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AD650S Datasheet(PDF) 6 Page - Analog Devices |
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AD650S Datasheet(HTML) 6 Page - Analog Devices |
6 / 12 page AD650 REV. C –6– BIPOLAR V/F Figure 4 shows how the internal bipolar current sink is used to provide a half-scale offset for a ±5 V signal range, while provid- ing 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 Pins 4 and 5. Thus, with the grounded 10 k Ω nominal resistance shown, a –5 V offset is developed at Pin 2. Since Pin 3 must also be at –5 V, the current through RIN is 10 V/40 k Ω = +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 Figure 4. Connections for ±5 V Bipolar V/F with 0 to 100 kHz TTL Output 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 Conditions section. As in the unipolar circuit, RIN and COS must have low tempera- ture coefficients to minimize the overall gain drift. The 1.24 k Ω 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 5 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 may be used since it only drives the noninverting integrator input. Typical input imped- ance at this terminal is 1 G Ω or higher. For V/F conversion of positive input signals using the connection diagram of Figure 1, the signal generator must be able to source the integration cur- rent to drive the AD650. For the negative V/F conversion circuit of Figure 5, 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 a previous sec- tion. For best operating results use component equations listed in that section. Figure 5. Connection Diagram for V/F Conversion, Negative Input Voltage F/V CONVERSION The AD650 also makes a very linear frequency-to-voltage converter. Figure 6 shows the connection diagram for F/V con- version 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 increase proportionately. The voltage across the inte- gration 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 which is proportional to the input frequency. Optimum performance can be obtained by selecting components using the same guidelines and equations listed in the V/F Conversion section. The reader is referred to Analog Devices' Application Note AN-279 where a more complete description of this application can be found. Figure 6. Connection Diagram for F/V Conversion 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 k Ω pull-down resistor from Pin 1 to –V S is required (see Figure 7). The additional current drawn through the pull- down resistor reduces the op amp’s output impedance and improves its transient response. |
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