LM231, LM331

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SNOSBI2C – JUNE 1999 – REVISED SEPTEMBER 2015

9.3 System Examples

9.3.1 F-to-V Converters

In these applications, a pulse input at fIN 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 IAVERAGE = i × (1.1 RtCt) × f.

In the simple circuit of Figure 18, this current is filtered in the network RL = 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.

10 kHz Full-Scale, ±0.06% Non-Linearity

*Use stable components with low temperature coefficients.

100 kHz Full-Scale, ±0.03% Non-Linearity

*Use stable components with low temperature coefficients.

**This resistor can be 5 k

Ω or 10 kΩ for VS=8V to 22V, but must be

10 k

Ω for VS=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

Figure 18. Simple Frequency-to-Voltage Converter

Converter

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