Time (40 ms/div)

C017

V

OUT

V

IN

16

OPA187, OPA2187, OPA4187

SBOS807 – DECEMBER 2016

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7.3 Feature Description

The OPAx187 is unity-gain stable and free from unexpected output phase reversal. This device uses a

proprietary, periodic autocalibration technique to provide ultra-low input offset voltage and near zero input offset

voltage drift over temp and temperature. For lowest offset voltage and precision performance, optimize circuit

layout and mechanical conditions. Avoid temperature gradients that create thermoelectric (Seebeck) effects in

the thermocouple junctions formed from connecting dissimilar conductors. Cancel these thermally-generated

potentials by making sure they are equal on both input pins. Other layout and design considerations include:

Use low thermoelectric-coefficient conditions (avoid dissimilar metals).

Thermally isolate components from power supplies or other heat sources.

Shield operational amplifier and input circuitry from air currents, such as cooling fans.

Follow these guidelines to reduce the likelihood of junctions being at different temperatures, which may cause

thermoelectric voltages of 0.1 µV/°C or higher, depending on the materials used.

7.3.1 Operating Characteristics

The OPAx187 device is specified for operation from 4.5 V to 36 V (±2.25 V to ±18 V). Many specifications apply

from –40°C to +125°C. Parameters that can exhibit significant variance with regard to operating voltage or

temperature are presented in the Typical Characteristics section.

7.3.2 Phase-Reversal Protection

The OPAx187 device has an internal phase-reversal protection. Many op amps exhibit a phase reversal when

the input is driven beyond its linear common-mode range. This condition is most often encountered in non-

inverting circuits when the input is driven beyond the specified common-mode voltage range, causing the output

to reverse into the opposite rail. The OPAx187 input prevents phase reversal with excessive common-mode

voltage. Instead, the output limits into the appropriate rail. Figure 37 shows this performance.

Figure 37. No Phase Reversal

7.3.3 Input Bias Current Clock Feedthrough

Zero-drift amplifiers, such as the OPAx187, use switching on their inputs to correct for the intrinsic offset and drift

of the amplifier. Charge injection from the integrated switches on the inputs can introduce very short transients in

the input bias current of the amplifier. The extremely short duration of these pulses prevents them from being

amplified, however they may be coupled to the output of the amplifier through the feedback network. The most

effective method to prevent transients in the input bias current from producing additional noise at the amplifier

output is to use a low-pass filter such as an RC network.

7.3.4 Internal Offset Correction

The OPAx187 op amp uses an auto-calibration technique with a time-continuous 125-kHz op amp in the signal

path. This amplifier is zero-corrected every 22 μs using a proprietary technique. Upon power-up, the amplifier