a FEATURES Low Cost Single or Dual Supply, 5 V to 36 V, ؎5 V to ؎18 V Full-Scale Frequency Up to 500 kHz Minimum Number of External Components Needed Versatile Input Amplifier Positive or Negative Voltage Modes Negative Current Mode High Input Impedance, Low Drift Low Power: 2.0 mA Quiescent Current Low Offset: 1 mV
Low Cost Monolithic Voltage-to-Frequency Converter AD654 FUNCTIONAL BLOCK DIAGRAM
The AD654 is a monolithic V/F converter consisting of an input amplifier, a precision oscillator system, and a high current output stage. A single RC network is all that is required to set up any full scale (FS) frequency up to 500 kHz and any FS input voltage up to ± 30 V. Linearity error is only 0.03% for a 250 kHz FS, and operation is guaranteed over an 80 dB dynamic range. The overall temperature coefficient (excluding the effects of external components) is typically ± 50 ppm/°C. The AD654 operates from a single supply of 5 V to 36 V and consumes only 2.0 mA quiescent current.
1. Packaged in both an 8-pin mini-DIP and an 8-pin SOIC package, the AD654 is a complete V/F converter requiring only an RC timing network to set the desired full-scale frequency and a selectable pullup resistor for the open-collector output stage. Any full scale input voltage range from 100 mV to 10 volts (or greater, depending on +VS) can be accommodated by proper selection of the timing resistor. The fullscale frequency is then set by the timing capacitor from the simple relationship, f = V/10 RC.
The low drift (4 µV/°C typ) input amplifier allows operation directly from small signals such as thermocouples or strain gauges while offering a high (250 MΩ) input resistance. Unlike most V/F converters, the AD654 provides a square-wave output, and can drive up to 12 TTL loads, optocouplers, long cables, or similar loads.
2. A minimum number of low cost external components are necessary. A single RC network is all that is required to set up any full scale frequency up to 500 kHz and any full-scale input voltage up to ± 30 V. 3. Plastic packaging allows low cost implementation of the standard VFC applications: A/D conversion, isolated signal transmission, F/V conversion, phase-locked loops, and tuning switched-capacitor filters. 4. Power supply requirements are minimal; only 2.0 mA of quiescent current is drawn from the single positive supply from 4.5 volts to 36 volts. In this mode, positive inputs can vary from 0 volts (ground) to (+VS –4) volts. Negative inputs can easily be connected for below ground operation. 5. The versatile open-collector output stage can sink more than 10 mA with a saturation voltage less than 0.4 volts. The Logic Common terminal can be connected to any level between ground (or –VS) and 4 volts below +VS. This allows easy direct interface to any logic family with either positive or negative logic levels.
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