OPA2333AIDGKR Datasheet, PDF (9/28 Pages) Texas Instruments – 1.8V, microPOWER CMOS OPERATIONAL AMPLIFIERS Zero-Drift Series

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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 19.
V+ = +5V
OPA333
V_OUT
V_IN
R_P = 20kW
Op Amp V- = GND
-5V
Additional
Negative
Supply
Figure 19. For V_OUT Range to Ground
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
OPA333
OPA2333
SBOS351C – MARCH 2006 – REVISED MAY 2007
works with some types of output stages. The
OPA333 and OPA2333 have been characterized to
perform with this technique; 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
+
0.1mF
REF3140
D1
4.096V
R₁
6.04kW
R₅
31.6kW
R₉
150kW
+5V
--
R₂
2.94kW
R₂
549W
0.1mF
++
R₆
OPA333
V_O
K-Type
200W
Thermocouple
R₄
R₃
Zero Adj.
40.7mV/°C
6.04kW 60.4W
Figure 20. Temperature Measurement
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