OPA333 Datasheet, PDF (8/18 Pages) Burr-Brown (TI) – 1.8V, microPOWER CMOS OPERATIONAL AMPLIFIERS Zer-PI Drift Series

OPA333
OPA2333
SBOS351 − MARCH 2006
ACHIEVING OUTPUT SWING TO THE OP
AMP NEGATIVE RAIL
Some applications require output voltage swings from 0V
to a positive full-scale voltage (such as +2.5V) with
excellent accuracy. With most single-supply op amps,
problems arise when the output signal approaches 0V,
near the lower output swing limit of a single-supply op amp.
A good single-supply op amp may swing close to
single-supply ground, but will not reach ground. The output
of the OPA333 and OPA2333 can be made to swing to
ground, or slightly below, on a single-supply power source.
To do so requires the use of another resistor and an
additional, more negative, power supply than the op amp
negative supply. A pull-down 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 2.
V+ = +5V
OPA333
V_OUT
V_IN
R_P = 20kΩ
Op Amp V− = Gnd
−5V
Additional
Negative
Supply
Figure 2. For V_OUT Range to Ground
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The OPA333 and OPA2333 have an output stage that
allows the output voltage to be pulled to its negative supply
rail, or slightly below, using the technique previously
described. This technique only works with some types of
output stages. The OPA333 and OPA2333 have been
characterized to perform with this technique; however, the
recommended resistor value is approximately 20kΩ. Note
that this configuration will increase the current
consumption by several hundreds of microamps.
Accuracy is excellent down to 0V and as low as −2mV.
Limiting and nonlinearity occurs below−2mV, but excellent
accuracy returns as the output is again driven above
−2mV. Lowering the resistance of the pull-down resistor
will allow the op amp to swing even further below the
negative rail. Resistances as low as 10kΩ can be used to
achieve excellent accuracy down to −10mV.
GENERAL LAYOUT GUIDELINES
Attention to good layout practices is always
recommended. Keep traces short and, when possible, use
a printed circuit board (PCB) ground plane with
surface-mount components placed as close to the device
pins as possible. Place a 0.1µF capacitor closely across
the supply pins. These guidelines should be applied
throughout the analog circuit to improve performance and
provide benefits such as reducing the EMI
(electromagnetic-interference) susceptibility.
Operational amplifiers vary in their susceptibility to radio
frequency interference (RFI). RFI can generally be
identified as a variation in offset voltage or dc signal levels
with changes in the interfering RF signal. The OPA333 has
been specifically designed to minimize susceptibility to
RFI and demonstrates remarkably low sensitivity
compared to previous generation devices. Strong RF
fields may still cause varying offset levels..
+5V
+
0.1µF
REF3140
D1
−−
4.096V
R₁
6.04kΩ
R₅
31.6kΩ
R₂
2.94kΩ
R₂
549Ω
R₉
150kΩ
+5V
0.1µF
++
K−Type
R₆
200Ω
OPA333
V_O
Thermocouple
40.7µV/_ C
R₄
6.04kΩ
R₃
60.4Ω
Zero Adj.
Figure 3. Temperature Measurement
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