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Design Solutions 11 November 1999 Testing Linearity of the LTC2400 24-Bit No Latency TM A/D Converter Help from the Nineteenth Century by Jim Williams Introduction follower is required to unload the KVD without introducing significant loading error. Now, our KVD looks like Figure 3. Verifying the linearity of the LTC2400 analog to digital converter requires special considerations. Typical nonlinearity is only 2ppm (0.0002%). Bench testing this 10k 2k 400 80 necessitates some form of voltage source that produces INPUT equal amplitude output steps for incremental digital in- puts. Additionally, for measurement confidence, it is de- sirable that the source be substantially more linear than the 2ppm requirement. This is, of course, a stringent demand and painfully close to the state of the art. The most linear "D to A" converter is also one of the oldest: Lord Kelvin's Kelvin-Varley divider (KVD), in its most developed form, is linear to 0.1ppm. This manually switched device features ten million individual dial settings ar- 80 OUT ranged in seven decades. It may be thought of as a 80 3-terminal potentiometer with fixed "end-to-end" resis- tance and a 7-decade switched wiper position (Figure 1). SEVEN-DECADE SWITCHED R = 100k WIPER POSITION PERMITS SETTING TO 0.1ppm LINEARITY DSOL11 F01 COMMON DSOL11 F02 Figure 1. Conceptual Kelvin-Varley Divider Figure 2. A 4-Decade Kelvin-Varley Divider. Additional Decades The actual construction of a 0.1ppm KVD is more artistry Are Implemented By Opening Last Switch, Deleting Two Associated 80 Values and Continuing