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Examining integrated multiplexed input ADC

15 Oct 2015  | Maithil Pachchigar

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Multi-channel precision data acquisition systems utilised 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 among 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.

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. The proposed low power solution using an integrated multiplexed input 8-channel, 16bit, 250kSPS 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 utilised 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.

Optical transceivers
The market for 100Gbps optical transceivers is uniquely positioned to grow in the next decade for high-speed coherent optical transmission. The key challenge for optical transceivers is to acquire and process wider bandwidth signals or multiplex number of input channels at lower power in a smaller footprint. The size, power, and cost structure of today's transceivers originally designed for long-haul applications limit their utilisation in more cost-sensitive metro networks. The metro networks include: metro regional 500km – 1000km, metro core 100km – 500km and metro access <100km applications. Due to fierce competition in metro networks, the space comes at a high premium, making line-card density extremely crucial and consequently, a path to a lower-cost optical line cards or pluggable modules in a smaller footprint has become increasingly important for coherent applications.

In optical networks, as bit rates per channel increases from 10Gbps towards 100Gbps and higher, the optical fibre non-idealities severely degrade signal quality and affect its transmission performance. Technical challenges arise also in long-haul optical networks when the penalties occur in terms of optical noise, non-linear effects, and dispersion due to optical fibre impairment.

To address these significant challenges, various manufacturers of 40G and 100Gbps optical transceivers use coherent technology that allow higher data rate connectivity with maximum reach at longer distances for metro long haul, long-haul and ultra-long-haul networks. The coherent technology generally combines multi-level signal formats and coherent detection using dual polarisation, quadrature, phase-shift keying (DP-QPSK) for optimised signal modulation, allowing immunity to fibre impairments at higher data rates and making 100Gbps transmission economically and technically feasible.

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