Ultra Long Time Constant Integrator

 This circuit is a complete ultra long time-constant charge integrator circuit. The integration current IIN is
produced by input VIN and is directly proportional to VIN, given by IIN = VIN/R. The feedback integration
capacitor C is charged with this integration current. Assuming there is no loss of charge at the negative input
terminal of the integrator Amplifier the time required in charging the integrating capacitor depends directly on
the magnitude of 1/VIN and is proportional to the product of R and C.  This integrator circuit utilizes 2 separate
relays to control the integration and the reset cycles. Initially, at the beginning of a charge cycle, Relay 1 is
closed to preset the negative input to the Operational Amplifier to a preset voltage. When sufficient time has
been allowed for the preset voltage to charge the integrating capacitor C, Relay 1 is opened. Then Relay 2 is
closed to start the integration cycle. In order to generate ultra long time-constant, select a large value for both
R and C.  Select a large value for R consistent with leakage currents permitted on the printed circuit board, and
select large value for C based on size, cost and internal leakage specification requirements. Key selection
criteria for an Operational Amplifier are a) extremely low input leakage currents b) low input offset voltage c)
sufficient slew rate and output current to charge the capacitor for the time-constant desired.
Recommended Components
ALD1712 ALD1721 ALD1722 ALD2711 ALD2722 ALD2726
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Ultra Long Time Constant Integrator application circuits
Ultra Long Time Constant Integrator application circuits

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