Performance sample and hold

  
When switch SWl is positive, the FET is turned on, and has a resistance of about 400 ohm. The input voltage charges up the capacitor through the FET. When SWl is negative, the FET is turned off (pinched off). To get a long storage time, the op amp must have a very low input bias current. For the CA3140, this current is about 10 pico amps.
Performance sample and hold  - schematic

The rate at which the capacitor will be discharged by this current is based on the equation, C (dv/dt) = i where dv/dt is the rate of change of voltage on the capacitor.




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