LMH6618 Datasheet, PDF (18/26 Pages) National Semiconductor (TI) – 130 MHz, 1.25 mA Rail-to-Rail Input and Output Operational Amplifier with Shutdown

Application Information
The LMH6618 is based on National Semiconductor’s propri-
etary VIP10 dielectrically isolated bipolar process. This de-
vice family architecture features the following:
• Complimentary bipolar devices with exceptionally high f_t
(∼8GHz) even under low supply voltage (2.7V) and low
bias current.
• Common emitter push-push output stage. This
architecture allows the output to reach within millivolts of
either supply rail.
• Consistent performance from any supply voltage (2.7V -
11V) with little variation with supply voltage for the most
important specifications (e.g. BW, SR, I_OUT.)
• Significant power saving compared to competitive devices
on the market with similar performance.
With 3V supplies and a common mode input voltage range
that extends beyond either supply rail, the LMH6618 is well
suited to many low voltage/low power applications. Even with
3V supplies, the −3 dB BW (at A_V = +1) is typically 120 MHz.
The LMH6618 is designed to avoid output phase reversal.
With input over-drive, the output is kept near the supply rail
(or as close to it as mandated by the closed loop gain setting
and the input voltage). Figure 1 shows the input and output
voltage when the input voltage significantly exceeds the sup-
ply voltages.
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FIGURE 1. Input and Output Shown with CMVR Exceeded
If the input voltage range is exceeded by more than a diode
drop beyond either rail, the internal ESD protection diodes will
start to conduct. The current flow in these ESD diodes should
be externally limited.
The LMH6618 can be shutdown by connecting the
DISABLE pin to a voltage 0.5V below the supply midpoint
which will reduce the supply current to typically less than 100
µA. The DISABLE pin is “active low” and should be connected
through a resistor to V⁺ for normal operation. Shutdown is
guaranteed when the DISABLE pin is 0.5V below the supply
midpoint at any operating supply voltage and temperature.
In the shutdown mode, essentially all internal device biasing
is turned off in order to minimize supply current flow and the
output goes into high impedance mode. During shutdown, the
input stage has an equivalent circuit as shown in Figure 2.
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FIGURE 2. Input Equivalent Circuit During Shutdown
When the LMH6618 is shutdown, there may be current flow
through the internal diodes shown, caused by input potential,
if present. This current may flow through the external feed-
back resistor and result in an apparent output signal. In most
shutdown applications the presence of this output is incon-
sequential. However, if the output is “forced” by another de-
vice, the other device will need to conduct the current
described in order to maintain the output potential.
To keep the output at or near ground during shutdown when
there is no other device to hold the output low, a switch using
a transistor can be used to shunt the output to ground.
SINGLE CHANNEL ADC DRIVER
The low noise and wide bandwidth make the LMH6618 an
excellent choice for driving a 12-bit ADC. Figure 3 shows the
schematic of the LMH6618 driving an ADC121S101. The AD-
C121S101 is a single channel 12-bit ADC. The LMH6618 is
set up in a 2nd order multiple-feedback configuration with a
gain of −1. The −3 db point is at 500 kHz and the −0.01 dB
point is at 100 kHz. Table 1 shows the performance data of
the LMH6618 and the ADC121S101.
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