Thermal Radiometer Signal Processing using Radiation Hard CMOS Application Specific Integrated Circuits for use in Harsh Planetary Environments

Abstract Thermal radiometers such as proposed for the E uropa Clipper flyby mission [1] require low noise signal processing for thermal imaging with immunity to T otal Ionizing D ose (TID) and S ingle E vent L atchup (SEL) . Described is asecond generation Multi-Channel Digitizer (MCD2G) pplication S pecific AIntegrated C ircuit (ASIC ) that accurately digitizes up to 40 thermopile pixels with greater than 50 Mrad (Si) immunity TID and 174 MeV-cm 2 /mg SEL. The MCD2G ASIC uses R adiation H ardened B y D esign (RHBD) techniques with a 180 nm CMOS process node. 1. Introduction Thermopile pixels provide a linear voltage response to infrared waveleng ths in the 7-200 µ m range making them suitable for thermal radiometers. A radiometer sensor with custom designed a 2D array and eight ASICs arranged in a hybrid F ocal P lane A ssembly (FPA) is shown in Figure 1The hybrid . package size is 86.3 mm x 63.5 mm x 8 mm. This implementation has 5 line arrays on one substrate giving a total of 320 pixels. Each pixel 64- line array has a unique passband. The spectral passbands are 7-10 µ m, 12.5–25 µ m, 2 5- 50 µ m, 50- 100 µ m and 100-200 µ m. The different passbands allow the measurement of endogenic heat flux, hotspot detection and day/night surface temperatures of an icy moon such as Europa. Sub-microvolt outputs from the pixels necessitate gains in the region of several thousand. The thermal noise of the pixel outputs and the 1/ f noise [2] of the amplifiers must be filtered in order to achieve adequate signal to noise ratio before digitization. The thermopiles are operated at cryogenic temperatures so as to minimize thermal noise. The front-end electronics must also operate at temperatures as low as 150 K in the high TID environment present in the Jovian orbital system. Sampled analog signal processing is used in the custom ASIC to accurately amplify and digitize the outputs of thermopile pixels .