A Radiation-Hardened CMOS 8-Bit Analog-to-Digital Converter

The increasing use of LSI circuits in advanced systems has required the incorporation of linear (analog) functions into digital Bulk CMOS LSI devices. This design required a radiation-hardened 8-bit successive approximation analog-to-digital converter (ADC) which would be implemented as a macrocell of a 24-channel data acquisition system on a single LSI circuit. The ADC and two operational amplifiers were first fabricated on a small chip for characterization testing. These parts were packaged and the characterization tests were successful, indicating better than 8-bit accuracy. Of these parts, six were irradiated at 2 × 106 rads (Si) per hour, each with different bias conditions. All but one of the parts were functional after 5 × 105 rads (Si) total dose and all but two were still accurate to 7-bit accuracy after 1 × 106 rads (Si). The one unit which failed was a gross linearity failure; the test results plus the bias conditions indicate that the offset cancellation switch became leaky, causing the failure. This portion of the circuit has been redesigned to eliminate the problem, and future tests are pending.