This article explores the impact of DC bias supplies on operational amplifiers (op amps) in sensitive analog applications, focusing on power sequencing and the influence of DC power on input offset voltage. It also introduces a practical method for implementing a trace-separated power supply using a linear regulator—typically without tracking capability—to mitigate some of the negative effects associated with DC bias power sources.
In many op amp circuits, the DC bias supply can significantly affect performance, especially when used with high-resolution analog-to-digital converters (ADCs) or in signal conditioning for sensitive sensor systems. The DC bias voltage directly influences the op amp’s input common-mode voltage and other critical specifications.
During power-up, it is crucial to coordinate the sequence of DC bias supplies to avoid latching, which can damage or prevent the op amp from functioning properly. This article emphasizes the importance of tracking power supplies for op amps and presents an easy way to implement a trace-separated power supply using a linear regulator that typically lacks tracking features.
There are two common methods for powering an op amp: a single positive supply or a dual (split) supply. A single-supply configuration is straightforward, as shown in Figure 1(a), while a split supply, depicted in Figure 1(b), provides both positive and negative voltages. Split supplies are particularly useful in analog circuits where signals may swing between positive and negative values.
Regardless of the power configuration, the input common-mode voltage is determined by the supply rails. For a single supply, the negative rail is grounded, so the common-mode voltage is simply half of the positive supply voltage. For a split supply, the common-mode voltage is zero.
Some op amps can operate in either single or split supply modes, and others can even handle asymmetric configurations. However, designers must ensure the chosen power configuration is supported by the op amp.
When using a symmetrical split supply, the positive and negative voltages must be tracked during power-up. A tracking power supply ensures that the output voltages remain equal in magnitude but opposite in polarity. If this is not maintained, the op amp may latch, leading to potential damage or malfunction.
Figure 2 illustrates a typical op amp power supply circuit, where a switching power supply provides ±18V, and low dropout (LDO) regulators adjust these to ±15V. LDOs help reduce noise and improve power supply rejection ratio (PSRR), which is essential for maintaining signal integrity.
However, the circuit in Figure 2 does not have tracking capability. During power-up, there is no guarantee that the LDO outputs will match in magnitude and polarity. Even after stabilization, small variations due to component tolerances can still occur.
These variations can affect the input common-mode voltage, introducing a compensation voltage at the op amp input. While the PSRR of the op amp reduces this effect, it is not complete, especially at higher frequencies. This can lead to measurable errors in ADC readings, as demonstrated in the example of a fully differential op amp connected to a 24-bit ADC.
To address these issues, an improved circuit design is introduced in Figure 4, incorporating an additional amplifier and resistors to track the LDO outputs. This ensures balanced power supply voltages and minimizes offset errors. Protection diodes are added to safeguard the LDO feedback pins during transient conditions.
By implementing such a tracking circuit, the accuracy and stability of the power supply for the op amp are significantly improved. This approach helps reduce the risk of latching, improves system reliability, and enhances overall performance in sensitive analog applications.
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