TLVx313 Low-Power, Rail-to-Rail In/Out, 500-µV Typical Offset, 1-MHz Operational Amplifier for Cost-Sensitive Systems

Table 1. Table of Graphs

8.3.1 Operating Voltage

The TLV313 series is fully specified and tested from 1.8 V to 5.5 V (±0.9 V to ±2.75 V). Parameters that vary with supply voltage are illustrated in the Typical Characteristics section.

8.3.2 Rail-to-Rail Input

The input common-mode voltage range of the TLV313 series extends 200 mV beyond the supply rails. This performance is achieved with a complementary input stage: an N-channel input differential pair in parallel with a P-channel differential pair, as shown in the Functional Block Diagram section. The N-channel pair is active for input voltages close to the positive rail, typically (V+) – 1.3 V to 200 mV above the positive supply, while the P-channel pair is on for inputs from 200 mV below the negative supply to approximately (V+) – 1.3 V. There is a small transition region, typically (V+) – 1.4 V to (V+) – 1.2 V, in which both pairs are on. This 200-mV transition region may vary up to 300 mV with process variation. Thus, the transition region (both stages on) may range from (V+) – 1.7 V to (V+) – 1.5 V on the low end, up to (V+) – 1.1 V to (V+) – 0.9 V on the high end. Within this transition region, PSRR, CMRR, offset voltage, offset drift, and THD may be degraded compared to device operation outside this region.

8.3.3 Rail-to-Rail Output

Designed as a micro-power, low-noise operational amplifier, the TLV313 device delivers a robust output drive capability. A class AB output stage with common-source transistors is used to achieve full rail-to-rail output swing capability. For resistive loads up to 100 kΩ, the output swings typically to within 5 mV of either supply rail regardless of the power-supply voltage applied. Different load conditions change the ability of the amplifier to swing close to the rails, as shown in Figure 12.

8.3.4 Common-Mode Rejection Ratio (CMRR)

CMRR for the TLV313 device is specified in several ways so the best match for a given application may be used; see the Electrical Characteristics. First, the CMRR of the device in the common-mode range below the transition region [V_CM < (V+) – 1.3 V] is given. This specification is the best indicator of the capability of the device when the application requires use of one of the differential input pairs. Second, the CMRR over the entire common-mode range is specified at (V_CM = –0.2 V to 5.7 V). This last value includes the variations seen through the transition region, as shown in Figure 4.

8.3.5 Capacitive Load and Stability

The TLV313 device is designed to be used in applications where driving a capacitive load is required. As with all op amps, there may be specific instances where the TLV313 device may become unstable. The particular op amp circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether or not an amplifier is stable in operation. An op amp in the unity-gain (+1-V/V) buffer configuration that drives a capacitive load exhibits a greater tendency to be unstable than an amplifier operated at a higher noise gain. The capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback loop that degrades the phase margin. The degradation of the phase margin increases as the capacitive loading increases. When operating in the unity-gain configuration, the TLV313 device remains stable with a pure capacitive load up to approximately 1 nF. The equivalent series resistance (ESR) of some capacitors (C_L greater than 1 μF) is sufficient to alter the phase characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when observing the overshoot response of the amplifier at higher voltage gains.

One technique for increasing the capacitive load drive capability of the amplifier when it operates in a unity-gain configuration is to insert a small resistor, typically 10 Ω to 20 Ω, in series with the output, as shown in Figure 19. This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. One possible problem with this technique is that a voltage divider is created with the added series resistor and any resistor connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that reduces the output swing.

Figure 19. Improving Capacitive Load Drive

8.3.6 EMI Susceptibility and Input Filtering

Operational amplifiers vary with regard to the susceptibility of the device to electromagnetic interference (EMI). If conducted EMI enters the op amp, the dc offset observed at the amplifier output may shift from the nominal value while EMI is present. This shift is a result of signal rectification associated with the internal semiconductor junctions. While all op amp pin functions may be affected by EMI, the signal input pins are likely to be the most susceptible. The TLV313 family incorporates an internal input low-pass filter that reduces the amplifiers response to EMI. Both common-mode and differential mode filtering are provided by this filter. The filter is designed for a cutoff frequency of approximately 35 MHz (–3 dB), with a roll-off of 20 dB per decade.

Texas Instruments has developed the ability to accurately measure and quantify the immunity of an operational amplifier over a broad frequency spectrum extending from 10 MHz to 6 GHz. The EMI rejection ratio (EMIRR) metric allows op amps to be directly compared by the EMI immunity. Figure 18 illustrates the results of this testing on the TLV313 family. Detailed information may be found in EMI Rejection Ratio of Operational Amplifiers (SBOA128), available for download from www.ti.com.

9.2.1 Design Requirements

The supply voltage must be chosen to be larger than the input voltage range and the desired output range. The limits of the input common-mode range (V_CM) and the output voltage swing to the rails (V_O) must also be considered. For instance, this application scales a signal of ±0.5 V (1 V) to ±1.8 V (3.6 V). Setting the supply at ±2.5 V is sufficient to accommodate this application.

9.2.2 Detailed Design Procedure

Determine the gain required by the inverting amplifier using Equation 1 and Equation 2:

Equation 1.

Equation 2.

When the desired gain is determined, choose a value for R_I or R_F. Choosing a value in the kilohm range is desirable for general-purpose applications because the amplifier circuit uses currents in the milliamp range. This milliamp current range ensures the device does not draw too much current. The trade-off is that very large resistors (100s of kilohms) draw the smallest current but generate the highest noise. Small resistors (100s of ohms) generate low noise but draw high current. This example uses 10 kΩ for R_I, meaning 36 kΩ is used for R_F. The values are determined by Equation 3:

Equation 3.

9.2.3 Application Curve

Figure 21. Inverting Amplifier Input and Output

12.1.1 Related Documentation

For related documentation, see the following:

• EMI Rejection Ratio of Operational Amplifiers.
• Circuit Board Layout Techniques.

Table 2. Related Links