As presented in a previous article, TSC9402 chip produced by Teledyne Semiconductor, may be particularly suitable for implementation, with a low cost frequency voltage converter and voltage-frequency converter.
For this electronic scheme input current is 10 uA, input voltage range is between 10 mV and 10 V, while the output frequency can be set between 10 Hz and 10 kHz.
Circuit has two outputs, one at pin 8 provides a short pulse whose period is proportional to input current, while that of the pin 10 delivers a square wave whose frequency is half the pulse to pin 8.
Calibration is accomplished by connecting a mounting a Frequencymeter to pin 8 and a voltage of exactly 10 mV,at input, and then will adjust P1 until you get exactly 10 Hz output. Then, connect a signal source, with exactly 10 V, input and verify that the output signal has a frequency of 10 kHz. If this frequency can not be reached, mount in parallel with C1 a trimer or replace R1 with a resistance of 820 k and 250 k semireglabil.