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OPA2277UG4 Datasheet(PDF) 9 Page - Burr-Brown (TI) |
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OPA2277UG4 Datasheet(HTML) 9 Page - Burr-Brown (TI) |
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9 / 25 page  OPA277, OPA2277, OPA4277 9 SBOS079A www.ti.com Op Amp (a) OPA277 (b) No bias current cancellation resistor (see text) Conventional op amp with external bias current cancellation resistor. OPA277 with no external bias current cancellation resistor. R 2 R 1 R 2 R 1 R B = R2 || R1 APPLICATIONS INFORMATION The OPA277 series is unity-gain stable and free from unex- pected output phase reversal, making it easy to use in a wide range of applications. Applications with noisy or high imped- ance power supplies may require decoupling capacitors close to the device pins. In most cases 0.1 µF capacitors are adequate. The OPA277 series has very low offset voltage and drift. To achieve highest performance, circuit layout and mechanical conditions should be optimized. Offset voltage and drift can be degraded by small thermoelectric potentials at the op amp inputs. Connections of dissimilar metals will generate thermal potential which can degrade the ultimate performance of the OPA277 series. These thermal potentials can be made to cancel by assuring that they are equal in both input terminals. • Keep thermal mass of the connections made to the two input terminals similar. • Locate heat sources as far as possible from the critical input circuitry. • Shield op amp and input circuitry from air currents such as cooling fans. OPERATING VOLTAGE OPA277 series op amp operate from ±2V to ±18V supplies with excellent performance. Unlike most op amps which are specified at only one supply voltage, the OPA277 series is specified for real-world applications; a single limit applies over the ±5V to ±15V supply range. This allows a customer operating at VS = ±10V to have the same assured perfor- mance as a customer using ±15V supplies. In addition, key parameters are assured over the specified temperature range, –40 °C to +85°C. Most behavior remains unchanged through the full operating voltage range ( ±2V to ±18V). Parameters which vary significantly with operating voltage or temperature are shown in typical performance curves. OFFSET VOLTAGE ADJUSTMENT The OPA277 series is laser-trimmed for very low offset voltage and drift so most circuits will not require external adjustment. However, offset voltage trim connections are provided on pins 1 and 8. Offset voltage can be adjusted by connecting a potentiometer as shown in Figure 1. This adjustment should be used only to null the offset of the op amp. This adjustment should not be used to compensate for offsets created elsewhere in a system since this can intro- duce additional temperature drift. INPUT PROTECTION The inputs of the OPA277 series are protected with 1k Ω series input resistors and diode clamps. The inputs can withstand ±30V differential inputs without damage. The pro- tection diodes will, of course, conduct current when the inputs are over-driven. This may disturb the slewing behavior of unity-gain follower applications, but will not damage the op amp. INPUT BIAS CURRENT CANCELLATION The input stage base current of the OPA277 series is internally compensated with an equal and opposite cancella- tion circuit. The resulting input bias current is the difference between the input stage base current and the cancellation current. This residual input bias current can be positive or negative. When the bias current is canceled in this manner, the input bias current and input offset current are approximately the same magnitude. As a result, it is not necessary to use a bias current cancellation resistor as is often done with other op amps (Figure 2). A resistor added to cancel input bias current errors may actually increase offset voltage and noise. FIGURE 2. Input Bias Current Cancellation. V+ V– 20k Ω OPA277 single op amp only. Use offset adjust pins only to null offset voltage of op amp—see text. Trim Range: Exceeds Offset Voltage Specification OPA277 6 7 8 4 3 2 1 0.1µF 0.1µF FIGURE 1. OPA277 Offset Voltage Trim Circuit. |
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