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AD650 데이터시트(PDF) 10 Page - Analog Devices |
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AD650 데이터시트(HTML) 10 Page - Analog Devices |
10 / 20 page AD650 Data Sheet Rev. E | Page 10 of 20 BIPOLAR V/F Figure 11 shows how the internal bipolar current sink is used to provide a half-scale offset for a ±5 V signal range, while providing a 100 kHz maximum output frequency. The nominally 0.5 mA (±10%) offset current sink is enabled when a 1.24 kΩ resistor is connected between Pin 4 and Pin 5. Thus, with the grounded 10 kΩ nominal resistance shown, a −5 V offset is developed at Pin 2. Because Pin 3 must also be at −5 V, the current through RIN is 10 V/40 kΩ = +0.25 mA at VIN = +5 V, and 0 mA at VIN = –5 V. Components are selected using the same guidelines outlined for the unipolar configuration with one alteration. The voltage across the total signal range must be equated to the maximum input voltage in the unipolar configuration. In other words, the value of the input resistor RIN is determined by the input voltage span, not the maximum input voltage. A diode from Pin 1 to ground is also recommended. This is further discussed in the Other Circuit Considerations section. As in the unipolar circuit, RIN and COS must have low temperature coefficients to minimize the overall gain drift. The 1.24 kΩ resistor used to activate the 0.5 mA offset current should also have a low temperature coefficient. The bipolar offset current has a temperature coefficient of approximately −200 ppm/°C. UNIPOLAR V/F, NEGATIVE INPUT VOLTAGE Figure 12 shows the connection diagram for V/F conversion of negative input voltages. In this configuration, full-scale output frequency occurs at negative full-scale input, and zero output frequency corresponds with zero input voltage. A very high impedance signal source can be used because it only drives the noninverting integrator input. Typical input impedance at this terminal is 1 GΩ or higher. For V/F conversion of positive input signals using the connection diagram of Figure 4, the signal generator must be able to source the integration current to drive the AD650. For the negative V/F conversion circuit of Figure 12, the integration current is drawn from ground through R1 and R3, and the active input is high impedance. Circuit operation for negative input voltages is very similar to positive input unipolar conversion described in the Unipolar Configuration section. For best operating results use Equation 7 and Equation 8 in the Component Selection section. F/V CONVERSION The AD650 also makes a very linear frequency-to-voltage converter. Figure 13 shows the connection diagram for F/V conversion with TTL input logic levels. Each time the input signal crosses the comparator threshold going negative, the one shot is activated and switches 1 mA into the integrator input for a measured time period (determined by COS). As the frequency increases, the amount of charge injected into the integration capacitor increases proportionately. The voltage across the integration capacitor is stabilized when the leakage current through R1 and R3 equals the average current being switched into the integrator. The net result of these two effects is an average output voltage that is proportional to the input frequency. Optimum performance can be obtained by selecting components using the same guidelines and equations listed in the Bipolar V/F section. For a more complete description of this application, refer to Analog Devices’ Application Note AN-279. HIGH FREQUENCY OPERATION Proper RF techniques must be observed when operating the AD650 at or near its maximum frequency of 1 MHz. Lead lengths must be kept as short as possible, especially on the one shot and integration capacitors, and at the integrator summing junction. In addition, at maximum output frequencies above 500 kHz, a 3.6 kΩ pull-down resistor from Pin 1 to −VS is required (see Figure 14). The additional current drawn through the pulldown resistor reduces the op amp’s output impedance and improves its transient response. OP AMP COMP IN FREQ OUT OUT ONE SHOT 8 9 10 11 13 12 7 6 5 4 3 2 1 14 INPUT OFFSET TRIM –0.6V AD650 –VS –VS 1mA S1 COS 330pF –15V 0.1µF VIN ±5V 1.24kΩ R3 37.4kΩ 10kΩ R1 5kΩ CINT 1000pF 20kΩ 250kΩ 0.1µF 1µF 1kΩ +15V +5V DIGITAL GND ANALOG GND FOUT Figure 11. Connections for ±5 V Bipolar V/F with 0 kHz to 100 kHz TTL Output |
유사한 부품 번호 - AD650_15 |
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유사한 설명 - AD650_15 |
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