A cheap 100KHZ VF converter made of a single IC

A cheap 100KHZ VF converter made of a single IC

Function of the circuit

In the past, when FF converters were mentioned, hybrid integrated components were often used. However, this circuit uses a monolithic VF converter, which is a cheap 100KHZ VF converter standard circuit. It converts the 0~10V input voltage into a 0~100KHZ pulse train. The analog signal can be isolated by using an optical coupler or optical fiber transmission. In addition, in order to improve the linearity of the VF converter, an OP amplifier A1 is added to the circuit.

How the Circuit Works

LM331 is a monolithic VF converter consisting of a 1.9V reference voltage, current switch, comparator, bistable multivibrator, etc. Even without an additional amplifier, a linearity of 0.03% can be obtained, but in order to expand the range, a FET input OP amplifier is added.

The reference current IR is determined by the resistor connected to lead 2. Because the internal reference voltage is 1.9V, IR=1.9/R, which can usually be selected in the range of 100~500UA. In addition, the current at the output of the current switch, its average value IO is equal to the input current IIN.

In the internal charging circuit, once the charging voltage is equal to 2/3 of the power supply voltage VCC, the circuit is reset, so the pulse width T=1.1C3.R4, because the average current I*1.1*C3.R4.FO is proportional to the oscillation frequency. Based on the above relationship, the oscillation frequency FO and the timing constant C3.R4 can be calculated by the following formula:

Considering the timing lag, a timing constant slightly smaller than 2.5us should be used, C3 is 330pF, and R4 is 6.8k. According to the above formula, fine-tuning any one of R1, R4, and R6 can adjust the full-scale frequency, but this circuit only changes R1 to a variable resistor for adjustment.

Capacitor C2 is used as an average capacitor. If the capacity increases, the response to the input voltage will become slower, so the capacity should be appropriate. D1 is a clamping diode to prevent the negative potential from being added to lead 7. Resistors R2 and R3 divide the power supply voltage by 1/2 to form a reference voltage, which is not necessarily 10K.

The output is open collector, so a load resistor must be added when driving a logic IC, and a current limiting resistor should be connected in series when directly driving an optocoupler.

Component Selection

The frequency stability of LM331 is very high, which is plus or minus 30PPM/degree. The timing capacitor can use mica capacitor with small temperature coefficient and be used under 10KHZ full range conditions. Because C3=0.01UF, styrene capacitor is suitable.

Adjustment

First, use variable resistor VR2 to adjust the offset. Connect a load resistor to the output end, use an oscilloscope to observe the oscillation output, adjust VR2 to reduce the oscillation frequency, and take the stop of oscillation as zero. Then connect a -10V constant voltage source to the input end, and adjust VR2 to make the oscillation frequency full scale. Then gradually reduce the input voltage and frequency value in units of 1V to check the linearity.