Transition-Edge Sensor Characteristics in a Modified Bias Circuit for Multiplexed Readout

The X-ray Integral Field Unit (X-IFU) on the NewAthena X-ray telescope will use a 1,504-pixel array of transition-edge sensor (TES) microcalorimeters, read out via time division multiplexing (TDM). To allow testing of multiplexer chips before TES integration, each unit cell includes a “TES-bypass” resistor that substitutes for a TES during characterization. After integration, this resistor remains as a high-resistance shunt: minimal for DC operation but can influence the frequency response due to its placement with the Nyquist inductor. We modeled the modified bias circuit, derived the small-signal response, and evaluated noise contributions, including Johnson noise from the bypass resistor. We analyzed the effects on pulse height, slew-rate, and energy resolution. Simulations were compared with measurements from prototype X-IFU microcalorimeters. Our results indicate that as long as the resistor value exceeds the TES dynamic resistance, performance is largely unaffected.