Inexpensive VFC features good linearity and dynamic range

Design Idea reveals a VFC (voltage-to-frequency converter) circuit that provides good performance at a low price. You can obtain all of the parts for a few dollars from a local electronics shop.

 

Figure 1.	Three inexpensive ICs and a few passive components make a VFC with good linearity, speed, and dynamic range.

The circuit has high input impedance, works with a single power supply, and connects directly to microcontrollers. The linearity error is less than 0.1% for frequencies as high as 700 kHz, and the dynamic range is 60 dB. The circuit exploits the integrator, comparator, and one-shot architecture (Figure 1). The output frequency is proportional to the input voltage:

 ,

where V_CC is the power supply, 5V, and t_OS is the duration of the pulse that the one-shot generates, according to the equation t_OS=0.7×R_OS×C_OS. You must filter and regulate the power supply, V_CC. If the magnitude of the power supply changes, the slope of the calibration curve also changes. The components you use for the integrator, C_INT and R_INT, do not participate in the equation so they need not be either accurate or stable. However, capacitors C_INT and C_OS must have low dielectric absorption.

You build a start-up circuit with switch S₁ and the timing network comprising R₁, C₁, and R₂. This step ensures that the circuit will oscillate with any value of input voltage. After you turn on the power supply, the switch stays closed for approximately 1 sec, keeping C_INT completely discharged. When the switch opens, C_INT starts charging by a fixed current, which the magnitude of the input voltage defines. The result is a rising ramp at the integrator’s output. When the ramp reaches 2.5V, IC₂ generates a pulse because 2.5V is the threshold level of the Schmitt trigger at the 1B input of IC₂. Because the pulse magnitude is larger than the input voltage, the current through C_INT reverses, and C_INT partially discharges (Figure 2).

 

Figure 2.	Because the pulse magnitude is larger than the input voltage, the current through CINT reverses, and CINT partially discharges.

When the pulse is over, the integrator starts another rising ramp, and the cycle repeats. Because of the built-in Schmitt trigger, the circuit requires no separate comparator IC. Most applications can go without any adjustment. You adjust the full-scale frequency using only the trimming potentiometer, which is part of R_OS in Figure 1.

You can select different frequency spans (Table 1), each requiring its own values for C_INT and R_OS. The spans have different linearity. The table shows the linearity error as a percentage of the full-scale frequency for 11 equally spaced values of the input value in a range from 2 mV to 2V.

 

Table 1. Performance at different frequency spans

Maximum frequency Duration of tOS ROS value CINT value Linearity (kHz) (μsec) (kΩ) (pF) (% of full-scale) 50 8 57.2 400 ±0.044 100 4 28.6 200 ±0.056 200 2 14.2 100 ±0.021 400 1 7.15 50 ±0.031 600 0.67 4.77 33 ±0.066 800 0.5 3.58 25 ±0.11 1000 0.4 2.86 20 ±0.42

References

1. Williams, Jim, “0.02% V/F converter consumes only 26 μA,” EDN, July 4, 1996.
2. Williams, Jim, “1-Hz to 100-MHz VFC features 160-dB dynamic range,” EDN, Sept 1, 2005, pg 82.
3. “LM231A/LM231/LM331A/LM331 precision voltage-to-frequency converters,” National Semiconductor, April 2006.
4. Pease, Robert A, “Wide-Range Current-to-Frequency converters,” AN-240, National Semiconductor, May 1980.
5. Pease, Robert A, “New Phase- Locked-Loops Have Advantages as Frequency-to-Voltage Converters (and more),” AN-210, National Semiconductor, April 1979.