V

OUT

R = 20 kW

P

Op Amp V = GND

-

Device

V

IN

V+ = +5 V

-5 V

Additional

Negative

Supply

CORE

-In

+In

Clamp

10 kW

10 kW

OPA317

OPA2317

OPA4317

SBOS682A – MAY 2013 – REVISED JUNE 2013

www.ti.com

Figure 19. Equivalent Input Circuit

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

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.

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 k

Ω. 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 k

Ω to achieve excellent accuracy down to –10 mV.

10

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