New England Photoconductor Corp. has
demonstrated a unique approach to multiplexing PbS and PbSe arrays
which fully integrates the signal from every detector. This
innovative concept provides a method for monitoring the signals from
a large number of photoresistive detectors. The result is minimal
degradation in signal to noise ratio due to multiplexor timing
requirements.
This multiplexing approach accumulates the
current from each detector within the array >99% of the time,
essentially removing the time dependent degradation of the serial
approach. The key to this new method is multiplexing current instead
of voltage. Each detector within the array has an associated
integration capacitor which accumulates charge at a rate dependent
upon the resistance of its pixel.
While this is occurring, the multiplexor
circuitry sequentially accesses each integrating capacitor,
transferring its charge to an output current to voltage converter.
This process discharges the capacitor, allowing the capacitor to
recharge for a full array scan period before being accessed again.
The multiplexing system also allows the output
leads from the multiplexor to be held at a constant voltage,
removing the effects of parasitic capacitance and substantially
reducing unwanted EMI effects. In essence, this approach provides a
single serial output signal from the array, with similar signal to
noise performance as achieved by operating a single detector.
The fully integrating multiplexing technique
has clock speeds which can be varied from 100 Hertz to 1.2 MHz. This
corresponds to an array scan time (i.e. a detector integration time)
of 0.05 milliseconds to 0.66 seconds (for a 64 element array). The
current system design is completely self-contained, requiring only
the application of ±12 volts ±15 volts and 5 volts @ 2 amps to
operate the cooler. The system outputs include a serial analog
signal with voltages corresponding to the detector signals, a clock
which indicates when data is good, and a start of array scan pulse
to register the data to the beginning of the array.
Many variations to this system can be provided
to allow custom interfacing of the fully integrating multiplexor
system to an end user's application. Features such as adjustable
clock speed, a user supplied clock (i.e. slave operation), active
modulation of the bias voltage to allow AC illumination operation,
and additional output signal processing are all available. Please
contact New England Photoconductor Corp. for further information on
applying this exciting new multiplexing approach to your
application.
