8.3.1 Operating Voltage

The OPA317 series of operational amplifiers can be used with single or dual supplies from an operating range of V_S = 1.8 V (±0.9 V) up to 5.5 V (±2.75 V).

See the Absolute Maximum Ratings table. Key parameters that vary over the supply voltage or temperature range are shown in the Typical Characteristics section.

8.3.2 Input Voltage

The OPA317, OPA2317, and OPA4317 input common-mode voltage range extends 0.1 V beyond the supply rails. The OPA317 device is designed to cover the full range without the troublesome transition region found in some other rail-to-rail amplifiers.

Typically, input bias current is about 200 pA; however, input voltages exceeding the power supplies can cause excessive current to flow into or out of the input pins. Momentary voltages greater than the power supply can be tolerated if the input current is limited to 10 mA. This limitation is easily accomplished with an input resistor, as shown in Figure 19.

NOTE: Current limiting resistor required if input voltage exceeds supply rails by ≥ 0.3 V.

Figure 19. Input Current Protection

8.3.3 Input Differential Voltage

The typical input bias current of the OPA317 during normal operation is approximately 200 pA. In overdriven conditions, the bias current can increase significantly (see Figure 17).The most common cause of an overdriven condition occurs when the operational amplifier is outside of the linear range of operation. When the output of the operational amplifier is driven to one of the supply rails, the feedback loop requirements cannot be satisfied, and a differential input voltage develops across the input pins. This differential input voltage results in activation of parasitic diodes inside the front-end input chopping switches that combine with 10-kΩ electromagnetic interference (EMI) filter resistors to create the equivalent circuit shown in Figure 20.

Figure 20. Equivalent Input Circuit

8.3.4 Internal Offset Correction

The OPA317, OPA2317, and OPA4317 operational amplifiers use an auto-calibration technique with a time-continuous, 125-kHz operational amplifier 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.

8.3.5 EMI Susceptibility and Input Filtering

Operational amplifiers vary in 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.