LM331 High accuracy in low frequencies

Thread Starter


Joined May 20, 2019
Hi team,

I'm trying to measure the frequency in the range from 100 Hz to 11 KHz. But I need high accuracy in the range from 100 Hz to 200 Hz (± 4 Hz).

I made "Figure 2" circuit from the application note SNOA734B from TI, which adds a PNP transistor to acts as a cascade in order to improve nonlinearity. It works very well but in low frequencies I get an error of 10Hz and I need more accuracy.

Any idea of how to get less error without adding an active filter?

PS: the output range goes from 0 V to 5 V so when I introduce a 100 Hz signal instead of getting 50 mV I get 55 mV. The voltimeter has an accuracy of 5 uV with 6 1/2 digits so thats not the problem.


Joined Mar 10, 2018
This can be done fairly easily with an accuracy < 1% with a microcontroller
and a crystal for time accuracy. Use its PWM or DAC to perform the V trans-
lation to the frequency

Regards, Dana.


Joined Apr 5, 2008

When I use the formula on the posted picture I get a value of 16.66 K.
Are Rt and Ct of high quality as described?



Joined Apr 7, 2016
I am missing some more numbers to determine if it's gain, offset or nonlinear error.

Output voltages at 0, 50, 100, 150, 200, 500, 1000, 2000 and 4000 Hz must be able to determine what the problem is.