AD7732_AD7734_AD7738_AD7739 in low Power Applications \AN-664, REV. 0\ Analog Devices, Inc.

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Specifications	AD7732_AD7734_AD7738_AD7739 in low Power Applications \AN-664, REV. 0\ Analog Devices, Inc.
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AN-664 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700

Specifications	AD7732_AD7734_AD7738_AD7739 in low Power Applications \AN-664, REV. 0\ Analog Devices, Inc.
Outline	INTRODUCTION SYSTEM ON A CHIP ADC POWER-DOWN CPU POWER-DOWN
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Content	AN-664 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com INTRODUCTION This application note refers only to the AD7739, but is also generally applicable to the AD7732, AD7734, and AD7738. The purpose of this application note is to explain how to optimize the use of these parts in power sensitive applications. Table I. AD7739 Power in Various Modes DVDD/ Clock Power (mW) AVDD (MHz) Mode Buffers Typ 5/5 6 Normal On 81.5 3/5 6 Normal On 71.0 3/5 6 Normal Off 45.5 3/5 4 Reduce On 53.0 3/5 4 Reduce Off 34.0 Standby 0.5 The AD7739 features can make this ADC competitive with seemingly lower power ADCs from a system perspective. This application note discusses some of these features. AD7732/AD7734/AD7738/AD7739 in Low Power Applications By Tom Meany SYSTEM ON A CHIP When comparing the AD7739, or other members of this family with lower power ADCs, the system on a chip features of the AD7739 must be taken into account. As shown in the block diagram, the AD7739 contains on-chip buffers, a multiplexer, crystal clock oscillator, 2-pin input/output port, high speed digital interface, and calibration logic. When compared as a system, these on-chip features may make the AD7739 competitive with lower power ADCs. The on-chip buffers in particular are worth noting. Unbuffered high speed 3-$ ADCs present a particularly difﬁcult load to any driving circuit. Typically the analog input of an unbuffered 3-$ ADC will look like a 4 pF to 40 pF capacitor, which is discharged and then needs to recharge to 16-bit accuracy at the modulator rate. For high speed 3-$ ADCs, the modulator rate can be several MHz. The on-chip buffers of the AD7739 are optimized to drive the AD7739 ADC, and all ADC speciﬁcations include the buffer performance. From a 5 V supply, the buffers consume about 25 mW. On the AD7738, the multiplexer output is made available off-chip. This means that it is possible to replace the on-chip buf- CALIBRATION CIRCUITRY SERIAL INTERFACE CONTROL LOGIC CLOCK GENERATOR MUX I/O PORT 3–$ ADC BUFFER REFERENCE DETECT AD7739 AIN0 AIN7 AINCOM/P0 SYNC/P1 AGNDAVDDMCLKDGNDDVDD SERIAL INTERFACE RDY RESET REFIN AVDD Figure 1. AD7739 Block Diagram REV. 0
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AD7732_AD7734_AD7738_AD7739 in low Power Applications \AN-664, REV. 0\ Analog Devices, Inc.

AD7732_AD7734_AD7738_AD7739 in low Power Applications \AN-664, REV. 0\ Analog Devices, Inc.

INTRODUCTION SYSTEM ON A CHIP ADC POWER-DOWN CPU POWER-DOWN