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AD7750 датащи(PDF) 7 Page - Analog Devices |
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AD7750 датащи(HTML) 7 Page - Analog Devices |
7 / 16 page AD7750 –7– REV. 0 ANALOG INPUTS The analog inputs of the AD7750 are high impedance bipolar voltage inputs. The four voltage inputs make up two truly differential voltage input channels called V1 and V2. As with any ADC, an antialiasing filter or low-pass filter is required on the analog input. The AD7750 is designed with a unique switched capacitor architecture that allows a bipolar analog input with a single 5 V power supply. The four analog inputs (V1+, V1–, V2+, V2–) each have a voltage range from –1.0 V to +1.0 V. This is an absolute voltage range and is relative to the ground (AGND) pin. This ground is nominally at a potential of 0 V relative to the board level ground. Figure 8 shows a very simplified diagram of the analog input structure. When the ana- log input voltage is sampled, the switch is closed and a very small sampling capacitor is charged up to the input voltage. The resistor in the diagram can be thought of as a lumped compo- nent made up of the on resistance of various switches. VIN R 1.4k C 2pF SAMPLING CAPACITOR Figure 8. Equivalent Analog Input Circuit Analog Inputs Protection Circuitry The analog input section also has protection circuitry. Since the power supply rails are 0 V to 5 V, the analog inputs can no longer be clamped to the supply rails by diodes. Thus, the inter- nal protection circuitry monitors the current paths during a fault condition and protects the device from continuous overvoltage, continuous undervoltage and ESD events. The maximum over- voltage the AD7750 analog inputs can withstand without caus- ing irreversible damage is ±6 V relative to AGND pin. In the case of continuous overvoltage and undervoltage the series resistance of the antialiasing filter can be used to limit input current. The total input current in the case of a fault should be limited to 10 mA. For normal operation of the AD7750 there are two further re- strictions on the signal levels presented to the analog inputs. 1. The voltage on any input relative to the AGND pin must not exceed ±1 V. 2. The differential voltage presented to the ADC (Analog Modulator) must not exceed ±2 V. In Figure 12, Channel 1 has a peak voltage on V1+ and V1– of ±1 V. These signals are not gained (G1 = 0) and so the differential signal presented to the modulator is ±2 V. However, Channel 2 has an associated gain of two and so care must be taken to ensure the modulator input does not exceed ±2 V. Therefore, the maximum signal voltage that can appear on V2+ and V2– is ±0.5 V. The difference between single-ended and complementary differential input schemes is shown in the diagram below, Figure 9. For a single-ended input scheme the V– input is held at the same potential as the AGND Pin. The maxi- mum voltages can then be applied to the V+ input are shown in Figures 10 and 11. An example of this input scheme uses a shunt resistor to convert the line current to a voltage that is then applied to the V1+ input of the AD7750. An example of the complementary differential input scheme uses a current transformer to convert the line current to a voltage that is then applied to V1+ and V1–. With this scheme the voltage on the V+ input is always equal to, but of opposite polarity to the voltage on V–. The maximum voltage that can be applied to the inputs of the AD7750 using this scheme is shown in Figures 12 and 13. Note that the common mode of the analog inputs must be driven. The output terminals of the CT are, therefore, referenced to ground. A CURRENT TRANSFORMER PROVIDES COMPLEMENTARY DIFFERENTIAL INPUTS TO THE AD7750 A CURRENT SENSE RESISTOR PROVIDES A SINGLE-ENDED INPUT TO THE AD7750 V+ V– V– V+ SHUNT RESISTOR Figure 9. Examples of Complementary and Single- Ended Input Schemes |
Аналогичный номер детали - AD7750 |
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Аналогичное описание - AD7750 |
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