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MICROPOWER_VOLTAGE_TO_FREQUENCY_CONVERTER

2015-03-22 18:40  
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MICROPOWER_VOLTAGE_TO_FREQUENCY_CONVERTER
This voltage-to-frequency converter takes full advantage of the LTC1441's low power consumption under dynamic conditions. A 0- to 5-V input produces a 0- to 10-kHz output with 0.02 percent linearity, 60 ppm/°C drift, and 40 ppm/V supply rejection. Maximum current consumption is only 26μA, 100 times lower than that of currently available circuits. C1 switches a charge pump, composed of Q5, Q6, and the 100-pF capacitor, to maintain its negative input at 0 V. The LT1004s and associated components form a temperature- compensated reference for the charge pump. The 100-pF capacitor charges to a fixed voltage; hence, the repetition rate is the circuit's only degree of freedom to maintain feedback. Comparator C1 pumps uniform packets of charge to its negative input at a repetition rate precisely proportional to the input voltage-derived current.This action ensures that circuit output frequency is strictly and solely determined by the input volt-age. Start-up or input overdrive can cause the circuit's ac-coupled feedback to latch. If this occurs, C1's output goes low; C2, detecting this via the 2.7-MΩ/0.1-μF lag, goes high. This lifts C1's positive input and grounds the negative input with Q7, initiating normal circuit action.