8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

The OPAx387 are unity-gain stable, precision, operational amplifiers featuring state-of-the-art, zero-drift

technology. The use of proprietary zero-drift circuitry gives the benefit of low input offset voltage over time

and temperature, as well as lower 1/f noise component. As a result of the high PSRR, the devices work well in

applications that run directly from battery power without regulation. The OPAx387 family is optimized for full rail-

to-rail input, allowing for low-voltage, single-supply operation or split-supply use. These miniature, high-precision,

low-noise amplifiers offer high-impedance inputs that have a common-mode range 100 mV beyond the supplies

without input crossover distortion, and a rail-to-rail output that swings within 5 mV of the supplies under normal

test conditions. The OPAx387 precision amplifiers are designed for upstream analog signal-chain applications in

low or high gains, as well as downstream signal-chain functions, such as DAC buffering.

8.1.1 Zero-Drift Clocking

The OPAx387 use an advanced zero-drift architecture to achieve ultra-low offset and offset drift. This

architecture uses a clock and switches internally to create a dc error-correction path. The clocking is filtered

internally, and typically not observable for most configurations. Take the following precautions to minimize clock

noise in the signal chain. The clocking creates a small charge-injection pulse at the input of the amplifier;

therefore, do not use high-value resistors (> 100 kΩ) in series with the inputs to avoid higher clock voltage noise

at the output. The charge injection pulses are minimized when the impedance to the input pins is matched. If

higher value resistors are used, then use matching impedances on both amplifier input pins.

8.2 Typical Applications

8.2.1 Bidirectional Current Sensing

This single-supply, low-side, bidirectional current-sensing design example detects load currents from –1 A to +1

A. The single-ended output spans from 110 mV to 3.19 V. This design uses the OPAx387 because of the device

low offset voltage and rail-to-rail input and output. One of the amplifiers is configured as a difference amplifier

and the other amplifier provides the reference voltage. Figure 8-1 shows the design example schematic.

+

+

–

I

LOAD

V

BUS

V

OUT

V

CC

V

CC

V

REF

+

–

V

SHUNT

V

CC

U1A

U1B

R

4

R

SHUNT

R

3

R

L

R

5

R

6

R

2

R

1

Figure 8-1. Bidirectional Current-Sensing Schematic

OPA387, OPA2387, OPA4387

SBOS984F – NOVEMBER 2020 – REVISED FEBRUARY 2022

www.ti.com

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