7.3.1 Offset Voltage and Drift

The OPAx22x series have very low offset voltage and drift. To achieve highest DC precision, circuit layout and mechanical conditions should be optimized. Connections of dissimilar metals can generate thermal potentials at the operational amplifier inputs, which can degrade the offset voltage and drift. These thermocouple effects can exceed the inherent drift of the amplifier and ultimately degrade its performance. The thermal potentials can be made to cancel by assuring that they are equal at both input terminals. In addition:

• 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 operational amplifier and input circuitry from air currents such as those created by cooling fans.

7.3.2 Operating Voltage

The OPAx22x series of operational amplifiers operate from ±2.5 V to ±18 V supplies with excellent performance. Unlike most operational amplifiers that are specified at only one supply voltage, the OPA227 series is specified for real-world applications; a single set of specifications applies over the ±5-V to ±15-V supply range. Specifications are assured for applications from ±5-V to ±15-V power supplies. Some applications do not require equal positive and negative output voltage swing. Power supply voltages do not need to be equal. The OPAx22x series can operate with as little as 5 V between the supplies and with up to 36 V between the supplies. For example, the positive supply could be set to 25 V with the negative supply at –5 V or vice-versa. In addition, key parameters are assured over the specified temperature range, –40°C to 85°C. Parameters which vary significantly with operating voltage or temperature are shown in the Typical Characteristics.

7.3.3 Offset Voltage Adjustment

The OPAx22x series are laser-trimmed for very low offset and drift so most applications will not require external adjustment. However, the OPA227 and OPA228 (single versions) provide offset voltage trim connections on pins 1 and 8. Offset voltage can be adjusted by connecting a potentiometer as shown in Figure 36. This adjustment should be used only to null the offset of the operational amplifier. This adjustment should not be used to compensate for offsets created elsewhere in the system because this can introduce additional temperature drift.

Figure 36. OPA227 Offset Voltage Trim Circuit

7.3.4 Input Protection

Back-to-back diodes (see Figure 37) are used for input protection on the OPAx22x. Exceeding the turnon threshold of these diodes, as in a pulse condition, can cause current to flow through the input protection diodes due to the amplifier’s finite slew rate. Without external current limiting resistors, the input devices can be destroyed. Sources of high-input current can cause subtle damage to the amplifier. Although the unit may still be functional, important parameters such as input offset voltage, drift, and noise may shift.

Figure 37. Pulsed Operation

When using the OPA227 as a unity-gain buffer (follower), the input current should be limited to 20 mA. This can be accomplished by inserting a feedback resistor or a resistor in series with the source. Use Equation 1 to calculate sufficient resistor size.

For example, for a 10-V pulse (V_S = 10 V), total loop resistance must be 500 Ω. If the source impedance is large enough to sufficiently limit the current on its own, no additional resistors are needed. The size of any external resistors must be carefully chosen because they will increase noise. See the Noise Performance section of this data sheet for further information on noise calculation. Figure 37 shows an example implementing a current limiting feedback resistor.

7.3.5 Input Bias Current Cancellation

The input bias current of the OPAx22x series is internally compensated with an equal and opposite cancellation current. The resulting input bias current is the difference between with input bias current and the cancellation current. The residual input bias current can be positive or negative.

When the bias current is cancelled in this manner, the input bias current and input offset current are approximately equal. A resistor added to cancel the effect of the input bias current (as shown in Figure 38) may actually increase offset and noise and is therefore not recommended.

Figure 38. Input Bias Current Cancellation

7.3.6 Noise Performance

Figure 39 shows total circuit noise for varying source impedances with the operational amplifier in a unity-gain configuration (no feedback resistor network, therefore no additional noise contributions). Two different operational amplifiers are shown with total circuit noise calculated. The OPA227 has very low voltage noise, making it ideal for low source impedances (less than 20 kΩ). A similar precision operational amplifier, the OPA277, has somewhat higher voltage noise but lower current noise. It provides excellent noise performance at moderate source impedance (10 kΩ to 100 kΩ). Above 100 kΩ, a FET-input operational amplifier such as the OPA132 (very low current noise) may provide improved performance. Use the equation in Figure 39 for calculating the total circuit noise. e_n = voltage noise, i_n = current noise, R_S = source impedance, k = Boltzmann’s constant = 1.38 × 10^–23 J/K and T is temperature in K. For more details on calculating noise, see Basic Noise Calculations.

Figure 39. Noise Performance of the OPA227 in Unity-Gain Buffer Configuration

7.3.7 Basic Noise Calculations

Design of low noise operational amplifier circuits requires careful consideration of a variety of possible noise contributors: noise from the signal source, noise generated in the operational amplifier, and noise from the feedback network resistors. The total noise of the circuit is the root-sum-square combination of all noise components.

The resistive portion of the source impedance produces thermal noise proportional to the square root of the resistance. This function is shown plotted in Figure 39. Because the source impedance is usually fixed, select the operational amplifier and the feedback resistors to minimize their contribution to the total noise.

Figure 39 shows total noise for varying source impedances with the operational amplifier in a unity-gain configuration (no feedback resistor network and therefore no additional noise contributions). The operational amplifier itself contributes both a voltage noise component and a current noise component. The voltage noise is commonly modeled as a time-varying component of the offset voltage. The current noise is modeled as the time-varying component of the input bias current and reacts with the source resistance to create a voltage component of noise. Consequently, the lowest noise operational amplifier for a given application depends on the source impedance. For low source impedance, current noise is negligible and voltage noise generally dominates. For high source impedance, current noise may dominate.

Figure 40 shows both inverting and noninverting operational amplifier circuit configurations with gain. In circuit configurations with gain, the feedback network resistors also contribute noise. The current noise of the operational amplifier reacts with the feedback resistors to create additional noise components. The feedback resistor values can generally be chosen to make these noise sources negligible. The equations for total noise are shown in the following images for both configurations.

Figure 40. Noise Calculation in Gain Configurations

Figure 41. 0.1 Hz to 10 Hz Bandpass Filter Used to Test Wideband Noise of the
OPAx22x Series

Figure 42. Noise Test Circuit

Figure 41 shows the 0.1 Hz 10 Hz bandpass filter used to test the noise of the OPA227 and OPA228. The filter circuit was designed using Texas Instruments’ FilterPro software (available at www.ti.com). Figure 42 shows the configuration of the OPA227 and OPA228 for noise testing.

7.3.8 EMI Rejection Ratio (EMIRR)

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 section 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:

1. Operational amplifier input pins are known to be the most sensitive to EMI, and typically rectify RF signals better than the supply or output pins.
2. The noninverting and inverting operational amplifier inputs have symmetrical physical layouts and exhibit nearly matching EMIRR performance.
3. EMIRR is easier to measure on noninverting pins than on other pins because the noninverting input terminal can be isolated on a printed-circuit-board (PCB). This isolation allows the RF signal to be applied directly to the noninverting input terminal with no complex interactions from other components or connecting PCB traces.

Figure 43. OPA227 EMIRR IN+ vs Frequency

If available, any dual and quad operational amplifier device versions have nearly similar EMIRR IN+ performance. The OPAx227 unity-gain bandwidth is 8 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 OPA227 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.

7.3.8.1 EMIRR IN+ Test Configuration

Figure 44 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). 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.

Figure 44. EMIRR IN+ Test Configuration Schematic