AFE Goes Biochemical

Analog Devices claims its AD5940 electrochemical and impedance measurement front end enables the next generation of vital sign monitoring devices and intelligent electrochemical sensors. The AD5940 analog front end (AFE) incorporates both potentiostat and electrochemical impedance spectroscopy (EIS) functionality on a single chip, allowing for sensor measurement in both time and frequency domains.  

The device features integrated hardware accelerators for advanced sensor diagnostics, low noise operation, and is designed for wearable always-on applications. It’s a viable choice for applications such as industrial gas sensing, liquid analysis, material sensing, vital signs monitoring, impedance spectroscopy and disease management. The on-chip potentiostat allows for a host of standard electrochemical-based measurement techniques, such as amperometric, voltametric, or impedance measurements.  

The AD5940 is designed to be used in healthcare-related bio-impedance systems for both skin impedance and body impedance measurements, and to work with the AD8233 AFE in a complete bioelectric/biopotential measurement system. The analog front-end chip can measure voltage, current and impedance. The device consists of two potentiostat loops, a low bandwidth loop with ability to generate AC signals up to 200 Hz and a high bandwidth loop with ability to generate AC signals up to 200kHz. The ultra-low power Potentiostat consumes 6.5 µA in biased mode. 

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The AD5940 measurement channel features a 16-bit, 800 ksamples/s, multichannel successive approximation register (SAR) analog-to-digital converter (ADC) with input buffers, an anti-alias filter, and a programmable gain amplifier. The ADC features an input voltage range of ±1.35 V. An input mux before the ADC allows the user to select an input channel for measurement. These input channels include multiple external current and voltage inputs, and internal voltage channels. The internal channels enable on chip diagnostic measurements of the internal supply voltages, die temperature, and reference voltages.  

The AD5940 measurement blocks can be controlled via direct register writes through the Serial Peripheral Interface (SPI) interface, or alternatively, by using a pre-programmable sequencer, which provides autonomous control of the AFE chip. 6 kB of static random-access memory (SRAM) is partitioned for a deep data first in, first out (FIFO) and command memory. Measurement commands are stored in the command memory and measurement results are stored in the data FIFO. Several FIFO related interrupts are available to indicate state of the FIFO.  

Price for the AD5940 in a 3.6 mm × 4.2 mm, 56 ball WLCSP is $4.17 each/1,000. For more details, peruse the AD5940 datasheet.

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