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AD537 データシート(PDF) 7 Page - Analog Devices |
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AD537 データシート(HTML) 7 Page - Analog Devices |
7 / 9 page AD537 REV. C –6– F-V CONVERTERS The AD537 can be used as a high linearity VCO in a phase- locked loop to accomplish frequency-to-voltage conversion. By operating the loop without a low-pass filter in the feedback path (first-order system), it can lock to any frequency from zero to an upper limit determined by the design, responding in three or four cycles to a step change of input frequency. In practice, the overall response time is determined by the characteristics of the averaging filter which follows the PLL. Figure 7 shows a connection using a low power TTL quad open-collector nand gate which serves as the phase comparator. The input signal should be a pulse train or square wave with characteristics similar to TTL or 5-volt CMOS outputs. Any duty cycle is acceptable, but the minimum pulse width is 40 µs. The output voltage is one volt for a 10 kHz input frequency. The output as shown here is at a fairly high impedance level; for many situations an additional buffer may be required. Trimming is similar to V-F application trimming. First set the VOS trimmer to mid-scale. Apply a 10 kHz input frequency and trim the 2 k Ω potentiometer for 1.00 volts out. Then apply a 10 Hz waveform and trim the VOS for 1 mV out. Finally, retrim the full-scale output at 10 kHz. Other frequency scales can be obtained by appropriate scaling of timing components. 1 2 14 13 5 6 7 10 9 8 3 4 12 11 AD537 DRIVER PRECISION VOLTAGE REFERENCE CURR -TO- FREQ CONV BUF 20k VOS fIN (0-10kHz) 2k 10k 0.001µF 1N4148 0.005µF 3.9k 120k OUTPUT 1V F.S. 0.33µF 10k 10k 74LO3 +5V 10k 9.09k Figure 7. 10 kHz F-V Converter TEMPERATURE-TO-FREQUENCY CONVERSION The linear temperature-proportional output of the AD537 can be used as shown in these applications to perform various direct temperature-to-frequency conversion functions; it can also be used with other external connections in a temperature sensing or compensation scheme. If the sensor output is used externally, it should be buffered through an op amp since loading that point will cause significant error in the sensor output as well as in the main V-F converter circuitry. An absolute temperature (Kelvin)-to-frequency converter is very easily accomplished, as shown in Figure 8. The 1 mV per K out- put serves as the input to the buffer amplifier, which then scales the oscillator drive current to a nominal 298 µA at +25°C (298K). Use of a 1000 pF capacitor results in a corresponding frequency of 2.98 kHz. Setting the single 2 k Ω trimmer for the correct frequency at a well-defined temperature near +25 °C will normally result in an accuracy of ±2°C from –55°C to +125°C (using an AD537S). An NPO ceramic capacitor is recom- mended to minimize nonlinearity due to capacitance drift. DRIVER CURR- TO-FREQ CONV PRECISION VOLTAGE REFERENCE VT VR AD537 +V VTEMP VREF LOGIC GND +VS –VS (CONNECTED TO CASE) 10 9 8 7 6 5 4 3 2 1 9.1k Ω 10k Ω f = 10Hz/K 1000pF 2k Ω BUF Figure 8. Absolute Temperature to Frequency Converter OFFSET TEMPERATURE SCALES Many other temperature scales can be set up by offsetting the temperature output with the voltage reference output. Such a scheme is shown by the Celsius-to-frequency converter in Figure 9. Corresponding component values for a Fahrenheit-to- frequency converter which give 10 Hz/ °F are given in parentheses. 1 2 14 13 5 6 7 10 9 8 3 4 12 11 AD537 10k fOUT 10Hz/ °C (10Hz/ °F) +5V 2.74k (4.02k) 500 Ω 3900 pF (1500pF) 2k 6.04k (10k) 49 Ω (205 Ω) BUF DRIVER CURR- TO-FREQ CONV PRECISION VOLTAGE REFERENCE VT VR Figure 9. Offset Temperature Scale Converters Centigrade and (Fahrenheit) to Frequency A simple calibration procedure which will provide ±2°C accu- racy requires substitution of a 7.27k resistor for the series com- bination of the 6.04k with the 2k trimmer; then simply set the 500 Ω trimmer to give 250 Hz at +25°C. High accuracy calibration procedure: 1. Measure room temperature in K. 2. Measure temperature output at Pin 6 at that temperature. 3. Calculate offset adjustment as follows: Offset Voltage ( mV ) = VTEMP ( Pin 6 ) ( mV ) Room temp ( K ) × 273.2 4. Temporarily disconnect 49 Ω resistor (or 500 Ω pot) and trim 2 k Ω pot to give the offset voltage at the indicated node. Reconnect 49 Ω resistor. 5. Adjust slope trimmer to give proper frequency at room tem- perature (+25 °C = 250 Hz). Adjustment for °F or any other scale is analogous. |
同様の部品番号 - AD537_17 |
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同様の説明 - AD537_17 |
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