LMH6658MA National Semiconductor, LMH6658MA Datasheet - Page 15

• Manufacturer Part Number: LMH6658MA
• Description: Op Amps
• Manufacturer: National Semiconductor

Specifications of LMH6658MA

• Number Of Channels: 2
• Voltage Gain Db: 95 dB
• Common Mode Rejection Ratio (min): 72 dB
• Input Offset Voltage: 5 mV at 5 V
• Operating Supply Voltage: 5 V, 9 V
• Supply Current: 17 mA at 5 V
• Maximum Operating Temperature: + 85 C
• Package / Case: SOIC-8 Narrow
• Maximum Dual Supply Voltage: +/- 6 V
• Minimum Operating Temperature: - 40 C

Application Section
LARGE SIGNAL BEHAVIOR
The LMH6657/6658 is specially designed to handle large
output swings, such as those encountered in video wave-
forms, without being slew rate limited. With 5V supply, the
LMH6657/6658 slew rate limit is larger than that might be
necessary to make full allowable output swing excursions.
Therefore, the large signal frequency response is dominated
by the small signal characteristics, rather than the conven-
tional limitation imposed by slew rate limit.
The LMH6657/6658 input stage is designed to provide ex-
cess overdrive when needed. This occurs when fast input
signal excursions cannot be followed by the output stage. In
these situations, the device encounters larger input signals
than would be encountered under normal closed loop con-
ditions. The LMH6657/6658 input stage is designed to take
advantage of this "input overdrive" condition. The larger the
amount of this overdrive, the greater is the speed with which
the output voltage can change. Here is a plot of how the
output slew rate limitation varies with respect to the amount
of overdrive imposed on the input:
To relate the explanation above to a practical example,
consider the following application example. Consider the
case of a closed loop amplifier with a gain of −1 amplifying a
sinusoidal waveform. From the plot of Output vs. Input (Typi-
cal Performance Characteristics section), with a 30MHz sig-
nal and 7V
be limited to a swing of 6.9V
sponse plot it can be seen that the inverting gain of −1 has a
−32˚ output phase shift at this frequency. It can be shown
that this setup will result in about 1.9V
voltage corresponding to 650V/µs of slew rate from Figure 1,
above (SR = V
overdrive appearing on the input for a given sinusoidal test
waveform is affected by the following:
• Output swing
• Gain setting
• Input/output phase relationship for the given test fre-
• Amplifier configuration (inverting or non-inverting)
FIGURE 1. Plot Showing the Relationship Between
quency
PP
Slew Rate and Input Overdrive
input signal, it can be seen that the output will
O
(pp)*π*f = 650V/µs). Note that the amount of
PP
. From the frequency Re-
PP
differential input
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Due to the higher frequency phase shift between input and
output, there is no closed form solution to input overdrive for
a given input. Therefore, Figure 1 is not very useful by itself
in determining the output swing.
The following plots aid in predicting the output transition time
based on the amount of swing required for a given gain
setting.
Beyond a gain of 5 or so, the LMH6657/6658 output transi-
tion would be limited by bandwidth. For example, with a gain
of 5, the −3dB BW would be around 30MHz corresponding to
a rise time of about 12ns (10% - 90%). Assuming a near
linear transition, the 20%-80% transition time would be
around 9ns which matches the measured results as shown
in Figure 2.
When the output is heavily loaded, output swing may be
limited by current capability of the device. Refer to "Output
Current Capability" section, below, for more details.
FIGURE 2. Output 20%-80% Transition vs. Output
FIGURE 3. Output 20%-80% Transition vs. Output
Voltage Swing (Non-Inverting Gain)
Voltage Swing (Inverting Gain)
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