One-chip data acquisition gives elbow room to analog signals

You can talk about digital fieldbus and wireless Ethernet. But there's nothing like good old analog signal over a 4-20 mA loop for transmitting data between process and control equipment—though engineers are using some unusual tools to help them create ever smaller and more powerful analog peripheral boards and boxes.


You can talk about digital fieldbus and wireless Ethernet. But there's nothing like good old analog signal over a 4-20 mA loop for transmitting data between process and control equipment-though engineers are using some unusual tools to help them create ever smaller and more powerful analog peripheral boards and boxes.

These peripherals are needed in process control applications to convert analog signals, so they can be read by digital PLCs, process control systems, or industrial PCs. However, snowballing needs by users and applications for more channels per board and smaller packages are leading to shortages in board space and power, which is fueling the search for more efficient peripherals. To aid this effort, Maxim Integrated Products (Munich, Germany) develops circuits and signal conditioning techniques that it reports can digitize up to eight channels on one chip.

DAS model to solution

Data acquisition systems (DAS) usually include: a multiplexer for switching between input channels; a signal conditioning circuit for gain and offset adjustment for different input ranges; and an analog-to-digital converter (ADC) with voltage reference (see diagram). Maxim used this model's basic pieces to produce a series of one-chip systems, Max196-199, which require few external components and can convert most standard signals. Maxim's most recent addition to its one-chip family are Max127 and Max128, which are the lower-speed (8 ksps), two-wire, serial versions of Max196-199. They consume even less board space.

Each one-chip system includes a 12-bit ADC, multiplexer, gain/offset correction, and a serial or parallel digital interface connection to most microprocessors, according to Thomas Kick, Maxim's central European applications director. The ADC is a successive-approximation type based on a capacitive DAC with an MSB capacitor that doubles as the hold capacitor in a track/hold circuit. Each device can operate with an external clock or the internal oscillator, says Mr. Kick.

Layout for Max197 shows differences between chips are mainly in the digital section, which connects to the microprocessor (see diagram). Each has six or eight single-ended analog input channels that connect to the internal ADC through a fault-protected multiplexer. The channels can withstand up to 16.5 V input. Mr. Kick says faults on any channel don't affect conversions to any other channel.

Each channel can be programmed independently for one of the standard input ranges (0-5 V, 0-10 V,

'Using WR pulses to start and stop an acquisition, Max196-199 devices in 'external acquisition mode' can offer a relatively long acquisition time without slowing the conversion. Their short aperture delay and low aperture jitter (&50ps in external clock/acquisition mode) enable precise control of acquisition time,' says Mr. Kick.

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