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 accuracy
requirements of many unipolar output sensor applications
occur at low signal levels. To meet this requirement,
Burr-Brown developed the adaptive delta modulation
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 programmable
gain amplification, A/D conversion, and digital
filtering to produce precision, wide dynamic range results.
The input signal can be a low level current connected
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 modulation
topology combines charge integration and digitization
functions. Oversampling and digital filtering reduce
system noise dramatically. Correlated Double Sampling
(CDS) captures and eliminates steady state and conversion
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.