OPA2377 Datasheet, PDF (10/26 Pages) Texas Instruments – 5MHz, Low-Noise, Single, Dual, Quad CMOS Operational Amplifiers

OPA377
OPA2377
OPA4377
SBOS504B – FEBRUARY 2010 – REVISED JANUARY 2011
INPUT AND ESD PROTECTION
The OPA377 family incorporates internal electrostatic
discharge (ESD) protection circuits on all pins. In the
case of input and output pins, this protection primarily
consists of current steering diodes connected
between the input and power-supply pins. These ESD
protection diodes also provide in-circuit, input
overdrive protection, as long as the current is limited
to 10mA as stated in the Absolute Maximum Ratings.
Figure 23 shows how a series input resistor may be
added to the driven input to limit the input current.
The added resistor contributes thermal noise at the
amplifier input and its value should be kept to a
minimum in noise-sensitive applications.
I_OVERLOAD
10mA max
V_IN
5kW
V+
OPA377
V_OUT
Figure 23. Input Current Protection
EMI SUSCEPTIBILITY AND INPUT FILTERING
Operational amplifiers vary in susceptibility to
electromagnetic interference (EMI). If conducted EMI
enters the operational amplifier, the dc offset
observed at the amplifier output may shift from the
nominal value while the EMI is present. This shift is a
result of signal rectification associated with the
internal semiconductor junctions. While all amplifier
pin functions can be affected by EMI, the input pins
are likely to be the most susceptible. The OPA377
operational amplifier family incorporates an internal
input low-pass filter that reduces the amplifier
response to EMI. Both common-mode and differential
mode filtering are provided by the input filter. The
filter is designed for a cutoff frequency of
approximately 75MHz (–3dB), with a roll-off of 20dB
per decade.
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CAPACITIVE LOAD AND STABILITY
The OPA377 series of amplifiers may be used in
applications where driving a capacitive load is
required. As with all op amps, there may be specific
instances where the OPAx377 can become unstable,
leading to oscillation. The particular op amp circuit
configuration, layout, gain, and output loading are
some of the factors to consider when establishing
whether an amplifier will be stable in operation. An op
amp in the unity-gain (+1V/V) buffer configuration and
driving a capacitive load exhibits a greater tendency
to be unstable than an amplifier operated at a higher
noise gain. The capacitive load, in conjunction with
the op amp output resistance, creates a pole within
the feedback loop that degrades the phase margin.
The degradation of the phase margin increases as
the capacitive loading increases.
The OPAx377 in a unity-gain configuration can
directly drive up to 250pF pure capacitive load.
Increasing the gain enhances the ability of the
amplifier to drive greater capacitive loads; see the
typical characteristic plot, Small-Signal Overshoot vs
Capacitive Load. In unity-gain configurations,
capacitive load drive can be improved by inserting a
small (10Ω to 20Ω) resistor, R_S, in series with the
output, as shown in Figure 24. This resistor
significantly reduces ringing while maintaining dc
performance for purely capacitive loads. However, if
there is a resistive load in parallel with the capacitive
load, a voltage divider is created, introducing a gain
error at the output and slightly reducing the output
swing. The error introduced is proportional to the ratio
R_S/R_L, and is generally negligible at low output
current levels.
V+
R_S
OPA377
V_OUT
V_IN
10W to
20W
R_L
C_L
Figure 24. Improving Capacitive Load Drive
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