Matching”.

The measurements were performed with SMT components using a feedback resistor of 300

Ω at a gain of +2V/V.

The LMH6715's amplifiers, built on the same die, provide the advantage of having tightly matched DC

characteristics.

SLEW RATE AND SETTLING TIME

One of the advantages of current-feedback topology is an inherently high slew rate which produces a wider full

power bandwidth. The LMH6715 has a typical slew rate of 1300V/µs. The required slew rate for a design can be

calculated by the following equation:

SR = 2

(2)

Careful attention to parasitic capacitances is critical to achieving the best settling time performance. The

LMH6715 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 capacitance will also affect settling time.

DC AND 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:

(3)

The input resistance is the resistance looking from the non-inverting input back toward the source. For inverting

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 Application Note 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 “Differential 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 techniques

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.

12

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