OPA363 Datasheet, PDF (9/27 Pages) Texas Instruments – 1.8V, 7MHz, 90dB CMRR, SINGLE-SUPPLY, RAIL-TO-RAIL I/O OPERATIONAL AMPLIFIER

ENABLE FUNCTION
The shutdown (enable) function of the OPA363 is referenced
to the negative supply voltage of the operational amplifier. A
logic level HIGH enables the op amp. A valid logic HIGH is
defined as voltage > 75% of the positive supply applied to the
enable pin. The valid logic HIGH signal can be as much as
5.5V above the negative supply, independent of the positive
supply voltage. A valid logic LOW is defined as < 0.8V above
the negative supply pin. If dual or split power supplies are
used, care should be taken to ensure logic input signals are
properly referred to the negative supply voltage. This pin
should be connected to a valid high or low voltage or driven,
not left open circuit.
The logic input is a high-impedance CMOS input. Dual op
amps are provided separate logic inputs. For battery-oper-
ated applications, this feature may be used to greatly reduce
the average current and extend battery life. The enable time
is 20µs; disable time is 1µs. When disabled, the output
assumes a high-impedance state. This allows the OPA363 to
be operated as a “gated” amplifier, or to have its output
multiplexed onto a common analog output bus.
CAPACITIVE LOAD
The OPA363 and OPA364 series op amps can drive a wide
range of capacitive loads. However, all op amps under
certain conditions may become unstable. Op amp configura-
tion, gain, and load value are just a few of the factors to
consider when determining stability. An op amp in unity-gain
configuration is the most susceptible to the effects of capaci-
tive load. The capacitive load reacts with the output resis-
tance of the op amp to create a pole in the small-signal
response, which degrades the phase margin.
In unity gain, the OPA363 and OPA364 series op amps
perform well with a pure capacitive load up to approximately
1000pF. The ESR (Equivalent Series Resistance) of the
loading capacitor may be sufficient to allow the OPA363 and
OPA364 to directly drive very large capacitive loads (> 1µF).
Increasing gain enhances the amplifier’s ability to drive more
capacitance. See the typical characteristic “Small-Signal Over-
shoot vs Capacitive Load.”
One method of improving capacitive load drive in the unity-
gain configuration is to insert a 10Ω to 20Ω resistor in series
with the output, as shown in Figure 3. This significantly
reduces ringing with large capacitive loads. However, if there
is a resistive load in parallel with the capacitive load, it
creates a voltage divider introducing a dc error at the output
and slightly reduces output swing. This error may be insignifi-
cant. For instance, with R_L = 10kΩ and R_S = 20Ω, there is
only about a 0.2% error at the output.
V+
OPAx363
OPAx364
V_IN
R_S
10Ω to
20Ω
R_L
V_OUT
C_L
FIGURE 3. Improving Capacitive Load Drive.
INPUT AND ESD PROTECTION
All OPA363 and OPA364 pins are static protected with
internal ESD protection diodes tied to the supplies. These
diodes will provide overdrive protection if the current is
externally limited to 10mA, as stated in the absolute maxi-
mum ratings and shown in Figure 4.
I_OVERLOAD
10mA max
V_IN
5kΩ
V+
OPAx363
V_OUT
FIGURE 4. Input Current Protection.
ACHIEVING OUTPUT SWING TO
THE OP AMP’S NEGATIVE RAIL
Some applications require an accurate output voltage swing
from 0V to a positive full-scale voltage. A good single supply op
amp may be able to swing within a few mV of single supply
ground, but as the output is driven toward 0V, the output stage
of the amplifier will prevent the output from reaching the
negative supply rail of the amplifier.
The output of the OPA363 or OPA364 can be made to swing to
ground, or slightly below, on a single supply power source. To do
so requires use of another resistor and an additional, more
negative power supply than the op amp’s negative supply. A
pulldown resistor may be connected between the output and the
additional negative supply to pull the output down below the value
that the output would otherwise achieve as shown in Figure 5.
V+ = +5V
OPA363
OPA364
V_OUT
V_IN
500µA
R_P = 10kΩ
Op Amp’s
Negative
Supply
Grounded
–V = –5V
(Additional
Negative Supply)
FIGURE 5. OPA363 and OPA364 Swing to Ground.
This technique will not work with all op amps. The output
stage of the OPA363 and OPA364 allows the output voltage
to be pulled below that of most op amps, if approximately
500µA is maintained through the output stage. To calculate
the appropriate value load resistor and negative supply,
R_L = –V/500µA. The OPA363 and OPA364 have been
characterized to perform well under the described conditions,
maintaining excellent accuracy down to 0V and as low as
–10mV. Limiting and nonlinearity occur below –10mV, with
linearity returning as the output is again driven above
–10mV.
OPA363, 2363, 364, 2364, 4364
9
SBOS259B
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