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AMICSA

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AMICSA

Methodologies for designing radiation

hardened Analog to digital converters

for space applications.

DUTH/SRL

George Kottaras, E.T. SarrisNick, Stamatopoulos

DUTH/SRL

GK

AMICSA

- 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

DUTH/SRL

GK

AMICSA

- 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.

DUTH/SRL

GK

AMICSA

- SOLUTIONS AGAINST TID
- Use deep submicron technologies &
- Use enclosed geometry transistors.

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.

DUTH/SRL

GK

AMICSA

- SOLUTIONS AGAINST TID
- Select appropriate topology for increasing radiation hardness.

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.

DUTH/SRL

GK

AMICSA

SOLUTIONS AGAINST TID

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

DUTH/SRL

GK

AMICSA

SOLUTIONS AGAINST TID

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.

DUTH/SRL

GK

AMICSA

SOLUTIONS AGAINST TID

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

DUTH/SRL

GK

AMICSA

SOLUTIONS AGAINST TID

Analog Autozeroing

DUTH/SRL

GK

AMICSA

SOLUTIONS AGAINST TID

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.

DUTH/SRL

GK

- A Rad-Hard Successive digitally autozeroed Successive Approximation ADC

AMICSA

DUTH/SRL

GK

AMICSA

- 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.

DUTH/SRL

GK

AMICSA

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

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

AMICSA

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

DUTH/SRL

GK

AMICSA

Use of the above architecture to produce a subranging converter

10+1 bits

11 bit

Successive

Approximation

ADC

VDAC

Large common mode so Analog AZ is required

Special AZ technique developed

Vin

DUTH/SRL

GK

AMICSA

- 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.

DUTH/SRL

GK