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Data Acquisition - Analog to Digital Convertors (ADC)

The MAX1408CAI+ integrated circuit (IC) from Maxim Integrated is a high-performance, low-power, 8-bit Analog-to-Digital Convertor (ADC) designed for data acquisition applications. It is designed to be used in consumer and industrial applications, and is ideal for applications such as process control, medical imaging, consumer electronics, communication control, and instrumentation systems. The MAX1408CAI+ operates from a single 2.7 V to 5.5 V power supply, has an 8-bit resolution, provides up to 10 MHz bandwidth, and provides direct connect to microcontrollers and DSPs with a SPI or 8-bit parallel interface.

Working Principle

The MAX1408CAI+ ADC is based on the traditional successive-approximation register (SAR) architecture. A successive-approximation scheme is a highly efficient and widely used approach to analog-digital conversion where successive bits of a conversion are estimated based on the comparison of a reference voltage and the input voltage. The input voltage is compared in each step by the internal reference (VREF+) and the output is updated. This process is repeated until all bits are determined.

The MAX1408CAI+ has a sample-and-hold stage that allows for a wide range of input signals from DC to frequencies up to 10 MHz, significantly higher than comparable parts. The sample-and-hold stage follows the input signal and holds it for the ADC to process. This is achieved by utilizing two switches—one for the sample and one for the hold state. The S/H has little effect on DC signals but can lead to additional gain and linearity errors on high-frequency signals.

When the input signal is complete, the start-conversion command is sent via the serial or parallel interface. The state machine is clocked at the interface rate, synchronous with the conversion. The start-conversion command is acknowledged and the ADC state machine starts a calibration sequence followed by a conversion sequence.

The time needed to complete the calibration depends on the program configuration, the number of bits of resolution, the conversion rate, the impedance of the ADC inputs, the reference voltage, and other internal parameters. The calibration sequence first calibrates the S/H, followed by calibration of the amplifier, and a coarse-gain calibration. The calibration time can be reduced by disabling certain internal calibrations or increasing the interface rate.

Following the calibration sequence, the conversion sequence is initiated. During the conversion sequence, the input signal is sampled and the successive-approximation register determines the 8-bit result. The 8-bit result is clocked out on the parallel interface or loaded into the serial shift register. The conversion duration depends on the interface rate, the number of bits of resolution, and other configuration parameters.

Features

The MAX1408CAI+ has several features which make it ideally suited for data acquisition applications. It features low-power consumption, which is attractive in battery-powered and consumer applications. The MAX1408CAI+ can be clocked as high as 10 MHz and offers excellent noise immunity, linearity, and dynamic performance even at high frequencies. The on-board S/H has no effect on DC signals and its linearity errors are below 2 least-significant-bit (LSB). The input range of the ADC can be extended to rail-to-rail.

The MAX1408CAI+ offers internal reference or external reference options for improved accuracy. The on-chip reference is a switched reference (SPM) with an internal bandgap. The external reference must provide a buffered, low-noise, low-impedance signal. A temperature sensor is also included, which can be used to measure temperature and provide thermal protection.

The MAX1408CAI+ can be configured via the SPI port or by a few external pins. By programming the configuration pins, various features such as the reference selection, ADC output mode, serial data rate, and manual calibration can be selected.

Conclusion

The MAX1408CAI+ is an ideal ADC for data acquisition applications such as process control, medical imaging, consumer electronics, communication control, and instrumentation systems. With its excellent dynamic range, high-speed, and low-power consumption, the MAX1408CAI+ is a great choice for applications requiring high-precision and reliable data conversion.

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