The Voltage Reference

The primary function of the voltage reference is to generate a low-noise ultra-stable reference for the input stage. The same voltage is used in the A/D converter, but requirements are less strict. The voltage reference has been optimized to give very similar performance at room temperature and at the planned operating temperature of -130°C, not to give the minimal temperature coefficient at the designed operating temperature. The plot in Fig. 4 shows excellent stability, ~0.1 mV/K at both room and operating temperature.

Figure 4. Voltage reference performance over temperature.

Since band-gap based voltage references are notoriously noisy, a trick was used to achieve the required noise performance. A sample and hold circuit was implemented at the output of the voltage reference. At the beginning of each pixel read the reference voltage is 'frozen' on the sample capacitor and the voltage reference is disconnected for the entire duration of the pixel read. Therefore the entire voltage reference noise is rejected by the CDS. Only the buffer amplifiers distributing the reference voltage over the chip contribute to the reference voltage noise.

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