Circuit Design of High-Performance Multiplexed Data Acquisition System

　　High-channel-density data acquisition systems used in medical imaging, industrial process control, automatic test equipment, and 40G/100G optical communication systems can multiplex the signals from numerous sensors into a small number of ADCs, which then convert each channel sequentially. Multiplexing allows fewer ADCs to be used per system, significantly reducing power, size, and cost. Successive-approximation ADCs—often called SAR ADCs for their successive-approximation registers—have low latency characteristics, making them suitable for multiplexed systems that require a fast response to a full-scale input step (worst case) without any settling time issues. Easy-to-use SAR ADCs offer low power and small size. This article focuses on the key design considerations, performance results, and application challenges associated with multiplexed data acquisition systems using high-performance precision SAR ADCs.

　　Multiplexed Data Acquisition System

　　Multiplexed data acquisition systems require wideband amplifiers that can settle quickly when driving the full-scale (FS) input range of the ADC. In addition, the switching and sequential sampling of the multiplexed channels must be synchronized with the ADC conversion cycle. The large voltage differences between adjacent inputs make these systems susceptible to channel-to-channel crosstalk. To avoid errors, the complete signal chain (including the multiplexer and amplifier) ​​must settle to the required accuracy - generally expressed as crosstalk error or settling error. The figure shows a block diagram of a data acquisition system that includes a multiplexer, ADC driver, and SAR ADC.

　　Multiplexed Data Acquisition Signal Chain Circuit

　　Figure 4 shows a simplified signal chain for a multiplexed data acquisition system. The ADG774 CMOS multiplexer is used to select one of the two differential channels. To evaluate this system, the positive and negative differential inputs of the ADG774 are continuously switched to generate a full-scale step. Two ultralow distortion ADA4899-1 op amps buffer the multiplexer outputs and drive the AD7960 18-bit, 5 MSPS PulSAR® ADC. The RC filter (33 Ω/56 pF) helps reduce the kickback from the AD7960 capacitive DAC inputs and limits the noise going into the AD7960 inputs.

　　High-performance, high-channel density, multiplexed data acquisition systems require reliable performance, flexible functionality, and high accuracy while meeting power, space, and thermal requirements. This article provides guidance on selecting multiplexed signal chain components based on key design considerations to achieve the expected performance, and how to trade off throughput, settling time, and noise. This signal chain achieves optimal performance with less than 0.01% crosstalk error at 5 MSPS over full-scale range.