LMH6682 datasheet(16/20 Pages) NSC

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Applications Section

LARGE SIGNAL BEHAVIOR

Amplifying high frequency signals with large amplitudes (as

in video applications) has some special aspects to look after.

The bandwidth of the Op Amp for large amplitudes is less

than the small signal bandwidth because of slew rate limita-

tions. While amplifying pulse shaped signals the slew rate

properties of the OpAmp become more important at higher

amplitude ranges. Due to the internal structure of an Op Amp

the output can only change with a limited voltage difference

per time unit (dV/dt). This can be explained as follows: To

keep it simple, assume that an Op Amp consists of two parts;

the input stage and the output stage. In order to stabilize the

Op Amp, the output stage has a compensation capacitor in

its feedback path. This Miller C integrates the current from

the input stage and determines the pulse response of the Op

Amp. The input stage must charge/discharge the feedback

capacitor, as can be seen in Figure 1.

When a voltage transient is applied to the non inverting input

of the Op Amp, the current from the input stage will charge

the capacitor and the output voltage will slope up. The

overall feedback will subtract the gradually increasing output

voltage from the input voltage. The decreasing differential

input voltage is converted into a current by the input stage

(Gm).

∆t=C*∆V

(1)

∆V/∆t = I/C

(2)

∆V*Gm

(3)

where I = current

t = time

C = capacitance

V = voltage

Gm = transconductance

Slew rate

∆V/∆t = volt/second

In most amplifier designs the current I is limited for high

differential voltages (Gm becomes zero). The slew rate will

than be limited as well:

∆V/∆t = Imax/C

(4)

The LMH6682/83 has a different setup of the input stage. It

has the property to deliver more current to the output stage

when the input voltage is higher (class AB input). The current

into the Miller capacitor exhibits an exponential character,

while this current in other Op Amp designs reaches a satu-

ration level at high input levels: (see Figure 2)

This property of the LMH6682/83 guaranties a higher slew

rate at higher differential input voltages.

∆V/∆t= ∆V*Gm/C

(5)

In Figure 3 one can see that a higher transient voltage than

will lead to a higher slew rate.

HANDLING VIDEO SIGNALS

When handling video signals, two aspects are very important

especially when cascading amplifiers in a NTSC- or PAL

video system. A composite video signal consists of both

amplitude and phase information. The amplitude represents

saturation while phase determines color (color burst is

3.59MHz for NTSC and 4.58MHz for PAL systems). In this

case it is not only important to have an accurate amplification

of the amplitude but also it is important not to add a varying

phase shift to the video signals. It is a known phenomena

that at different dc levels over a certain load the phase of the

amplified signal will vary a little bit. In a video chain many

amplifiers will be cascaded and all errors will be added

together. For this reason, it is necessary to have strict re-

quirements for the variation in gain and phase in conjunction

to different dc levels. As can be seen in the tables the

number for the differential gain for the LMH6682/83 is only

0.01% and for the differential phase it is only 0.08˚ at a

supply voltage of ±5V. Note that the phase is very depen-

20059060

FIGURE 1.

20059061

FIGURE 2.

20059062

FIGURE 3.

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