9.1.1 Hold Capacitor

Hold step, acquisition time, and droop rate are the major trade-offs in the selection of a hold capacitor value. Size and cost may also become important for larger values. Use of the curves included with this data sheet should be helpful in selecting a reasonable value of capacitance. Keep in mind that for fast repetition rates or tracking fast signals, the capacitor drive currents may cause a significant temperature rise in the LF198-N.

A significant source of error in an accurate sample and hold circuit is dielectric absorption in the hold capacitor. A mylar cap, for instance, may sag back up to 0.2% after a quick change in voltage. A long sample time is required before the circuit can be put back into the hold mode with this type of capacitor. Dielectrics with very low hysteresis are polystyrene, polypropylene, and Teflon. Other types such as mica and polycarbonate are not nearly as good. The advantage of polypropylene over polystyrene is that it extends the maximum ambient temperature from 85°C to 100°C. Most ceramic capacitors are unusable with > 1% hysteresis. Ceramic NPO or COG capacitors are now available for 125°C operation and also have low dielectric absorption. For more exact data, see Figure 2. The hysteresis numbers on the curve are final values, taken after full relaxation. The hysteresis error can be significantly reduced if the output of the LF198-N is digitized quickly after the hold mode is initiated. The hysteresis relaxation time constant in polypropylene, for instance, is 10 to 50 ms. If A-to-D conversion can be made within 1 ms, hysteresis error will be reduced by a factor of ten.