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Analog Front End Family Provides Input Channel Options For Precision Measurement Applications
| The manufacturer says . . . | ChipCenter's Paul O'Shea says . . . |
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A new family of 24-bit Sigma-Delta analog-to-digital converters (ADCs) for high-resolution multiplexing applications is now available from Analog Devices. The AD7732 (2 fully differential input channels), the AD7734 (4 single-ended input channels) and the AD7738 (4 fully differential/8 single-ended input channels) high-throughput precision analog front ends are ideal for low-frequency signals in a variety of applications including programmable logic controllers, distributed control systems, process control, and medical and industrial instrumentation. The AD7738 offers up to an 8 kHz channel switching rate, the industry's fastest in a Sigma-Delta architecture. Key features common to these new converters are: 0.0015 percent nonlinearity, programmable conversion rates up to 12 kHz, under- and over-range detection capability, three wire serial interface (SPI(TM), QSPI(TM), MICROWIRE(TM) and DSP-compatible), and single-supply operation (+5 V analog supply and +3 or +5 V digital supply). The AD7732 and AD7734 provide unipolar and bipolar input ranges of +/- 10 V, +/- 5 V, 0 to 10 V and 0 to 5 V, while the AD7738 offers input ranges of +/- 2.5 V, +/- 1.25 V, +/- 0.625 V, 0 to 1.25 V and 0 to 0.625 V. Additionally, true 16-bit peak-to-peak resolution with 500 microsecond conversion time is provided by the AD7732 and AD7734.
Analog Devices, Inc., 804 Woburn Street, Wilmington, MA 01887. Tel: 800-ANALOGD (262-5643)
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When you think of Sigma-Delta analog-to-digital converters you probably think of precision but not speed. Analog Devices wants to change your mind. The introduction of this part pushes the envelope on the throughput while maintaining the resolution. The company focused on high resolution at high speeds and also on a converter with multichannel capabilities. You get channel switching speeds from 2kHz to 8kHz (for the AD7738 itýs 8kHz and the AD7732/34 offers 2kHz), and you still get 16-bit peak-to-peak resolution. According to the company, the AD7732 and AD7734 are slower than the AD7738 because they have on-chip resistors that allow the converters to accept an over-voltage input. The over-voltage is not trivial either. The AD7732 and AD7734 can accept +/- 10V as standard but can accept an over voltage up to 16.5V. The focus is to allow someone to connect directly and not worry about over-voltage or over-voltage protection. However, the lower input range AD7738 does not have the over-voltage protection. The company says that these chips have the fastest sigma-delta switching rate available. How was the switching rate accomplished so that it is the fastest sigma-delta converter? It was achieved with technology breakthroughs including on-chip third-order modulation clocked at 3 MHz speed with an optimized filter design for fast channel switching. Some competitors also have 24-bit sigma delta converters but they donýt have on-chip resistors (AD7732/34) that provide +/- 10V and allow for over-voltage protection. Other design features that help set these sigma-delta converters apart from other competitors are the on-chip components such as the multiplexer, buffer, oscillator, Schmitt trigger and the individual calibration registers. These integrated parts are helpful to the end user by reducing cost as well as shrinking the form factor. For example, having a multiplexer on-chip allows for multiple inputs, and the input buffer means you donýt have to add an external input buffer. Analog Devices also saw enough interest by customers for a higher temperature range so they extended it from the typical 85 degrees C to 105 degrees C. This moves the part into the industrial environment such as a control process that uses a 4-20 mA loop in a harsh industrial setting. Presently, the AD7738 has been sampling the most because of its multichannel capabilities. The on-chip components such as the buffer, helps reduce how many times the circuit needs to transfer data or download from a microprocessor when switching among channels. It reduces the amount of data that you need to transfer. The new chips also have a reference detect for reference voltage detection and an on-chip clock generator, which means you only need a ceramic resonator of 4.9152 MHz or a crystal. These converters have signal scaling too, which typically need additional tight-precision external resistors that must be matched when they are off-chip. Since this design has onboard resistors that are made with the same process, they are already matched to the other resistors for the precision you need. The AD7732/34/38 are priced at $7.77/$7.77/$6.89 per unit, respectively, in 1,000-piece quantities. Available in 28-pin TSSOP packages, the three ADCs are specified for operation over the -40 degrees C to +105 degrees C industrial temperature range.
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