DDC101

The DDC101 is a precision, wide dynamic range, charge digitizing A/D converter with 20-bit resolution. Low level current output devices, such as photosensors, can be directly connected to its input. The most stringent accu-racy requirements of many unipolar output sensor appli-cations occur at low signal levels. To meet this require-ment, Burr-Brown developed the adaptive delta modula-tion architecture of the DDC101 to provide linearly
improving noise and linearity errors as the input signal level decreases. The DDC101 combines the functions of current-to-voltage conversion, integration, input program-mable gain amplification, A/D conversion, and digital filtering to produce precision, wide dynamic range re-sults. The input signal can be a low level current con-nected directly into the unit or a voltage connected through a user selected resistor. Although the DDC101 is
optimized for unipolar signals, it can also accurately digitize bipolar input signals. The patented delta modula-tion topology combines charge integration and digitiza-tion functions. Oversampling and digital filtering reduce system noise dramatically. Correlated Double Sampling (CDS) captures and eliminates steady state and conver- sion cycle dependent offset and switching errors that are not eliminated with conventional analog circuits.

The DDC101 block diagram is shown below. During conversion, the input signal is collected on the internal integration capacitance for a user determined integration period. A high precision, autozeroed comparator samples the analog input node. Tracking logic updates the internal
high resolution D/A converter at a 2MHz rate to maintain the analog input at virtual ground. A user programmable digital filter oversamples the tracking logic's output. The digital filter passes a low noise, high resolution digital output to the serial I/O register. The serial outputs of multiple DDC101 units can be easily connected together in series or parallel if desired to minimize interconnections.