OPA388 Datasheet, PDF (17/32 Pages) Texas Instruments – Precision, Zero-Drift, Zero-Crossover, True Rail-to-Rail Input/Output, Operational Amplifiers

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OPA388, OPA2388, OPA4388
SBOS777 – DECEMBER 2016
Feature Description (continued)
Typically, input bias current is approximately ±30 pA. Input voltages exceeding the power supplies, however, 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 39.
Current-limiting resistor
required if input voltage
exceeds supply rails by
> 0.3V.
+5V
IOVERLOAD
10 mA max
VIN
5 NŸ
VOUT
Copyright © 2016, Texas Instruments Incorporated
Figure 39. Input Current Protection
7.3.3 Input Differential Voltage
The typical input bias current of the OPAx388 during normal operation is approximately 30 pA. In overdriven
conditions, the bias current can increase significantly. 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 40. Notice that the input bias
current remains within specification in the linear region.
100 W _Clamp
+In
CORE
-In
100 W
Copyright © 2016, Texas Instruments Incorporated
Figure 40. Equivalent Input Circuit
7.3.4 Internal Offset Correction
The OPA388 family of operational amplifiers uses an auto-calibration technique with a time-continuous, 200-kHz
operational amplifier in the signal path. This amplifier is zero-corrected every 5 µs using a proprietary technique.
At power-up, the amplifier requires approximately 1 ms to achieve the specified V_OS accuracy. This design has
no aliasing or flicker noise.
7.3.5 EMI Susceptibility and Input Filtering
Operational amplifiers vary in susceptibility to EMI. If conducted EMI enters the operational amplifier, the dc
offset at the amplifier output can shift from its nominal value when EMI is present. This shift is a result of signal
rectification associated with the internal semiconductor junctions. Although all operational amplifier pin functions
can be affected by EMI, the input pins are likely to be the most susceptible. The OPAx388 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 20 MHz (–3 dB), with a rolloff of 20 dB per decade.
7.4 Device Functional Modes
The OPA388 has a single functional mode and is operational when the power-supply voltage is greater than
2.5 V (±1.25 V). The maximum specified power-supply voltage for the OPAx388 is 5.5 V (±2.75 V).
Copyright © 2016, Texas Instruments Incorporated
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Product Folder Links: OPA388 OPA2388 OPA4388