Data acquisition

Characterising the PSRR of data acquisition μModule devices with internal bypass capacitors

When optimising data acquisition (DAQ) systems, designers must carefully consider the effects of power supplies on high precision performance. Often, the power supply circuitry includes a combination of low dropout linear regulators and dc-to-dc switch-mode converters. One drawback of switch-mode converters is that they produce an output ripple. While the ripple magnitudes are relatively low, they can couple into key components in the analog signal path that could corrupt measurements and compromise performance. Power components must typically be low noise with adequate supply decoupling at multiple locations on the PCB to prevent performance degradation of the signal chain. Power supply rejection ratio (PSRR) is a metric that quantifies a system’s ability to reject supply noise and perturbations. As DAQ solutions evolve to be more complete signal chain solutions via system-in- package (SiP) technology, the supply decoupling can be co- packaged with the precision signal chain to improve the overall system PSRR. Here, Analog Devices’ Naveed Naeem, product test development engineer and Samantha fontaine, product engineer, investigate

calculate PSRR (AV2

voltage to the output voltage expressed in dB. The following equation defines how to is the voltage gain).

P PSRR is a key parameter that quantifies a

circuit’s sensitivity to supply noise and perturbations and how it affects the circuit’s output. It is typically measured over a wide frequency range from dc up to several MHz, with PSRR tending to degrade at higher frequencies. System designers often add decoupling

capacitors to the power supply nodes of their circuit to reduce noise and glitches that could couple into sensitive components. For amplifiers, 0.1 µF ceramic capacitors are placed as close to the power supply pins as possible to reduce high frequency coupling. In addition, to provide low frequency decoupling, large 10 µF tantalum capacitors are connected in parallel, and typically placed closer to the supply source.

PSRR MOtivAtiON The desire for power efficiency is one reason why some system designers cannot tolerate high power, low noise power conversion components. Battery-powered DAQ systems are an example of an application that requires high performance at low power—a significant motivator for designing DAQs with less sensitivity to power supply noise. Modern devices often include several systems

powered from the same battery. If the current consumption for one system or device increases under certain conditions, the battery voltage, and therefore any supply voltage to other devices powered from said battery, can vary. For these reasons, dc PSRR is important when designing the battery management circuitry of a

ower supply rejection ratio, also referred to as power supply ripple rejection, is essentially the ratio of change in supply

system. Depending on how sensitive the system is, a designer can use LDO regulators to help combat voltage drops. AC PSRR is also an important specification in battery-powered systems if a ripple-inducing buck, boost, or inverting regulator is required. For industrial applications, system noise is a key

specification. For example, electromagnetic interference (EMI) from nearby sources can couple to the supplies, causing noise spurs and other errors. To help minimise these noise spurs, it is important to use decoupling capacitors and proper PCB design techniques such as grounding, shielding, and proper component placement. Figure 1 shows a typical precision data

acquisition system signal chain. Each component is affected by power supply noise to a different degree. Adding proper decoupling capacitance improves PSRR performance at higher frequencies for each of the components in the signal chain shown in Figure 1. Analog Devices signal chain µModule data

acquisition solutions help solve some of the power design pain points, such as optimised layout of traces, the addition of decoupling capacitors, and in some cases, power management components such as LDO regulators. The ADAQ4003 is a µModule data acquisition solution that includes decoupling capacitors on all supplies to reduce their sensitivity to perturbations. The ADAQ7980/ ADAQ7988 µModule data acquisition systems include decoupling capacitors and an LDO regulator. The integrated LDO regulator further simplifies design—system designers only need to provide one clean supply to power the µModule device and they are free to bypass the LDO regulator if required.

CuRReNt MetHOD fOR teStiNg PSRR ON DiSCRete COMPONeNtS PSRR testing of discrete components is a common feature in a characterisation plan, as it

Figure 1. Typical precision data acquisition signal chain. 38 November 2020 Instrumentation Monthly

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