Using Power to Improve Signal-Path Performance
Bob Nguyen, Applications Engineer

System-level hardware designers pay careful attention to picking the right analog signal path ICs for their specific applications. Whether designing consumer MP3 players or medical ultrasound equipment, engineers care about making the right trade-offs while maintaining high performance. After reviewing and prioritizing specifications such as Signal-to-Noise Ratio (SNR), resolution, Total Harmonic Distortion (THD), input offset voltage, slew rate, jitter, and supply current, the analog signal path ICs can be selected. Picking the right signal path ICs leads to the best sight and sound experience for the user. Each of these signal path ICs need "clean" power. Often, power management is the last part of the system design. Figure 1 shows an example system of how the signal path is powered.

Not surprisingly, the power supply affects the analog signal integrity which ultimately affects overall system performance. An easy way to improve signal path performance is by selecting the right power supply. When selecting a power supply a key parameter affecting the analog signal path performance is noise or ripple on the power supply line. Noise or ripple on the power supply can couple into the output of an op amp, increase jitter on a Phase-Locked Loop (PLL) or Voltage-Controlled Oscillator (VCO), or degrade the SNR in an ADC. Low noise and ripple on the power supply will also improve signal path performance.

Figure 1. Power Supply Design Affects the Signal Path IC Performance

Noise and ripple on the power supply line can come from a number of sources. High-speed data and high-frequency signals within the system itself create noise because Printed Circuit Board (PCB) traces and wiring elements can act like an antenna if not carefully attended to. Digital ICs such as microcontrollers, Field Programmable Gate Arrays (FPGAs) and Complex Programmable Logic Devices (CPLDs) have fast edge rates that draw varying amounts of current and radiate Electromagnetic Interference (EMI) into the system. Silicon ICs generate thermal noise internally which is caused by the random motion and collision of molecules at temperatures above absolute zero Kelvin.

There are three common ways to minimize noise and ripple in the signal path: careful attention to system PCB layout, proper supply bypassing, and choice of the right power supply. Though system-dependent, PCB layout considerations include proper component placement, minimization of signal path trace length, and having a solid ground.

Bypassing the supply rail is a common practice, often recommended in analog ICs datasheets to filter out noise. Signal path ICs can have separate analog, digital, and PLL power supply inputs, each with its own suggested bypassing. The PLL supply and analog supplies are the most sensitive to noise and ripple. Bypass capacitors, resistor-capacitor (RC) filters and EMI suppression filters minimize noise and ripple into the signal path power supply.

The right power supply can reduce noise and ripple to the signal path IC. When choosing a power supply the designer first makes a basic choice between a switching converter and a linear regulator. A switching converter provides higher efficiency which translates to less power consumption in the overall system. A linear regulator provides an easy-to-use solution while lowering the noise /ripple on the supply rail. Use a linear regulator to improve signal path performance by reducing noise and ripple.

Analog signal path ICs specify how well the part rejects noise and ripple on the power supply line. The parameter is called Power Supply Rejection Ratio (PSRR); see Figure 2. It is the ratio of the noise/ripple coming into the device to the noise/ripple going out of the device. PSRR is measured in decibels (dB) and the equation is:

where V1 is the change in input voltage and V2 is the change in output voltage. Using this equation yields a negative value. Don’t be concerned if you see suppliers specifying PSRR as a positive value; this means they used the ratio V2/V1 instead of V1/V2. Take the absolute value of PSRR to avoid confusion. Designers want the highest absolute value PSRR which correlates to low noise/ripple on the output. As an example 80 dB (output ripple is 10,000x smaller than the input ripple) is better than 20 dB (output ripple is 10x smaller than the input ripple).

Figure 3. LMP7711 Single, High Precision Op Amp PSRR Over Frequency		Figure 4. LP5900 100 mA Low Dropout Linear Regulator PSRR Over Frequency

Figure 3 shows a PSRR graph for the LMP7711 high precision op amp. The LMP7711 is a high precision op amp with a PSRR of 90 dB at 1 kHz. Two things to note are PSRR exists for both the positive and negative rail and also PSRR degrades (absolute value decreases) as the frequency of the ripple increases.

Using a linear regulator to power the signal path reduces noise/ripple because a linear regulator also rejects noise/ripple from its input to its output. The LP5900 is a low noise 100 mA LDO with a PSRR of 85 dB at 1 kHz (see Figure 4). Using the LP5900 (low noise LDO) to power the LMP2011 (high precision op amp) reduces the noise/ripple on the power supply (Figure 5). 1 kHz ripple/noise is rejected by the LDO at 85 dB and then also rejected by the precision op amp at 90 dB.

Figure 5. Using a Linear Regulator to Power an Amplifier Reduces
Noise/Ripple in the Analog Signal Path

A simple way to improve the signal path performance in an application is to use a linear regulator to power the signal path ICs. Linear regulators provide an easy way to generate the needed power supply rails while also rejecting noise/ripple on the power supply line.

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