A method is presented for generating linear or nonlinear functions digitally. The nonlinear analog-to-digital conversation (NLADC) is accomplished using the pulsewidth modulation of the analog input voltage. The conversion is done accordingly to a special quantizing characteristic function (QCF), which depends on the specific application. This special QCF, sampled, quantized, and coded, has been stored in an EPROM (erasable programmable read-only memory). The quantizing characteristic can be any monotonically increasing function of any type, resulting in a very flexible linear or nonlinear analog-to-digital converter. Such an NLADC could be used for the expansion or compression of the dynamic range in nuclear science measurements or in robotics for code modulation planning. >

Nonlinear ADC with digitally selectable quantizing characteristic

A method for finding non-linear functions digitally is described. The design is based on pulse width modulating (PWM) the analogue signal by applying a specially shaped reference waveform in the modulator. The number of pulses of a high frequency clock entering in a binary counter during the high state period of the PWM will provide a digital number proportional to the first, square, cubic or forth order root at the input voltage. Exponential form functions also can be found digitally.

Non-linear analogue-to-digital conversion through PWM

In background noise limited instrumentation systems, an analog-to-digital converter with a square root transfer function will allow a maximum dynamic range with a fixed number of data bits. The converter described herein was developed to digitize the output from a CCD in a space environment where telemetry is limited. The goals of low power dissipation, moderately fast conversion time, and reliability have been achieved using standard components and avoiding non-linear elements.

A Quadratic Analog-to-Digital Converter

A method is introduced for finding non-linear functions digitally. The design is based on voltage-to-frequency conversion (VFC) and decimal rate multiplication (DRM). A DC signal is applied to the VFC and analogue comparator, producing a digital number proportional to the square of the input voltage. For the second half-period, this digital number is applied together with the VFC output clock frequency to the DRM, producing a digital number proportional to the cubic and fourth powers of the input DC signal. Any of the three digital numbers can be selected and displayed.

Flexible non-linear analogue-to-digital converter

The arrival of photons at a given location is a Poisson process with an associated shot noise which rises with the square root of the number of photons received. An analog-to-digital converter (ADC) with a square root transfer function can quantize photonic signals with LSB size kept constant with respect to the photon shot noise. In imaging applications, this can greatly reduce the number of bits needed to characterize a signal compared to a linear ADC without detrimental effects to image quality. Such a device, based on the Analogic MuSIC chip, was designed and tested for the needs of a medical computed tomography (CT) device. The experimental setup increases the MuSIC sampling frequency from 3kHz to 7kHz, while reducing the amount of data necessary for reconstruction. A constant quantization noise to photon shot noise ratio acceptable for CT is maintained by sizing each LSB to be one half the rms noise level. Results show an INL of 2.5 LSB, which is reduced to 0.27 LSB after a correction scheme. Thesis Supervisor: Charles G. Sodini Title: Clarence J. Lebel Professor of Electrical Engineering

A square root analog to digital converter to optimally convert photonic signals for computed tomography

Two instruments, a White Light Coronagraph and an X-ray XUV telescope built into the same housing, share several electronic functions. Each instrument uses a CCD as an imaging detector, but due to different spectral requirements, each uses a different type. Hardware reduction, required by the stringent weight and volume allocations of the interplanetary mission, is made possible by the use of a microprocessor. Most instrument functions are software controlled with the end use circuits treated as peripherals to the microprocessor. The instruments are being developed for the International Solar Polar Mission.

M. H. Staples

Papers

A Quadratic Analog-to-Digital Converter

CCD Camera Systems and Support Electronics for a White Light Coronagraph and X-Ray XUV Solar Telescope