为K~+—ISFET设计Ref—ISFET

There are three methods for measuring the ISFET response: [1] 1). constant V_G operation, 2) constant I_D operation, 3) differential measurement. The differential measurement has two advantages: first, the probc may be made extremely small and full-solid by combining the ISFET and reference-ISFET on a same chip; second, this measurement can make temperature compensation and noise reduction possible. But how to make the Ref-ISFET