AMICSA DUTH/SRL GK Methodologies for designing radiation hardened Analog to digital converters for space applications. DUTH/SRL George Kottaras, E.T. SarrisNick, Stamatopoulos

AMICSA DUTH/SRL GK ADCs are very critical in space applications. (needed in every subsystem, telemetry, housekeeping, instrumentation, etc) Currently there is a high demand for ADCs and at the same time a lack of availability Commercial ADCs suffer from radiation effects TID effects SEE effects

AMICSA DUTH/SRL GK Main Radiation Effects in the ADCs. Analog components suffer from TID effects Digital components suffer from SEE effects. RADIATION EFFECTS ON ADCs ARE VERY TECHNOLOGY DEPENDENT.

SOLUTIONS AGAINST TID Use deep submicron technologies & Use enclosed geometry transistors. AMICSA DUTH/SRL GK TID effects are nearly cancelled. Potential Problems Deep submicron technologies need low Vdds Enclosed geometry transistors design rules 90deg bend gate is not allowed by most technologies=> More complex designs Matching of ELTs is an important issue.

SOLUTIONS AGAINST TID Select appropriate topology for increasing radiation hardness. AMICSA DUTH/SRL GK DESIGN RULES FOR SELECTING A TOPOLOGY Minimum number of analog comparators (SA, ΣΔ), if possible. The comparator is most sensitive circuit in the ADC design. Needs autozeroing to cancel the effects. Rely on passive components for voltage/current division Resistors are preferred since they are immune to TID. Perform autozeroing in the digital domain, if possible. The autozeroing circuits will be more immune to TID.

SOLUTIONS AGAINST TID AMICSA DUTH/SRL GK DESIGN RULES FOR ADC peripherals Usually all ADCs are accompanied by some peripherals such as S/H amplifiers, instrumentation amplifiers, voltage references, etc. Design having in mind that the smaller the common mode variation of the input of the amplifiers, the more TID immune the design is going to be. For example, Amplifier A will be more radhard than amplifier B. Constant CM Non Constant CM

SOLUTIONS AGAINST TID AMICSA DUTH/SRL GK Autozeroing Radiation Induced errors are inevitable no matter of the design. The strategy is to compensate for them. This step is called autozeroing. Two possible ways to autozero Digital Autozero Somehow the TID induced offset is quantized and then digitally removed from the output code of the ADC. Analog Autozero Perform autozero function on the comparator/s by means of an error amplifier.

SOLUTIONS AGAINST TID AMICSA DUTH/SRL GK Analog Autozeroing Analog autozeroing involves adding an offset value in the comparator input so as to cancel that TID induced offset. It is frequently used in Flash ADCs Pipeline ADCs An extra phase is required so that autozeroing can be performed.

SOLUTIONS AGAINST TID AMICSA DUTH/SRL GK Analog Autozeroing

SOLUTIONS AGAINST TID AMICSA DUTH/SRL GK Digital Autozeroing Digital Autozeroing is applied on the output code of the ADC. Can be easily applied in ΣΔ ADCs Successive Approximation ADCs Current cyclic ADCs. Usually oversampling is involved.

AMICSA DUTH/SRL GK A Rad-Hard Successive digitally autozeroed Successive Approximation ADC

AMICSA DUTH/SRL GK It consists of 2 DACs sharing the same resistive string a comparator a successive approximation state machine (SASM), a temporary register and a subtractor circuit. The comparator, along with the SASM and one of the two DACs form the successive approximation ADC. The temporary register, the subtractor and the auto zeroing DAC (AZ-DAC) are utilized to perform the digital auto-zeroing function on the ADC.

AMICSA DUTH/SRL GK When no auto-zeroing is selected the device works as a nominal 10-bit successive approximation ADC. When auto-zeroing is selected, two analogue to digital conversions take place sequentially and the final result is a combination of the two conversions. In the first analogue to digital conversion, the analogue input is converted by the successive approximation ADC to a digital word (OC1), stored to a temporary memory location, after being shifted one position left, and fed to the AZ-DAC. In the second A/D conversion the analogue input is isolated and the output of the AZ-DAC, which is the result of the first A/D, is converted to a digital word (OC2) by the successive approximation ADC. The output code (OC) comes from the following subtraction OC=2OC1-OC2

AMICSA DUTH/SRL GK A Rad-Hard Successive digitally autozeroed Successive Approximation ADC

AMICSA DUTH/SRL GK A Rad-Hard Successive digitally autozeroed Successive Approximation ADC Assume that in the CM of interest the comparator has developed offset (in LSBs) off. The result from the first conversion would be : OC1=OCideal+off The second conversion would yiled a code equal to OC2=OCideal+off+off The final result wuld be eual to OC=2OC1-OC2=OCideal Offset has been cancelled

AMICSA Use of the above architecture to produce a subranging converter DUTH/SRL GK 11 bit Successive Approximation ADC Vin V DAC Large common mode so Analog AZ is required Special AZ technique developed 10+1 bits

SOLUTIONS AGAINST SEEs Large FFs in the state machine P-N good separation => full custom layout Use of guardrings => full custom layout Watchdog counters to reset the ADC incase of SEUs. AMICSA DUTH/SRL GK