The 428 is an integrating analogue to digital converter which is used to digitise the output from dc or lock-in amplifiers and pass the result to a computer using the USB interface incorporated in a 417.

It uses a voltage to frequency converter followed by a counter. The count accumulated during the conversion period represents the true average of the analogue input during that time. The 428 has a number for conversion periods which range from 25ms up to 250ms. To achieve longer conversion periods the readings are averaged by the controlling software.

The count accumulated from a number of conversions thus represents, after normalisation for total time, the true average of the signal over a longer period. The 428 therefore behaves as a digital signal averager with the averaging period programmable in conversion period increments.

The 428 has two other special features which enhance its usefulness in light measurement systems:

Firstly, the input to the ADC is offset, giving the unit a small negative range. This ensures that negative-going noise peaks, occurring in near zero signals, are correctly averaged while retaining most of the available resolution for positive-going signals.

Secondly, the ADC provides information to the computer, indicating that a transient overload has occurred at some point during the conversion period. This information is essential if accurate measurements are to be made on pulsed light sources such as CRT monitors.

Specification
Resolution	16-bit
Conversion	25ms to 250ms
Input range	-0.2V to 9.8V
Linearity	<0.025 departure from linearity from zero to full scale
Inputs	2-channel