9.1.1 Simplified Voltage-to-Frequency Converter

The operation of these blocks is best understood by going through the operating cycle of the basic V-to-F converter, Figure 13, which consists of the simplified block diagram of the LMx31 and the various resistors and capacitors connected to it.

The voltage comparator compares a positive input voltage, V1, at pin 7 to the voltage, V_x, at pin 6. If V1 is greater, the comparator will trigger the 1-shot timer. The output of the timer will turn ON both the frequency output transistor and the switched current source for a period t = 1.1 R_tC_t. During this period, the current i will flow out of the switched current source and provide a fixed amount of charge, Q = i × t, into the capacitor, C_L. This will normally charge V_x up to a higher level than V1. At the end of the timing period, the current i will turn OFF, and the timer will reset itself.

Now there is no current flowing from pin 1, and the capacitor C_L will be gradually discharged by R_L until V_x falls to the level of V1. Then the comparator will trigger the timer and start another cycle.

The current flowing into C_L is exactly I_AVE = i × (1.1×R_tC_t) × f, and the current flowing out of C_L is exactly V_x/R_L ≃ V_IN/R_L. If V_IN is doubled, the frequency will double to maintain this balance. Even a simple V-to-F converter can provide a frequency precisely proportional to its input voltage over a wide range of frequencies.

9.1.2 Principles of Operation

The LMx31 are monolithic circuits designed for accuracy and versatile operation when applied as voltage-to-frequency (V-to-F) converters or as frequency-to-voltage (F-to-V) converters. A simplified block diagram of the LMx31 is shown in Figure 13 and consists of a switched current source, input comparator, and 1-shot timer.

Figure 13. Simplified Block Diagram of Stand-Alone
Voltage-to-Frequency Converter and
External Components

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)

9.2.1.3 Application Curve

Figure 15. Output Non-Linearity Error vs. Frequency

9.2.2 Precision V-To-F Converter

In this circuit, integration is performed by using a conventional operational amplifier and feedback capacitor, C_F. When the integrator's output crosses the nominal threshold level at pin 6 of the LMx31, the timing cycle is initiated.

The average current fed into the summing point of the op-amp (pin 2) is i × (1.1 R_tC_t) × f which is perfectly balanced with −V_IN/R_IN. In this circuit, the voltage offset of the LMx31 input comparator does not affect the offset or accuracy of the V-to-F converter as it does in the stand-alone V-to-F converter; nor does the LM231/331 bias current or offset current. Instead, the offset voltage and offset current of the operational amplifier are the only limits on how small the signal can be accurately converted. Since op-amps with voltage offset well below 1 mV and offset currents well below 2 nA are available at low cost, this circuit is recommended for best accuracy for small signals. This circuit also responds immediately to any change of input signal (which a stand-alone circuit does not) so that the output frequency will be an accurate representation of V_IN, as quickly as the spacing of the 2 output pulses can be measured.

In the precision mode, excellent linearity is obtained because the current source (pin 1) is always at ground potential and that voltage does not vary with V_IN or f_OUT. (In the stand-alone V-to-F converter, a major cause of non-linearity is the output impedance at pin 1 which causes i to change as a function of V_IN).

The circuit of Figure 17 operates in the same way as Figure 16, but with the necessary changes for high-speed operation.

*Use stable components with low temperature coefficients.
**This resistor can be 5 kΩ or 10 kΩ for V_S = 8 V to 22 V, but must be 10 kΩ for V_S = 4.5 V to 8 V.
***Use low offset voltage and low offset current op-amps for A1: recommended type LF411A

Figure 16. Standard Test Circuit and Applications Circuit, Precision Voltage-to-Frequency Converter

9.3.1 F-to-V Converters

In these applications, a pulse input at f_IN is differentiated by a C-R network and the negative-going edge at pin 6 causes the input comparator to trigger the timer circuit. Just as with a V-to-F converter, the average current flowing out of pin 1 is I_AVERAGE = i × (1.1 R_tC_t) × f.

In the simple circuit of Figure 18, this current is filtered in the network R_L = 100 kΩ and 1 μF. The ripple will be less than 10-mV peak, but the response will be slow, with a 0.1 second time constant, and settling of 0.7 second to 0.1% accuracy.

In the precision circuit, an operational amplifier provides a buffered output and also acts as a 2-pole filter. The ripple will be less than 5-mV peak for all frequencies above 1 kHz, and the response time will be much quicker than in Figure 18. However, for input frequencies below 200 Hz, this circuit will have worse ripple than Figure 18. The engineering of the filter time-constants to get adequate response and small enough ripple simply requires a study of the compromises to be made. Inherently, V-to-F converter response can be fast, but F-to-V response can not.

100 kHz Full-Scale, ±0.03% Non-Linearity
*Use stable components with low temperature coefficients.
**This resistor can be 5 kΩ or 10 kΩ for V_S=8V to 22V, but must be 10 kΩ for V_S=4.5V to 8V.
***Use low offset voltage and low offset current op-amps for A1: recommended types LF411A or LF356.

Figure 17. Precision Voltage-to-Frequency Converter

10 kHz Full-Scale With 2-Pole Filter, ±0.01% Non-Linearity Maximum
*Use stable components with low temperature coefficients.

Figure 19. Precision Frequency-to-Voltage Converter,

Figure 21. Temperature to Frequency Converter

Figure 23. Basic Analog-to-Digital Converter Using
Voltage-to-Frequency Converter

Figure 25. Remote Voltage-to-Frequency Converter With 2-Wire Transmitter and Receiver

Figure 27. Voltage-to-Frequency Converter With Isolators

Figure 29. Voltage-to-Frequency Converter With Isolators

10 kHz Full-Scale, ±0.06% Non-Linearity
*Use stable components with low temperature coefficients.

Figure 18. Simple Frequency-to-Voltage Converter

*L14F-1, L14G-1 or L14H-1, photo transistor (General Electric Co.) or similar

Figure 20. Light Intensity to Frequency Converter

Figure 22. Long-Term Digital Integrator Using VFC

Figure 24. Analog-to-Digital Converter With Microprocessor

Figure 26. Voltage-to-Frequency Converter With Square-Wave Output Using ÷ 2 Flip-Flop

Figure 28. Voltage-to-Frequency Converter With Isolators

Figure 30. Voltage-to-Frequency Converter With Isolators