Electronic Design

Capacitor-Switching Algorithm Makes Delta-Sigma ADCs A Cinch To Drive

While some established chipmakers are developing new proprietary process technologies for mixed-signal and analog devices (see "'Sample Wars' And Silicon Energize ADCs," p. 47), other companies are using clever chip design to enhance the performance of established architectures on well-stabilized, mature process technologies.

One of these companies, Linear Technology, has eliminated most of the problems of switched-capacitor inputs in delta-sigma analog-to-digital converters (ADCs). The project began when Linear looked at what happens in a switchedcapacitor network where capacitors are switched rapidly between the input, reference, and ground as a function of the final output code.

Switching these capacitors to the ADC input generates a pattern of charging/ discharging current pulses, making it difficult for the preceding operational amplifier to drive. External RC networks that don't completely settle during each sample period cause large dc errors.

Linear's LTC248x family eliminates this problem with a proprietary switching algorithm known as Easy Drive. It forces the average differential input current to zero independently of external settling errors. This enables accurate direct digitization of high-impedance sensors without buffers.

Easy Drive also accounts for errors resulting from mismatched leakage currents by forcing the average input current on the positive input to equal the average input current on the negative input. Over the complete conversion cycle, the average differential input current is zero. At the same time, the common-mode input current is proportional to the difference between the common-mode input voltage and the commonmode reference voltage.

This arrangement is attractive in many types of delta-sigma applications. With balance bridges, where the input common-mode voltage is equal to the reference common-mode voltage, the differential and common-mode input current both are zero. Also, settling errors and mismatches in source impedances between the differential inputs have no effect on accuracy.

Similar analyses hold for cases where the input common-mode voltage is constant but different from the reference common-mode voltage, as well as where the input common-mode voltage is constant but different from the reference, varying with the input signal level.

Thousand-quantity unit pricing for the 16-bit LTC2480 begins at $1.85. The LTC2482 lacks the internal temperature sensor and programmable gain. The 24-bit LTC2484 has the temperature sensor but lacks programmable gain.

Linear Technology
www.linear.com

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