The OPAx277 series precision operational amplifiers replace the industry standard OP-177. They offer improved noise, wider output voltage swing, and are twice as fast with half the quiescent current. Features include ultralow offset voltage and drift, low bias current, high common-mode rejection, and high power supply rejection. Single, dual, and quad versions have identical specifications, for maximum design flexibility.
The OPAx277 series is unity-gain stable and free from unexpected output phase reversal, making it easy to use in a wide range of applications. Applications with noisy or high-impedance power supplies may require decoupling capacitors close to the device pins. In most cases 0.1-μF capacitors are adequate.
The OPAx277 series has low offset voltage and drift. To achieve highest performance, the circuit layout and mechanical conditions should be optimized. Offset voltage and drift can be degraded by small thermoelectric potentials at the operational amplifier inputs. Connections of dissimilar metals generate thermal potential, which can degrade the ultimate performance of the OPAx277 series. These thermal potentials can be made to cancel by assuring that they are equal in both input terminals.
OPAx277 series operational amplifiers operate from ±2-V to ±18-V supplies with excellent performance. Unlike most operational amplifiers, which are specified at only one supply voltage, the OPA277 series is specified for real-world applications; a single limit applies over the ±5-V to ±15-V supply range. This allows a customer operating at VS = ±10 V to have the same assured performance as a customer using ±15-V 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 (±2 V to ±18 V). Parameters which vary significantly with operating voltage or temperature are shown in Typical Characteristics.
The OPAx277 series is laser-trimmed for low offset voltage and drift, so most circuits do 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 24. Only use this adjustment to null the offset of the operational amplifier. This adjustment should not be used to compensate for offsets created elsewhere in a system, because this can introduce additional temperature drift.
The inputs of the OPAx277 series are protected with 1-kΩ series input resistors and diode clamps. The inputs can withstand ±30-V differential inputs without damage. The protection diodes conduct current when the inputs are over-driven. This may disturb the slewing behavior of unity-gain follower applications, but will not damage the operational amplifier.
The input stage base current of the OPAx277 series is internally compensated with an equal and opposite cancellation 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 operational amplifiers (see Figure 26). A resistor added to cancel input bias current errors may actually increase offset voltage and noise.
The electromagnetic interference (EMI) rejection ratio, or EMIRR, describes the EMI immunity of operational amplifiers. An adverse effect that is common to many operational amplifiers is a change in the offset voltage as a result of RF signal rectification. An operational amplifier that is more efficient at rejecting this change in offset as a result of EMI has a higher EMIRR and is quantified by a decibel value. Measuring EMIRR can be performed in many ways, but this report provides the EMIRR IN+, which specifically describes the EMIRR performance when the RF signal is applied to the noninverting input pin of the operational amplifier. In general, only the noninverting input is tested for EMIRR for the following three reasons:
If available, any dual and quad operational amplifier device versions have nearly similar EMIRR IN+ performance. The OPA277 unity-gain bandwidth is 1 MHz. EMIRR performance below this frequency denotes interfering signals that fall within the operational amplifier bandwidth.
Table 1 shows the EMIRR IN+ values for the OPA277 at particular frequencies commonly encountered in real-world applications. Applications listed in Table 1 may be centered on or operated near the particular frequency shown. This information may be of special interest to designers working with these types of applications, or working in other fields likely to encounter RF interference from broad sources, such as the industrial, scientific, and medical (ISM) radio band.
|400 MHz||Mobile radio, mobile satellite/space operation, weather, radar, UHF||59.1 dB|
|900 MHz||GSM, radio com/nav./GPS (to 1.6 GHz), ISM, aeronautical mobile, UHF||77.9 dB|
|1.8 GHz||GSM, mobile personal comm. broadband, satellite, L-band||91.3 dB|
|2.4 GHz||802.11b/g/n, Bluetooth™, mobile personal comm., ISM, amateur radio/satellite, S-band||93.3 dB|
|3.6 GHz||Radiolocation, aero comm./nav., satellite, mobile, S-band||105.9 dB|
|5.0 GHz||802.11a/n, aero comm./nav., mobile comm., space/satellite operation, C-band||107.5 dB|
Figure 28 shows the circuit configuration for testing the EMIRR IN+. An RF source is connected to the operational amplifier noninverting input terminal using a transmission line. The operational amplifier is configured in a unity gain buffer topology with the output connected to a low-pass filter (LPF) and a digital multimeter (DMM). Note that a large impedance mismatch at the operational amplifier input causes a voltage reflection; however, this effect is characterized and accounted for when determining the EMIRR IN+. The resulting dc offset voltage is sampled and measured by the multimeter. The LPF isolates the multimeter from residual RF signals that may interfere with multimeter accuracy. Refer to SBOA128 for more details.
The OPAx277 has a single functional mode and is operational when the power-supply voltage is greater than 4 V (±2 V). The maximum power supply voltage for the OPAx277 is 36 V (±18 V).