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How to increase the resolution of comparator in SAR ADC?

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pz5921087

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Hi,
I am designing a SAR ADC with 12 bit and 2kS/s rate. The structure is general and simple.
But when I am considering the comparator(preamp+latch+buffer), I found it only can sense about 5mV voltage difference. In 12bit ADC it needs to sense about 0.3mV voltage difference, so how to increase the resolution of this comparator?

The figure below is my comparator circuit

 

You need to add gain to your preamplifier, for instance cascading multiple preamplifier stages or adding a cross-coupled load instead of a simple current mirror load.
 

Gain might not help in offset (unless the succeeding stages are the ones causing the offset). One way would be to use techniques such as auto-zeroing, correlated double sampling etc.
 

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