lmh6732mfx National Semiconductor Corporation, lmh6732mfx Datasheet - Page 16

• Manufacturer Part Number: lmh6732mfx
• Description: High Speed Op Amp With Adjustable Bandwidth
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information:
"POPLESS OUTPUT" & OFF CONDITION OUTPUT
STATE
The LMH6732 has been especially designed to have mini-
mum glitches during turn-on and turn-off. This is advanta-
geous in situations where the LMH6732 output is fed to
another stage which could experience false auto-ranging, or
even worse reset operation, due to these transient glitches.
Example of this application would be an AGC circuit or an
ADC with multiple ranges set to accommodate the largest
input amplitude. For the LMH6732, these sorts of transients
are typically less than 50mV in amplitude (see Electrical
Characteristics Tables for Typical values). Applications de-
signed to utilize the CLC505’s low output glitch would benefit
from using the LMH6732 instead since the LMH6732’s out-
put glitch is improved to be even lower than the CLC505’s. In
the "Off State", the output stage is turned off and is in effect
put into a high-Z state. In this sate, output can be forced by
other active devices. No significant current will flow through
the device output pin in this mode of operation.
MUX APPLICATION
Since The LMH6732’s output is essentially open in the “off”
state, it is a good candidate for a fast 2:1 MUX. Figure 6
shows one such application along with the output waveform
in Figure 7 displaying the switching between a continuous
triangle wave and a single cycle sine wave (signals trigger
locked to each other for stable scope photo). Switching
speed of the MUX will be less than 50 ns and is governed by
the “Ton" and “Toff” times for U1 and U2 at the supply current
set by R
CMOS logic level.
P1
FIGURE 6. 50 ns 2:1 MUX Schematic
and R
P2
. Note that the “Control” input is a 5V
(Continued)
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DIFFERENTIAL GAIN AND PHASE
Differential gain and phase are measurements useful prima-
rily in composite video channels. They are measured by
monitoring the gain and phase changes of a high frequency
carrier (3.58MHz for NTSC and 4.43MHz for PAL systems)
as the output of the amplifier is swept over a range of DC
voltages. Specifications for the LMH6732 include differential
gain and phase. Test signals used are based on a 1V
video level. Test conditions used are the following:
DC sweep range: 0 to 100 IRE units (black to white)
Carrier: 4.43MHz at 40 IRE units peak to peak
A
SOURCE IMPEDANCE
For best results, source impedance in the non-inverting cir-
cuit configuration (see Figure 1) should be kept below 5kΩ.
Above 5kΩ it is possible for oscillation to occur, depending
on other circuit board parasitics. For high signal source
impedances, a resistor with a value of less than 5kΩ may be
used to terminate the non-inverting input to ground.
FEEDBACK RESISTOR
In current-feedback op amps, the value of the feedback
resistor plays a major role in determining amplifier dynamics.
It is important to select the correct value. The LMH6732
provides optimum performance with feedback resistors as
shown in Table 2 below. Selection of an incorrect value can
lead to severe rolloff in frequency response, (if the resistor
value is too large) or , peaking or oscillation (if the value is
too low).
V
= +2, R
FIGURE 7. MUX “V
L
= 75Ω + 75Ω
OUT
” and “Control” Waveform
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