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OPA350
OPA2350
OPA4350
SBOS099C − SEPTEMBER 2000 − REVISED JANUARY 2005
APPLICATIONS INFORMATION
OPA350 series op amps are fabricated on a
state-of-the-art 0.6 micron CMOS process. They are
unity-gain stable and suitable for a wide range of
general-purpose applications. Rail-to-rail input/output
make them ideal for driving sampling A/D converters.
They are also well-suited for controlling the output
power in cell phones. These applications often require
high speed and low noise. In addition, the OPA350
series offers a low-cost solution for general-purpose
and consumer video applications (75Ω drive capability).
Excellent ac performance makes the OPA350 series
well-suited for audio applications. Their bandwidth,
slew rate, low noise (5nV/√Hz), low THD (0.0006%),
and small package options are ideal for these
applications. The class AB output stage is capable of
driving 600Ω loads connected to any point between V+
and ground.
Rail-to-rail input and output swing significantly
increases dynamic range, especially in low voltage
supply applications. Figure 1 shows the input and
output waveforms for the OPA350 in unity-gain
configuration. Operation is from a single +5V supply
with a 1kΩ load connected to V
S
/2. The input is a 5V
PP
sinusoid. Output voltage swing is approximately
4.95V
PP
.
Power supply pins should be bypassed with 0.01µF
ceramic capacitors.
OPERATING VOLTAGE
OPA350 series op amps are fully specified from +2.7V
to +5.5V. However, supply voltage may range from
+2.5V to +5.5V. Parameters are tested over the
specified supply range—a unique feature of the
OPA350 series. In addition, many specifications apply
from −40°C to +85°C. Most behavior remains virtually
unchanged throughout the full operating voltage range.
Parameters that vary significantly with operating
voltage or temperature are shown in the typical
characteristics.
RAIL-TO-RAIL INPUT
The tested input common-mode voltage range of the
OPA350 series extends 100mV beyond the supply rails.
This is achieved with a complementary input stage—an
N-channel input differential pair in parallel with a
P-channel differential pair, as shown in Figure 2. The
N-channel pair is active for input voltages close to the
positive rail, typically (V+) – 1.8V to 100mV above the
positive supply, while the P-channel pair is on for inputs
from 100mV below the negative supply to
approximately (V+) – 1.8V. There is a small transition
region, typically (V+) – 2V to (V+) – 1.6V, in which both
pairs are on. This 400mV transition region can vary
±400mV
with process variation. Thus, the transition
region (both input stages on) can range from (V+) –
2.4V to (V+) – 2.0V on the low end, up to (V+) – 1.6V
to (V+) – 1.2V on the high end.
OPA350 series op amps are laser-trimmed to reduce
offset voltage difference between the N-channel and
P-channel input stages, resulting in improved
common-mode rejection and a smooth transition
between the N-channel pair and the P-channel pair.
However, within the 400mV transition region PSRR,
CMRR, offset voltage, offset drift, and THD may be
degraded compared to operation outside this region.
A double-folded cascode adds the signal from the two
input pairs and presents a differential signal to the class
AB output stage. Normally, input bias current is
approximately 500fA. However, large inputs (greater
than 300mV beyond the supply rails) can turn on the
OPA350’s input protection diodes, causing excessive
current to flow in or out of the input pins. Momentary
voltages greater than 300mV beyond the power supply
can be tolerated if the current on the input pins is limited
to 10mA. This is easily accomplished with an input
resistor, as shown in Figure 3. Many input signals are
inherently current-limited to less than 10mA; therefore,
a limiting resistor is not required.
V
S
= +5, G = +1, R
L
= 1kΩ
5V
V
IN
0
5V
1.25V/div
V
OUT
0
Figure 1. Rail-to-Rail Input and Output
9
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