What are the requirements of a 12-bit (250KHz) SAR ADC's opamp？

Q1: What is the requirement of a 12bit (250KHz) SAR ADC's opamp?
Q2: How to determine the unity-gain frequency of 12bit (250KHz) SAR ADC's opamp?
Q3: How to determine the open-loop gain of 12bit (250KHz) SAR ADC's opamp?
Supply voltage=3.3V
Vref=1.65

Help Me, please

作业?
opamp 拿来干嘛用?

250KHz 好慢啊，你说的是S/H中的OP?，小编还是多花心思用在如何减小C的尺寸，如何提高线性度上吧

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What are the requirements of a 12-bit (250KHz) SAR ADC's opamp