Integrated multiplexed input ADC saves PCB area, power, and cost for high channel count systems

October 20, 2015 // By Maithil Pachchigar, Analog Devices
This article highlights the design considerations for multiplexed data acquisition systems and focuses on an integrated multiplexed input ADC solution to address these technical challenges for space-constrained applications such as optical transceivers, wearable medical devices and other portable instruments.

Multi-channel precision data acquisition systems used in industrial, instrumentation, optical communication and healthcare applications are driving the demand for high channel count, low power and compact form factors to address the increased printed circuit board (PCB) density and thermal power consumption challenges. System designers make trade-offs in performance, thermal stability, and PCB density to maintain optimum balance and they are continually pressed to find innovative ways to tackle these challenges while minimising overall bill of material (BOM) cost.

The proposed low power solution using an integrated multiplexed input 8-channel, 16-bit, 250 ksample/sec PulSAR ADC AD7689 available in a miniature, wafer level chip scale package (WLCSP) footprint saves over 60% board space for these applications while offering precision performance and flexible configuration.

Multichannel data acquisition systems typically employ either discrete or integrated multiplexed and simultaneously sampled analogue signal chains for interfacing with various sensor types such as temperature, pressure, optical, vibration, and many more based on the application requirements. Some applications require simultaneous sampling to obtain increased sampling rate per channel and to preserve the phase information among different channels. The key benefit of multiplexing is fewer number of ADCs per channel required, resulting in reduced space, power and cost. However, the achievable throughput rate in a multiplexed system is the single ADC throughput rate divided by the number of channels being sampled.

The Successive Approximation Register (SAR) analogue-to-digital Converters (ADCs) offer inherent merits of low latency and dynamic power scaling with throughput. They are often used in channel multiplexed architectures ideally suited for sensing and monitoring functions. Multiplexed data acquisition systems used in optical transceiver modules need high channel density and wearable medical devices require small form factors and low power, where the signals from multiple sensors need to be monitored and multiplex many input channels in to a single or several ADCs.

One of the main challenges with multiplexed data acquisition systems is that when the input is switched to next the channel, it requires fast response to step input near full scale amplitude without any settling time or crosstalk issue. The following section presents a real world use case of SAR architecture based multiplexed input ADC for optical transceivers and indicates why the ADC is ideally suited for this type of application.