Tracking power supply improves signal chain performance

This article explains the effects of DC bias supplies on operational amplifiers (op amps) used in sensitive analog applications. It also covers power supply sequencing and the effects of DC supplies on input offset voltage. It also shows a method to easily implement tracking split supplies using linear regulators (which typically do not have tracking capabilities) to help minimize some of the adverse effects of DC bias supplies.

In many op amp circuits, the DC bias supply affects the performance of the op amp, especially when used with high-bit count analog-to-digital converters (ADCs) or for signal conditioning in sensitive sensor circuits. The DC bias supply voltage determines the input common-mode voltage of the amplifier, as well as many other specifications.

During power-up, the sequencing of the DC bias supplies must be coordinated to prevent the op amp from latching up. This can destroy, damage, or prevent the op amp from operating properly. This article explains the importance of tracking supplies for op amps and describes a method to easily implement a tracking split supply using linear regulators that do not typically have tracking capabilities.

There are two common ways to power an op amp. The first and simplest method is to use a single positive supply, as shown in Figure 1 (a). The second method is to use a split (dual) supply, as shown in Figure 1 (b), which has both a positive and a negative voltage. This split supply is very useful in many analog circuits because it allows input signals to include zero voltage potential or input signals that swing between positive and negative.

Figure 1. Op amp power supply options

Regardless of which method is used, the input common-mode voltage is determined by the supply voltage. The input common-mode voltage is simply the arithmetic mean of the two voltages. Equation 1 can be used to calculate the input common-mode voltage, where VP is the value of the positive voltage rail and VN is the value of the negative voltage rail.

For a single-supply system, VN is always zero because the negative rail of the op amp is connected to ground potential.

Using the values ​​shown in Figure 1, the single-supply op amp has an input common-mode voltage of 7.5V, while the split-supply op amp has an input common-mode voltage of 0V.

Some op amps can operate in either a single-supply configuration or a split-supply configuration. Some op amps can even operate with asymmetrical split supplies (VP ​​is not the same size as VN). In all cases, the designer needs to verify that the op amp can support the desired power configuration.

Additionally, many op amps feature split supplies, so if an op amp is designed for split-supply operation in a single-supply configuration, there may be some performance differences.

When using symmetrical split supplies, the positive and negative voltages must track each other, especially when the circuit is first powered up. A tracking supply is a power supply that regulates its output voltage to another voltage or signal. For most op amps, the positive supply voltage should always be equal and opposite to the negative supply voltage.

Alternatively, you can adjust the negative supply to be equal in magnitude and opposite in polarity to the positive supply. Either method will produce the same power-up waveform.

The op amp can latch up during power-up if the two supplies are not equal in magnitude and opposite in polarity. Latch-up can destroy, damage, or prevent the op amp from operating properly.

Figure 2 shows a schematic diagram of a typical op amp power supply circuit . Here, a switching power supply provides a positive 18V and a negative 18V. Two low dropout (LDO) linear regulators further regulate the ±18V to ±15V. The LDOs are typically installed between the power supply and the op amp to reduce the high-frequency switching noise generated by the switching power supply. The LDOs have high power supply rejection (expressed as a ratio, PSRR), which attenuates the noise at the LDO input over a wide frequency bandwidth.

Figure 2 Typical power supply structure of an operational amplifier

This helps provide a low noise power supply to the op amp. The op amp also has its own PSRR, which is typically above 80dB. However, the op amp only has a high PSRR over a few kilohertz bandwidth, so the LDO is used to provide a high PSRR up to hundreds of kilohertz bandwidth.

The circuit shown in Figure 2 has no inherent tracking capability. During power-up, there is no guarantee that each LDO will be equal in magnitude and opposite in polarity to the other LDO. During power-up, the output voltage of each LDO is determined by all the soft-start circuitry, current limiting, load capacitance, load current, and input voltage.

Therefore, it is possible for the two voltages to be different in magnitude and polarity at startup. Also, after the LDO is powered up and providing a steady-state DC output, they may still be unequal in magnitude because each LDO has its own output voltage accuracy and the feedback resistors may vary slightly due to their tolerances.

In addition to latch-up issues during power-up, each power supply can have an impact on system performance if its final operating DC voltage varies over time. Power supply output will vary with line voltage, load current variations, and temperature variations. Power supply output will vary within its accuracy specification, which is typically 3% to 5% of the rated output voltage.

Although these changes in supply voltages are small, they change the op amp's input common-mode voltage point, which is usually modeled as an additional compensation voltage at the op amp input. Because op amps have high PSRR, the modeled compensation voltage is equal to the input common-mode voltage change divided by the op amp's PSRR. Equation 2 can be used to calculate the compensation voltage at the op amp input due to supply changes.

The PSRR is expressed in decibels as shown in Equation 2, which can be found in most op amp data sheets. Equation 2 gives the compensation voltage referenced to the op amp input. By multiplying the result of Equation 2 by the op amp gain, the op amp output is referenced to the compensation voltage.

Because the op amp's PSRR further reduces small changes in the power supplies, you might incorrectly conclude that small changes in the power supply voltage have little or no effect in the system. As a quantitative example, we can analyze a fully differential op amp that buffers a signal to a 24-bit ADC.