SQUID MUX Issues Series addressing scheme Balanced SQUID

SQUID MUX Issues - Series addressing scheme - Balanced SQUID pairs for crosstalk reduction - Input transformer (for more inductance in smaller area. ) - Candidate SCUBA-2 SQUID MUX pixel layout

Conventional “Parallel Address” SQUID MUX VSQUID ~ 100 m. V ISQUID ~ 100 m. A IOFF < 1/3 ISQUID R|| ~ 3 W RA = 80 R|| = 240 W for 450 mm l/2 D PMUX = 2 RA(ISQUID)2 x 80 x 4 = 1. 5 m. W !!!

Photometric response of multiplexed 8 -pixel TES array

NIS address elements to reduce power - High ‘off’ resistance for low crosstalk, leakage - Low ‘on’ resistance for low power

Old NIS Addressed Multiplexer For 450 mm l/2 D For Nb NIS junctions D=1. 5 m. V P=2 D ISQUIDx 2 x 80 x 4 = 192 m. W For Al NIS junctions D=0. 18 m. V P=2 D ISQUIDx 2 x 80 x 4 = 23 m. W

But. . . Even with the NIS element, the “parallel” MUX has some issues: - Complicated fabrication: integration of additional NIS steps into our process - A full row of address elements has low resistance (~ 0. 1 W). Speed limitation. - High current source required … perhaps tens of milliamps - NIS junctions can’t be tested at 4 K. What we really want is a current-biased, series-address scheme, but putting the elements in series leads to ‘network effects’ that prevent operation.

Series MUX Rs Rs

Series-Address SQUID MUX Instead of adding voltages, add magnetic flux. Then address lines can be series instead of parallel.

Series Address vs. Parallel NIS Address Power Fabrication Address lines Crosstalk Address complexity Resistance Test temperature Parallel NIS Series few 10’s m. W Very hard 10’s of m. A Hard voltage bias Complex, but doable < 0. 1 W 0. 1 K few 10’s m. W Somewhat less hard < 1 m. A Hard current bias Simpler ~ 20 W (faster) 4 K

32 -Channel Series-Address SQUID MUX Chip In fabrication 32 address pairs 32 input bond-pad pairs

Feedback-Input Coupling Signal is sent down comp coil to cancel feedback coil – input Coil coupling. Comp coil Feedback line Feedback to the pixel which is ‘on’ charges up the input inductors on ‘off’ pixels: a crosstalk mechanism for distant pixels Two solutions: 1) A common ‘Comp coil’ for the column can be adjusted to cancel input -feedback coupling (FIBRE). TES V 2) A balanced SQUID pair with opposite winding will geometrically cancel. Only one SQUID is turned on: the other is just a dummy.

FIBRE Comp Coil 8 channels Nyquist inductor Comp coil

Input Transformer Input More inductance can be put in a smaller area by coupling through a transformer. The input transformer is wound as a gradiometer over a ground plane with a dumbell slit to reduce crosstalk

SCUBA-2 SQUID MUX Pixel Layout Comp coil? Feedback line Flux output line to bump bonds Address Line Input Transformer Balanced SQUID pair; only one is turned on. to bump bonds

SCUBA-2 SQUID MUX Physical Pixel Layout Bump bond? Input Transformer SQUID Input Transformer Output Transformer Dummy SQUID Bump bond?