Voltage to Frequency / Frequency to Voltage Converter

www.DataSheet4U.com AN795 Voltage-to-Frequency/Frequency-to-Voltage Converter Author: Michael O. Paiva, Microchip Technology, Inc. RATIOMETRIC MEASUREMENT (ANALOG DIVISION) One of the most difficult circuits to build is one which will divide one analog signal by another. Two voltage-to-frequency (V/F) converters can do such division with ease. The numerator is counted directly as a signal, while the denominator forms the time base. Latch V1 V2 TC9400 V/F TC9400 V/F ÷N One Shot Counter One Shot Reset Latch V Output = N 1 V2 FIGURE 1: Ratiometric measurement (analog division). RPM/SPEED INDICATOR Flow rates and revolutions per second are nothing more than frequency signals, since they measure the number of events per time period. Optical and magnetic sensors will convert these flows and revolutions into a digital signal which, in turn, can be converted to a proportional voltage by the use of a frequency-to-voltage (F/V) converter. A simple voltmeter will then give a visual indication of the speed. Analog Display Speed Sensor TC9400 F/V RPM DVM Display (Optical or Magnetic) RPM FIGURE 2: RPM/speed indicator. © 2002 Microchip Technology, Inc. DS00795A-page 1 www.DataSheet4U.com AN795 MOTOR SPEED CONTROL The motor's speed is measured with the F/V converter, which converts RPM into a proportional voltage. This voltage is used in a negative feedback system to maintain the motor at the controlled setting. V+ Speed Set Motor Op Amp TC9400 F/V FIGURE 3: Motor speed control. PROPORTIONAL FLOW-RATE CONTROLLER A TC9400 F/V converter can be used to regulate the amount of liquid or gas flowing through a pipeline. The flow-rate detector generates a pulse train whose frequency is proportional to the rate of flow through it. The F/V converts this frequency to a proportional analog voltage which is used to drive the valve controller. The valve controller regulates the valve so that the flow is steady, even though pipeline pressure goes up and down. A voltmeter connected to the F/V converter output will indicate the actual instantaneous flow rate. Valve Flow Set Valve Controller Flow Rate Meter FIGURE 4: Proportional flow-rate controller. DS00795A-page 2 – + Pulse Type Tachometer (Optical or Magnetic) Flow Rate Detector Pulse Output TC9400 F/V © 2002 Microchip Technology, Inc. www.DataSheet4U.com AN795 TEMPERATURE METER A temperature meter using the voltage output of a probe, such as one of the three shown, can be economically and straightforwardly implemented with the TC9400 V/F converter. The V/F output is simply counted to display the temperature. For long-distance data transmission, the TC9400 can be used to modulate an RF transmitter. Preamp Temp Probe TC9400 V/F Gate 50/60Hz One Shot Temperature Display Gate Latch Latch One Shot Reset Reset Temperature Probes A. Thermocouple Preamp B. Thermistor Preamp C. Transistor Junction Preamp FIGURE 5: Temperature meter. A/D CONVERSION WITH A MICROCONTROLLER There are two schemes that can be utilized to accomplish A/D conversion with a microcontroller: 1. 2. Depending on the number of digits of resolution required, VIN is measured by counting the FOUT frequency for 1ms, 10ms, 100ms, or 1 second. The final count is then directly proportional to VIN. (The microcontroller provides the time base.) VIN is measured by determining the time between two pulses (negative edges). FOUT is used as a gate for counting the microcontroller's clock. The final count will then be inversely proportional to VIN. By taking the one's complement (changing 1's to 0's and 0's to 1's) of the final binary count, a value directly proportional to VIN will result. This technique will give a faster conversion time when resolution is very important, but dynamic range is limited. VIN TC9400 V/F FOUT PIC Microcontroller Digital Output FIGURE 6: A/D conversion with a microcontroller. © 2002 Microchip Technology, Inc. DS00795A-page 3 www.DataSheet4U.com AN795 13-BIT A/D CONVERTER A 13-bit binary A/D converter can be built by combining the TC9400 V/F converter with a counter, latch, and time base. When the V/F converter is set up for 10kHz full scale, a 1-second time base will provide one conversion per second. 1MΩ Gate TC9400 V/F 13-bit Binary Counter Reset Latch VIN Time Base 13-bit Latch Bit 12 1110 9 8 7 6 5 4 3 2 1 0 FIGURE 7: 13-Bit A/D converter. 4-DIGIT VOLTMETER WITH OPTOISOLATED INPUT The use of a frequency counter will give a display of the V/F converter's frequency, which is directly proportional to the input voltage. When the V/F converter is running at 10kHz full scale, a 1-second time base will give 4-digit resolution with 1 reading per second. The optoisolator is used for transmitting the frequency, so there is no DC path to the frequency counter. This is especially useful in medical applications, where a voltage probe should not be directly connected to the human body. V+ 1MΩ + VIN TC9400 V/F Battery or Transformer Isolated Supply Frequency Counter FIGURE 8: 4-Digit voltmeter with optoisolated input. LONG-TERM INTEGRATOR WITH INFINITE HOLD This system will integrate an input signal for minutes or days, and hold its output indefinitely. The data is held in a digital counter and stays there until the counter is reset. Typical applications involve controlling the amount of surface metal deposited in a plating system or how much charge a battery has taken on. Digital Display VIN TC9400 V/F VOUT α Binary or BCD Counter Reset ∫ t o VIN dt D/A Converter VOUT FIGURE 9: Long-term integrator with infinite hold. DS00795A-page 4 © 2002 Microchip Technology, Inc. www.DataSheet4U.com AN795 LONG-TERM INTEGRATOR FOR BIPOLAR (±) SIGNALS When the input signal is negative as well as positive, there has to be a way of generating "negative" frequencies. An absolute value circuit accomplishes this by giving the V/F converter a positive voltage only; and also telling the counter to count up for a positive voltage and to count down for a negative voltage. 1MΩ 47kΩ 500kΩ TC9400 V/F Up/Down Counter Up/Down Reset VIN 47kΩ – + Op Amp – + Op Amp Absolute Value Circuit FIGURE 10: Long-term integrator for bipolar (±) signals. ANALOG SIGNAL TRANSMISSION OVER TELEPHONE LINES The TC9400's square-wave output is ideal for transmitting analog data over telephone lines. A square wave is actually preferred over a pulse waveform for data transmission, since the square wave takes up less frequency spectrum. The square wave's spectrum can be further reduced by use of low-pass filters. At the other end of the telephone line, the TC9400 converts the frequency signal back into a voltage output linearly proportional to the original input voltage. VIN 9400 V/F Telephone Telephone TC9400 F/V VOUT System Linearity ~ 0.03% FIGURE 11: Analog signal transmission over telephone lines. © 2002 Microchip Technology, Inc. DS00795A-page 5 www.DataSheet4U.com AN795 TELEMETRY In a telemetry system, the TC9400 converts the analog input (VIN) into frequencies (10Hz to 100kHz) which can be used to modulate an RF transmitter. At the other end, a receiver picks up the RF signal and demodulates it back into the 10Hz to 100kHz spectrum. A frequency counter connected to this signal then gives a count linearly proportional to the original analog voltage (VIN). If a linearly-proportional analog output voltage is required, the counter can be replaced by a TC9400 used in the F/V mode. Digital Display VIN TC9400 V/F RF Transmitter RF Receiver Counter Gate Latch Reset Time Base FIGURE 12: Telemetry. HIGH NOISE IMMUNITY DATA TRANSMISSION When transmitting analog data over long distances, it is advantageous to convert the analog signal into a digital signal, which is less susceptible to noise pick-up. In the system shown below, the TC9400 converts the input voltage into a pulse or square wave which is transmitted on a pair of wires by use of a line driver and receiver. At the other end, the original voltage (VIN), can be digitally displayed on a frequency counter or converted back to an analog voltage by use of a TC9400 F/V converter. Digital Display VIN TC9400 V/F Differential Driver Twisted Pair Cable Counter Differential Line Receiver Gate Latch Reset Time Base Analog Display 9400 F/V FIGURE 13: High noise immunity data transmission. DS00795A-page 6 © 2002 Microchip Technology, Inc. www.DataSheet4U.com AN795 DC RESPONSE DATA RECORDING SYSTEM Low-frequency analog data (DC to 10kHz) can be recorded anywhere, stored, and then reproduced. By varying the playback speed, the frequency spectrum of the original data can be shifted up or down. V1 TC9400 L V/F L TC9400 F/V V1 V2 TC9400 R V/F Cassette or Reel-to-Reel Recorder R TC9400 F/V V2 FIGURE 14: DC response data recording system. FREQUENCY SHIFT KEYING (FSK) GENERATION AND DECODING Frequency Shift Keying (FSK) is a simple means of transmitting digital data over a signal path (two wires, telephone lines, AM or FM transmitters). Typically, only two frequencies are transmitted. One corresponds to a logical "0," the other to a logical "1." A TC9400 V/F converter will generate these two frequencies when connected as shown below. The potentiometer sets the V/F converter to the lower frequency. The digital input then determines which frequency is selected. A "0" selects the lower frequency, a "1" selects the upper frequency. The digital frequency signal is converted back into a digital format by a TC9400 used in the F/V mode. V+ Center Frequency V+ Frequency Offset Input 0 1 00 1 0 0 TC9400 V/F TC9400 F/V Digital Output 0 00 0 1 1 FIGURE 15: Frequency Shift Keying (FSK) generation and decoding. ULTRALINEAR FREQUENCY MODULATOR Since the TC9400 is a very linear V/F converter, an FM modulator is very easy to build. The potentiometer determines the center frequency, while VIN determines the amount of modulation (FM deviation) around the center frequency. VIN can be negative as well as positive. V+