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 F 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.