This Idea for Design was originally published May 13, 1993, p. 70.
A single-supply, 5-V powered, voltage-to-frequency (V/F) converter can be built with low-power ICs and just a handful of passive components (see the figure). A precision high-impedance, 0-V, common-mode-capable input voltage-to-current interface is provided by U1 (which is an AD820 JFET-input, single-supply op amp). The 2-pA bias current of U1 allows megohm-range source impedances with negligible dc error, while the single-supply U2 is used for V/F conversion. These measures combine to keep the overall current budget typically at about 3 mA, operating from a 5-V supply.
U2 (an AD654 V/F converter) is a precision emitter-coupled multivibrator that operates on a single supply. In V/F operation, the output current of Q1, It, drives U2 to produce a controlled output frequency.
At the same time, the components around U1 act to scale this drive current proportional to the input voltage VIN. With the loop operating, the V/F converter follows VIN over a 3-decade or more range with low nonlinearity. The expression for output frequency Fout is:
Fout = VIN /[10 × C1 × (R1 + R2)]
As scaled here, a 500-mV full-scale input produces 50-kHz frequency (100 Hz/mV scale factor), using a 1-nF C1. Stable low-TC components should be used for C1 (such as NP0 or C0G) and R1 (±50 ppm/°C metal film). Overall scaling is trimmed by R2 (Full-scale adjust), a multiturn film trimmer.
The circuit operates over nearly 3 decades without offset trim, as defined by the U1 1-mV offset. But, with trimmer R2 used (Low-frequency adjust), range can be extended to 4 decades, so this trim will obtain the best results.
The circuit performs quite well, as is evident by its nonlinearity of ±0.02% of full scale over the 0.5-mV-to-0.5-V range after trim. Supply sensitivity measures about 0.01% of full-scale frequency shift for a 100-mV supply change.
Input sources to the V/F converter can be high in impedance as the noninverting input to U1 loads the source by only 2 pA. Therefore, while the circuit has a basic 0.5-V full-scale input range, scaling for higher input ranges is provided simply by adding an appropriate input driver. For example, a standard 10-MΩ, 10/1 divider will work well for a 5-V full-scale range, and can retain the low-source-loading feature.
Negative input voltage ranges can be accommodated by grounding the U1 (+) input and applying a negative voltage as noted in the figure; similar steps are taken for calibration.
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