  We characterize a new x-ray Mixed-Mode Pixel Array Detector (MM-PAD-2.1)
Application Specific Integrated Circuit (ASIC). Using an integrating pixel
front-end with dynamic charge removal architecture, the MM-PAD-2.1 ASIC extends
the maximum measurable x-ray signal (in 20 keV photon units) to > 10$^{7}$
x-rays/pixel/frame while maintaining a low read noise across the full dynamic
range, all while imaging continuously at a frame rate of up to 10 kHz. The
in-pixel dynamic charge removal mechanism prevents saturation of the input
amplifier and proceeds in parallel with signal integration to achieve
deadtime-less measurements with incident x-ray rates of > 10$^{10}$
x-rays/pixel/s. The ASIC format consists of 128$\times$128 square pixels each
150 $\mu$m on a side and is designed to be 3-side buttable so large arrays can
be effectively tiled. Here we use both laboratory x-ray sources and the Cornell
High Energy Synchrotron Source (CHESS) to characterize two single ASIC
prototype detectors for both low (single x-ray) and high incident flux
detection. In the first detector the ASIC was solder bump-bonded to a 500
$\mu$m thick Si sensor for efficient detection of x-rays below ~20 keV, whereas
the second detector used a 750 $\mu$m thick CdTe sensor for x-rays above $\sim$
20 keV.
