LM231, LM331

SNOSBI2C – JUNE 1999 – REVISED SEPTEMBER 2015

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9.2 Typical Applications

9.2.1 Basic Voltage-to-Frequency Converter

The simple stand-alone V-to-F converter shown in Figure 14 includes all the basic circuitry of Figure 13 plus a

few components for improved performance.

*Use stable components with low temperature coefficients. See Application Information.

**0.1

μF or 1 μF, See Typical Applications.

Figure 14. Simple Stand-Alone V-to-F Converter

with ±0.03% Typical Linearity (f = 10 Hz to 11 kHz)

9.2.1.1 Design Requirements

For this example, the system requirements are 0.05% linearity over an output frequency range of 10 Hz to 4 kHz

with an input voltage range of 25 mV to 12.5 V. The available supply voltage is 15.0 V.

9.2.1.2 Detailed Design Procedure

A capacitor CIN is added from pin 7 to ground to act as a filter for VIN, use of a 0.1 μF is appropriate for this

application. A value of 0.01

μF to 0.1 μF will be adequate in most cases; however, in cases where better filtering

is required, a 1-

μF capacitor can be used. When the RC time constants are matched at pin 6 and pin 7, a voltage

step at VIN will cause a step change in fOUT. If CIN is much less than CL, a step at VIN may cause fOUT to stop

momentarily.

Next, we cancel the comparator bias current by setting RIN to 100 kΩ to match RL. This will help to minimize any

frequency offset.

For best results, all the components should be stable low-temperature-coefficient components, such as metal-film

resistors. The capacitor should have low dielectric absorption; depending on the temperature characteristics

desired, NPO ceramic, polystyrene, Teflon or polypropylene are best suited.

The resistance RS at pin 2 is made up of a 12-kΩ fixed resistor plus a 5-kΩ (cermet, preferably) gain adjust

rheostat. The function of this adjustment is to trim out the gain tolerance of the LMx31, and the tolerance of Rt,

RL and Ct.

12

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