How to use AD654 voltage to frequency converter

　　A voltage frequency converter (VFC) is an oscillator that outputs a square wave whose frequency is linearly proportional to its input voltage. The output square wave can be fed directly to a microcontroller's digital pin to accurately measure the DC input voltage, meaning the input voltage can be measured using an 8051 or any other microcontroller without a built-in ADC.

　　VFCs are often mistaken for voltage-controlled oscillators (VCOs), but VFCs have many advantages and enhanced performance specifications that (VCOs) do not have, such as dynamic range, low linearity error, temperature and supply voltage stability, and more.

　　Here IC AD654 is used in this circuit to demonstrate operation, this is a monolithic voltage to frequency converter. An oscilloscope is also used to display the output square wave.

　　Integrated circuit AD654

　　AD654 is a voltage-to-frequency converter IC in an 8-pin DIP package. It consists of an input amplifier, a very accurate built-in oscillator and a high current open collector output driver, which allows the IC to drive up to 12 TTL loads, optocouplers, long cables or similar loads and can 5-30) Volts. Another thing to mention is that unlike other ICs, the AD654 IC outputs a square wave, so it is easy for the microcontroller to measure the readings. Some of the chip's most interesting features are listed below.

　　feature:

　　Wide input voltage ±30 V

　　Full scale frequency up to 500 kHz

　　High input impedance of 125MΩ,

　　Low drift (4 µV/°C)

　　2.0mA quiescent current

　　Low offset 1 mV

　　Minimum requirements for external components

　　Required components

　　Schematic diagram, schematic diagram

　　The schematic of this voltage to frequency converter circuit is taken from the datasheet and some external components are added to modify the circuit for this demonstration

　　The circuit is built on a solderless breadboard with its components as shown in the picture. For demonstration purposes, a potentiometer is added to the input section of the amplifier to vary the input voltage, whereby we can observe the change in the output.

　　notes! All components are placed as close as possible to reduce the inductance and resistance of parasitic capacitance.

　　How does the device function?

　　An internal op amp is used as the input, and when 1mA drive current is supplied to the frequency converter, it converts the input voltage into drive current for the NPN follower. It charges an external timing capacitor, a scheme that allows the oscillator to provide nonlinearity over a total voltage range of 100 nA to 2mA. This output also goes into an output driver which is just a NPN power transistor with an open collector from which we get the output

　　calculate

　　To theoretically calculate the output frequency of the circuit, the following formula can be used

　　Fout = Vin / 10*Rt*Ct

　　Where,

　　Fout is the output frequency

　　Vin is the input voltage of the circuit,

　　Rt is the resistance of the RC oscillator

　　Ct is the capacitance of the Rc oscillator

　　For example,

　　Vin is 0.1V or 100mV

　　Rt is 10000K or 10K

　　Ct is 0.001uF or 1000pF

　　Output = 0.1 / (10 * 10 * 0.001)

　　Output = 1 KHz

　　So if 0.1V is applied to the input of the circuit we will get 1kHz at the output

　　Voltage to frequency converter testing

　　In order to test the circuit, use the following tools

　　12V switching power supply (SMPS)

　　Meco 108B+ multimeter

　　Hantech 600BE USB PC Oscilloscope

　　To build the circuit, 1% metal film resistors were used and the tolerances of the capacitors were not taken into account. The room temperature during the test was 22 degrees Celsius

　　Test setup

　　As you can see the DC input voltage is 11.73 V

　　The voltage at the IC input is 104.8 mV

　　Here you can see the output on my DSO is 1.045 kHz. Where multiple inputs are given and the frequency changes with the ratio of the input voltages.

　　further enhance

　　Stability can be improved by making the circuit on a PCB, and accuracy can be improved by using resistors and capacitors with 0.5% tolerance. The most important part of this circuit is the RC oscillator section, so the RC oscillator must be placed as close as possible to the input pins, otherwise the PCB traces or the startup capacitance and resistance of the components may reduce the accuracy of the circuit.

　　application

　　This is a very useful IC that can be used in many applications, some of which are listed below

　　AD654 VFC as ADC

　　Frequency multiplier

　　Temperature sensor with thermocouple

　　Strain gauge

　　function generator

　　Self-biased precision clock