LMH6682 Datasheet, PDF (16/20 Pages) National Semiconductor (TI) – 190MHz Single Supply, Dual and Triple Operational Amplifiers

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.
20059061
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.
20059060
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).
I*∆t = C *∆V
(1)
∆V/∆t = I/C
(2)
I=∆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)
FIGURE 3.
20059062
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-
www.national.com
16