Voltage-To-Frequency Converter Ii
Posted on Jun 2, 2012 4109
This voltage-to-frequency converter (VFC) accepts the bipolar-ac inputs. For -10- to + 10-V inputs, the c
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onverter produces a proportional 0- to 10-kHz output. Linearity is 0.04%, and temperature coefficient (TC) measures about 50 ppm/Â°C. To understand the circuit, assume that its input sees a bipolar square wave. During the input"s positive phase, ICl"s output swings negative and drives current through CI via the full-wave diode bridge. ICl"s current causes Cl"s voltage to ramp up linearity. Instrumentation amplifier IC2 operates at a gain of 10 and measures the differential voltage across CI. IC2"s output biases comparator IC3"s negative input. When IC2"s output crosses zero, IC3 fires ac positive feedback to IC3"s positive input and hangs up IC3"s output for about 20 /as. The Q1 level shifter drives ground-referred inverters IC5A and IC5B to deliver biphase drive to LT1004 switch IC6. IC6, configured as a charge pump, places C2 across CI each time the inverters switch, which resets CI to a lower voltage. The LT1004 reference (Dl), along with C2"s value, determines how much charge the charge pump removes from CI each time the charge pump cycles. Thus, each time IC2"s output tries to cross zero, the charge pump switches C2 across CI, which resets CI to a small negative voltage and forces IC1 to begin recharging CI. The frequency of this oscillatory behavior is directly proportional to the input-derived current into IC1. During the time that CI is ramping toward zero, IC6, places C2 across the reference diode (Dl), and prepares C2 for the next discharge cycle. The action is the same for negative-input excursions, except that ICl"s output phasing is reversed. IC2, looking differentially across ICl"s diode bridge, sees the same signal as it does for positive inputs; therefore, the circuit"s action is identical. IC4, detecting ICl"s output polarity, provides a signal bid output.