Stable differential voltage to frequency converter with low supply voltage and frequency offset control

In this paper, the circuit for a new bipolar differential voltage-to-frequency converter is presented. The circuit operation and the calculation of the operating frequency are described. The circuit design is also realized using Zetex transistor array integrated circuits and a 3.3-V power supply. This circuit incorporates an adjustable operating frequency via an external capacitor. The operating frequency varied from 15 to 368 kHz by changing the external capacitor from 1 μF to 22 pF. The circuit was tested with an applied differential voltage of ±15 mV. The deviation about the center frequency changed from ~±2.1 Hz to ~±38.22 kHz as the external capacitor was varied over the same range. The frequency offset control feature was implemented using a 4-bit current DAC (iDAC). As the iDAC input code was increased from 0 to 1111, the operating frequency varied quite linearly from ~156.5 to ~77.9 kHz. Thermal simulations with worst-case analysis were performed in PSPICE in order to estimate the thermal coefficient of frequency for the circuit. These simulations yielded a TC of ±192 ppm/°C for the operating frequency