The current buffer

  

Figure 8: Current buffer

  

Figure 9: Format of a readout burst

The current buffer is a differential voltage input/single current output (``transconductance'') amplifier (fig. 8) that converts the voltage signals delivered by the multiplexer. Linearity is achieved by a current feedback topology. The two multiplexer output voltages AnalogOut and AnalogOutDummy are amplified by two separate buffers sharing the common reference voltage Voffset. In case of radiation damage

• Idriver should be kept constant.
• Voffset should be adjusted so that the output offset current stays constant (probably the level must be raised).

Figure 9 shows how the chip outputs its data after receiving a trigger. Synchronously to the falling edge of RClk data are asserted, indicated by DataValid going high. The current output signals AnalogOut and AnalogOutDummy are plotted as they can be observed at the chip output shorted to ground. The 128 amplifier channels are output channel 0 first, followed by channel 1 etc. up to channel 127. Analog data are followed immediately by an 8 bit trailer showing the pipeline column number the event has been stored in. This number is coded LSB first, a "1" bit coded as $\approx$+2 MIP$_{\rm Si}$, a "0" bit as $\approx$-2 MIP$_{\rm Si}$.AnalogOutDummy should be subtracted from AnalogOut to cancel offset, common mode and clock feed through.