This article explores the impact of DC bias supplies on operational amplifiers (op amps) in sensitive analog applications, focusing on power sequencing and how DC power affects input offset voltage. It also introduces a practical method for implementing a trace-separated power supply using a linear regulator—typically without tracking capabilities—to reduce the negative effects of DC bias.
In many op amp circuits, especially those involving high-resolution ADCs or signal conditioning for sensitive sensors, the DC bias supply plays a crucial role. It directly influences the input common-mode voltage and other critical specifications of the amplifier. During power-up, the sequence of DC supplies must be carefully managed to avoid latching, which can damage or disable the op amp. The article emphasizes the importance of tracking power supplies and presents a simple solution using a linear regulator to achieve better performance.
There are two main ways to power an op amp: either with a single positive supply or with a dual (split) supply that includes both positive and negative voltages. A split supply is particularly useful when dealing with signals that swing around zero. Regardless of the configuration, the input common-mode voltage is determined by the supply rails. For a single-supply system, the negative rail is typically grounded, while a split supply allows the common-mode voltage to be centered at zero.
Some op amps are designed to work with both single and split supplies, while others may exhibit different behaviors depending on the power configuration. When using a split supply, it's essential that the positive and negative rails are well-matched in magnitude and opposite in polarity. This ensures stable operation during power-up and prevents latching issues.
Figure 2 illustrates a typical power supply setup where a switching power supply provides ±18V, and low dropout (LDO) regulators step these down to ±15V. LDOs help reduce noise from the switching power supply and improve overall signal integrity. However, without tracking, the LDO outputs may not be perfectly matched, leading to potential performance issues.
The article explains that even small variations in supply voltage can affect the op amp’s input offset voltage through its power supply rejection ratio (PSRR). For example, a 3% change in LDO output could introduce a measurable offset at the ADC input, causing errors in the digital reading. To address this, the article proposes a tracking circuit that uses an additional op amp and resistors to force the LDOs to remain balanced.
By adding components like U1, R1-R4, and a clamping diode, the circuit ensures that the LDOs track each other accurately, improving stability and reducing DC errors. This approach enhances the accuracy and reliability of the power supply, making it more suitable for high-precision applications.
In conclusion, understanding the effects of DC bias on op amps is essential for designing robust analog systems. By applying the methods discussed, engineers can minimize unwanted offsets, improve power supply tracking, and achieve better overall performance in their designs.
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