OPA317-Q1 Datasheet, PDF (16/29 Pages) Texas Instruments – Zero-Drift, Low-Offset, Rail-to-Rail I/O Operational Amplifier

OPA317-Q1, OPA2317-Q1, OPA4317-Q1
SLOS914 – JULY 2016
9 Application and Implementation
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NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
The OPA317-Q1, OPA2317-Q1, and OPA4317-Q1 are unity-gain stable, precision operational amplifiers free
from unexpected output and phase reversal. Proprietary Zerø-Drift circuitry gives the benefit of low input offset
voltage over time and temperature, as well as lowering the 1/f noise component. As a result of the high PSRR,
these devices work well in applications that run directly from battery power without regulation. The OPA317-Q1
family is optimized for low-voltage, single-supply operation. These miniature, high-precision, low quiescent
current amplifiers offer high impedance inputs that have a common-mode range 100 mV beyond the supplies,
and a rail-to-rail output that swings within 100 mV of the supplies under normal test conditions. The OPA317-Q1
series are precision amplifiers for cost-sensitive applications.
9.1.1 Achieving Output Swing to the Op Amp Negative Rail
Some applications require output voltage swings from 0 V to a positive full-scale voltage (such as 2.5 V) with
excellent accuracy. With most single-supply operational amplifiers, problems arise when the output signal
approaches 0 V, near the lower output swing limit of a single-supply operational amplifier. A good single-supply
operational amplifier may swing close to single-supply ground, but does not reach ground. The output of the
OPA317-Q1, OPA2317-Q1, and OPA4317-Q1 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 operational amplifier negative supply. A pulldown resistor can 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 21.
V+ = 5 V
Device
V_OUT
V_IN
R_P = 20 kW
Op Amp V- = GND
-5 V
Additional
Negative
Supply
Figure 21. For V_OUT Range to Ground
The OPA317-Q1, OPA2317-Q1, and OPA4317-Q1 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 OPA317-Q1, OPA2317-Q1, and OPA4317-Q1 have been
characterized to perform with this technique; the recommended resistor value is approximately 20 kΩ. This
configuration increases the current consumption by several hundreds of microamps. Accuracy is excellent down
to 0 V and as low as –2 mV. Limiting and nonlinearity occur below –2 mV, but excellent accuracy returns as the
output drives back up above –2 mV. Lowering the resistance of the pulldown resistor allows the operational
amplifier to swing even further below the negative rail. Use resistances as low as 10 kΩ to achieve excellent
accuracy down to –10 mV.
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