DC Accuracy in a Fast Boxcar Circuit Via a Comparator

A capacitor-storage boxcar (i.e., zero-order hold) circuit is described, which has a one-half μsec sampling time and a 0-to 6-volt dynamic range, with less than 10-mv absolute error for a hold time of one second or less. No operational amplifier is used. In the new boxcar, feedback is applied via a difference amplifier comparator during sampling to make the readout voltage, not the voltage actually across the capacitor, equal to the input voltage. This effectively cancels any long-term voltage errors across either sampling switch or readout isolation amplifier. Eliminated are both offset voltage errors due to circuit assembly tolerances and also long-term offset drifts occurring over more than one storage interval, e.g., as due to temperature changes. The actual circuit given is all solid state, has no inductive or critical elements and has been easily reproduced several times. This comparator boxcar concept is not limited to capacitor storage.