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AD650B Folha de dados(PDF) 10 Page - Analog Devices |
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AD650B Folha de dados(HTML) 10 Page - Analog Devices |
10 / 12 page AD650 REV. C –10– Figure 11. Bipolar Offset Current vs. External Resistor APPLICATIONS DIFFERENTIAL VOLTAGE-TO-FREQUENCY CONVERSION The circuit of Figure 12 accepts a true floating differential input signal. The common-mode input, VCM, may be in the range +15 to –5 volts with respect to analog ground. The signal input, VIN, may be ±5 volts with respect to the common-mode input. Both inputs are low impedance: the source which drives the common-mode input must supply the 0.5 mA drawn by the bipolar offset current source and the source which drives the signal input must supply the integration current. If less common-mode voltage range is required, a lower voltage Zener may be used. For example, if a 5 volt Zener is used, the VCM input may be in the range +10 to –5 volt. If the Zener is not used at all, the common-mode range will be ±5 volts with respect to analog ground. If no Zener is used, the 10k pulldown resistor is not needed and the integrator output (Pin 1) is con- nected directly to the comparator input (Pin 9). Figure 12. Differential Input AUTOZERO CIRCUIT In order to exploit the full dynamic range of the AD650 VFC, very small input voltages will need to be converted. For example, a six decade dynamic range based on a full scale of 10 volts will require accurate measurement of signals down to 10 µV. In these situations a well-controlled input offset voltage is imperative. A constant offset voltage will not affect dynamic range but simply shift all of the frequency readings by a few hertz. However, if the offset should change, then it will not be possible to distinguish between a small change in a small input voltage and a drift of the offset voltage. Hence, the usable dynamic range is less. The circuit shown in Figure 13 provides automatic adjustment of the op amp offset voltage. The circuit uses an AD582 sample and hold amplifier to control the offset and the input voltage to the VFC is switched between ground and the signal to be measured via an AD7512DI analog switch. The offset of the AD650 is adjusted by injecting a current into or drawing a current out of Pin 13. Note that only one of the offset null pins is used. During the “VFC Norm” mode, the SHA is in the hold mode and the hold capacitor is very large, 0.1 µF, to hold the AD650 offset constant for a long period of time. Figure 13. Autozero Circuit When the circuit is in the “Autozero” mode the SHA is in sample mode and behaves like an op amp. The circuit is a varia- tion of the classical two amplifier servo loop, where the output of the Device Under Test (DUT)—here the DUT is the AD650 op amp—is forced to ground by the feedback action of the con- trol amplifier—the SHA. Since the input of the VFC circuit is connected to ground during the autozero mode, the input cur- rent which can flow is determined by the offset voltage of the AD650 op amp. Since the output of the integrator stage is forced to ground it is known that the voltage is not changing (it is equal to ground potential). Hence if the output of the integra- tor is constant, its input current must be zero, so the offset voltage has been forced to be zero. Note that the output of the DUT could have been forced to any convenient voltage other than ground. All that is required is that the output voltage be known to be constant. Note also that the effect of the bias current at the inverting input of the AD650 op amp is also nulled in this circuit. The 1000 pF capacitor shunting the 200 k Ω resistor is compensation for the two amplifier servo loop. Two integra- tors in a loop requires a single zero for compensation. Note that the 3.6 k Ω resistor from Pin 1 of the AD650 to the negative sup- ply is not part of the autozero circuit, but rather it is required for VFC operation at 1 MHz. PHASE LOCKED LOOP F/V CONVERSION Although the F/V conversion technique shown in Figure 6 is quite accurate and uses only a few extra components, it is very limited in terms of signal frequency response and carrier feed-through. If the carrier (or input) frequency changes instantaneously, the |
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