OPA2377 datasheet(10/26 Pages) TI1

10 / 26 page

5kW

OPA377

10mAmax

V+

OUT

OVERLOAD

20W

V+

OUT

OPA377

OPA2377

OPA4377

SBOS504B

– FEBRUARY 2010 – REVISED JANUARY 2011

www.ti.com

INPUT AND ESD PROTECTION

CAPACITIVE LOAD AND STABILITY

The OPA377 family incorporates internal electrostatic

The OPA377 series of amplifiers may be used in

discharge (ESD) protection circuits on all pins. In the

applications where driving a capacitive load is

case of input and output pins, this protection primarily

required. As with all op amps, there may be specific

consists

current

steering

diodes

connected

instances where the OPAx377 can become unstable,

between the input and power-supply pins. These ESD

leading to oscillation. The particular op amp circuit

protection

diodes

also

provide

in-circuit,

input

configuration, layout, gain, and output loading are

overdrive protection, as long as the current is limited

some of the factors to consider when establishing

to 10mA as stated in the Absolute Maximum Ratings.

whether an amplifier will be stable in operation. An op

Figure 23 shows how a series input resistor may be

amp in the unity-gain (+1V/V) buffer configuration and

added to the driven input to limit the input current.

driving a capacitive load exhibits a greater tendency

The added resistor contributes thermal noise at the

to be unstable than an amplifier operated at a higher

amplifier input and its value should be kept to a

noise gain. The capacitive load, in conjunction with

minimum in noise-sensitive applications.

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.

unity-gain

configurations,

Figure 23. Input Current Protection

capacitive load drive can be improved by inserting a

small (10

Ω to 20Ω) resistor, RS, in series with the

output,

shown

Figure

24.

This

resistor

EMI SUSCEPTIBILITY AND INPUT FILTERING

significantly reduces ringing while maintaining dc

performance for purely capacitive loads. However, if

Operational

amplifiers

vary

susceptibility

there is a resistive load in parallel with the capacitive

electromagnetic interference (EMI). If conducted EMI

load, a voltage divider is created, introducing a gain

enters

the

operational

amplifier,

the

offset

error at the output and slightly reducing the output

observed at the amplifier output may shift from the

swing. The error introduced is proportional to the ratio

nominal value while the EMI is present. This shift is a

RS/RL, and is generally negligible at low output

result of signal rectification associated with the

current levels.

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

designed

for

cutoff

frequency

approximately 75MHz (

–3dB), with a roll-off of 20dB

per decade.

Figure 24. Improving Capacitive Load Drive

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