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AD622 Datenblatt(PDF) 11 Page - Analog Devices |
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AD622 Datenblatt(HTML) 11 Page - Analog Devices |
11 / 16 page AD622 Rev. D | Page 11 of 16 GAIN SELECTION The AD622 gain is resistor programmed by RG or, more precisely, by whatever impedance appears between Pin 1 and Pin 8. The AD622 is designed to offer gains as close as possible to popular integer values using standard 1% resistors. Table 5 shows required values of RG for various gains. Note that for G = 1, the RG pins are unconnected (RG = ∞). For any arbitrary gain, RG can be calculated by using the formula 1 k 5 . 50 − Ω = G R G To minimize gain error, avoid high parasitic resistance in series with RG. To minimize gain drift, RG should have a low temperature coefficient less than 10 ppm/°C for the best performance. Table 5. Required Values of Gain Resistors Desired Gain 1% Std Table Value of RG, Ω Calculated Gain 2 51.1 k 1.988 5 12.7 k 4.976 10 5.62 k 9.986 20 2.67 k 19.91 33 1.58 k 32.96 40 1.3 k 39.85 50 1.02 k 50.50 65 787 65.17 100 511 99.83 200 255 199.0 500 102 496.1 1000 51.1 989.3 INPUT AND OUTPUT OFFSET VOLTAGE The low errors of the AD622 are attributable to two sources: input and output errors. The output error is divided by G when referred to the input. In practice, the input errors dominate at high gains and the output errors dominate at low gains. The total VOS for a given gain is calculated as follows: Total Error RTI = input error + (output error/G) Total Error RTO = (input error × G) + output error REFERENCE TERMINAL The reference terminal potential defines the zero output voltage and is especially useful when the load does not share a precise ground with the rest of the system. The reference terminal provides a direct means of injecting a precise offset to the output, with an allowable range of 2 V within the supply voltages. Parasitic resistance should be kept to a minimum for optimum CMR. INPUT PROTECTION The AD622 features 400 Ω of series thin film resistance at its inputs and safely withstands input overloads of up to ±15 V or ±60 mA for up to an hour at room temperature. This is true for all gains and power on and off, which is particularly important because the signal source and amplifier can be powered separately. For longer time periods, the input current should not exceed 6 mA. For input overloads beyond the supplies, clamping the inputs to the supplies (using a diode such as a BAV199) reduces the required resistance, yielding lower noise. Large Input Voltages at Large Gains When operating at high gain, large differential input voltages may cause more than 6 mA of current to flow into the inputs. This condition occurs when the maximum differential voltage exceeds the following critical voltage: VCRITICAL = (400 + RG) × (6 mA) This is true for differential voltages of either polarity. The maximum allowed differential voltage can be increased by adding an input protection resistor in series with each input. The value of each protection resistor should be as follows: RPROTECT = (VDIFF_MAX − VCRITICAL)/6 mA RF INTERFERENCE RF rectification is often a problem when amplifiers are used in applications where there are strong RF signals. The disturbance may appear as a small dc offset voltage. High frequency signals can be filtered with a low-pass, RC network placed at the input of the instrumentation amplifier, as shown in Figure 18. In addition, this RC input network also provides additional input overload protection (see the Input Protection section). RG REF VOUT +IN –IN AD622 + 0.1µF 10µF + 0.1µF 10µF +VS –VS CC 1nF CD 47nF CC 1nF R 4.02kΩ R 4.02kΩ Figure 18. RFI Suppression Circuit for AD622 Series In-Amps The filter limits the input signal bandwidth to the following cutoff frequencies: ) (2 2 1 C D DIFF C C R FilterFreq + π = C CM RC FilterFreq π = 2 1 where CD ≥ 10CC. |
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Ähnliche Beschreibung - AD622 |
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