7 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.

7.1 Application Information

The OPA177 is unity-gain stable, making this device easy to use and free from oscillations in the widest range of

circuitry. Applications with noisy or high-impedance power-supply lines can require decoupling capacitors close

to the device pins. In most cases, 0.1

‑µF ceramic capacitors are adequate.

The OPA177 has very low offset voltage and drift. To achieve highest performance, optimize circuit layout

and mechanical conditions. Offset voltage and drift degrades because of small thermoelectric potentials at

the op-amp inputs. Connections of dissimilar metals generate thermal potential that can degrade the ultimate

performance of the OPA177. To cancel these thermal potentials, make sure the thermal potentials are equal in

both input pins.

1. Keep connections made to the two input pins close together.

2. Locate heat sources as far as possible from the critical input circuitry.

3. Shield the op amp and input circuitry from air currents, such as cooling fans.

7.1.1 Offset Voltage Adjustment

The OPA177 has been laser-trimmed for low offset voltage and drift; therefore, most circuits do not require

external adjustment. Figure 7-1 shows the optional connection of an external potentiometer to adjust offset

voltage. Do not use this adjustment to compensate for offsets created elsewhere in a system because this

adjustment can introduce excessive temperature drift.

7.1.2 Input Protection

The inputs of the OPA177 are protected with 500

‑Ω series input resistors and diode clamps as shown in the

simplified circuit diagram on the front page. The inputs can withstand ±30

‑V differential inputs without damage.

The protection diodes conduct current when the inputs are overdriven. This conducted current can disturb the

slewing behavior of unity-gain follower applications, but does not damage the op amp. Some applications, such

as programmable logic controllers (PLCs) require a robust input-protection design. An input-protection circuit can

be implemented using four Schottky diodes; however, the temperature and voltage dependent leakage of the

diodes can present undesirable nonlinear errors at the input. For applications requiring high precision and robust

input protection, the OPAx206 family of op amps are an excellent choice, offering integrated input overvoltage

protection that eliminates the need for external clamping circuits.

7.1.3 Noise Performance

The OPA177 noise performance is optimized for a circuit impedance range of 2 kΩ to 50 kΩ. Total noise in

an application is a combination of the op-amp input voltage noise and input bias current noise reacting with

circuit impedance. For applications with higher source impedance, the OPAx828 and OPAx140 FET-input op

amps generally provide lower noise due to the inherently low input current noise. For low-impedance, low-noise

applications, the OPAx210 is an excellent choice because of the exceptionally low op-amp input-voltage noise.

OPA177

SBOS008A – SEPTEMBER 2000 – REVISED SEPTEMBER 2023

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