[christianw.]

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Zitat:When using the LTC2495?s internal oscillator, the input capacitor array is switched at 123kHz. The effect of the charge transfer depends on the circuitry driving the input/reference pins.

If the total external RC time constant is less than 580ns the errors introduced by the sampling process are negligible since complete settling occurs.

Typically, the reference inputs are driven from a low impedance source. In this case, complete settling occurs even with large external bypass capacitors. The inputs (CH0-CH15, COM), on the other hand, are typically driven from larger source resistances. Source resistances up to 10k may interface directly to the LTC2495 and settle completely; however, the addition of external capacitors at the input terminals in order to filter unwanted noise (anti-aliasing) results in incomplete settling.

The LTC2495 offers two methods of removing these errors. The first is automatic differential input current cancellation (Easy Drive) and the second is the insertion of an external buffer between the MUXOUT and ADCIN pins, thus isolating the input switching from the source resistance.

Automatic Differential Input Current Cancellation In applications where the sensor output impedance is low (up to 10kW with no external bypass capacitor or up to 500W with 0.001uF bypass), complete settling of the input occurs. In this case, no errors are introduced and direct digitization is possible. For many applications, the sensor output impedance combined with external input bypass capacitors produces RC time constants much greater than the 580ns required for 1ppm accuracy. For example, a 10kW bridge driving a 0.1uF capacitor has a time constant an order of magnitude greater than the required maximum.