OPA4317 Datasheet, PDF (10/30 Pages) Texas Instruments – Low-Offset, Rail-to-Rail I/O Operational Amplifier

OPA317
OPA2317
OPA4317
SBOS682A – MAY 2013 – REVISED JUNE 2013
10 kW Clamp
+In
CORE
-In
10 kW
Figure 19. Equivalent Input Circuit
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INTERNAL OFFSET CORRECTION
The OPA317, OPA2317, and OPA4317 op amps use an auto-calibration technique with a time-continuous, 125-
kHz op amp in the signal path. This amplifier is zero-corrected every 8 μs using a proprietary technique. Upon
power-up, the amplifier requires approximately 100 μs to achieve specified V_OS accuracy. This design has no
aliasing or flicker noise.
EMI SUSCEPTIBILITY AND INPUT FILTERING
Operational amplifiers vary in their susceptibility to EMI. If conducted EMI enters the operational amplifier, the dc
offset observed at the amplifier output may shift from its nominal value while the EMI is present. This shift is a
result of signal rectification associated with the internal semiconductor junctions. While all operational amplifier
pin functions can be affected by EMI, the input pins are likely to be the most susceptible. The OPA317
operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier response to
EMI. Both common-mode and differential mode filtering are provided by the input filter. The filter is designed for a
cutoff frequency of approximately 8 MHz (–3 dB), with a roll-off of 20 dB per decade.
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 op amps, problems arise when the output signal approaches 0 V,
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 OPA317, OPA2317, and OPA4317 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 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 20.
V+ = +5 V
Device
V_OUT
V_IN
R_P = 20 kW
Op Amp V- = GND
-5 V
Additional
Negative
Supply
Figure 20. For V_OUT Range to Ground
The OPA317, OPA2317, and OPA4317 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, OPA2317, and OPA4317 have been characterized to perform with
this technique; the recommended resistor value is approximately 20 kΩ. Note that 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 occurs below –2 mV, but excellent accuracy returns as the output drives back up
above –2 mV. Lowering the resistance of the pull-down resistor allows the op amp 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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