Frequently Asked Questions about FV Converters

The approximate resolution of FV conversion is shown in the table below.

I will explain using the example of 1500 Hz.

The clock for periodic measurement is 20 MHz. The count value for the input frequency is;

20000000 ÷ (input frequency) = count value

At 1500Hz, 20000000 ÷ 1500 = 13333.33

The count value is an integer, so the resolution will be either 13333 or 13334. When this count value is converted back to frequency;

20000000÷13333＝1500.0375
20000000÷13334＝1499.925

This difference is 1500.0375 - 1499.925 = 0.1125 Hz

Therefore, it can be said that there is a resolution of approximately 0.12 Hz around 1500 Hz. It is important to note that there is a measurement error of 0.0375 Hz at 1500 Hz. Taking into account the precision of the crystal oscillator, the accuracy becomes 0.12 Hz plus an additional factor.

In the case of deviation output

In the case of deviation output, the resolution is the same, but the output voltage per unit of resolution can be larger.

Low frequency accuracy

The inverse of the count value is converted to voltage and output using a 16-bit DA converter.
The frequency at which the 16-bit count value becomes full is 305.1 Hz.
At frequencies below 305.1 Hz, it exceeds 16 bits, so the lower bits of the binary counter are cut off, and calculations are made using the upper 16 bits.
Therefore, the accuracy in this case is;

Accuracy = Input Frequency ÷ 2 ¹⁵

will become.

Accuracy as stated in the catalog.

Voltage output 0.1 % FS
Current output 0.7 % FS

The precision is worst at 20 kHz, so the resolution and the accuracy of the analog circuit at this time are documented.

It can be modified to an input frequency of up to 50 kHz. In this case, the resolution will also be considered similarly.