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AD8420ARMZ-R7 数据表(PDF) 23 Page - Analog Devices |
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AD8420ARMZ-R7 数据表(HTML) 23 Page - Analog Devices |
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23 / 27 page ![]() AD8420 Data Sheet Rev. A | Page 22 of 26 CAPACITIVELY COUPLED +VS C R R C –VS AD8420 1 fHIGH-PASS = 2πRC THERMOCOUPLE +VS –VS 10MΩ AD8420 TRANSFORMER +VS –VS AD8420 CORRECT VOUT VOUT THERMOCOUPLE +VS –VS AD8420 CAPACITIVELY COUPLED +VS C C –VS AD8420 TRANSFORMER +VS –VS AD8420 INCORRECT VOUT VOUT VOUT VOUT Figure 65. Creating an IBIAS Path INPUT BIAS CURRENT RETURN PATH The input bias current of the AD8420 must have a return path to ground. When the source, such as a thermocouple, cannot provide a return current path, create one, as shown in Figure 65. RADIO FREQUENCY INTERFERENCE (RFI) All instrumentation amplifiers can rectify high frequency out-of- band signals. Once rectified, these signals appear as dc offset errors at the output. High frequency signals can be filtered with a low-pass RC network placed at the input of the instrumentation amplifier, as shown in Figure 66. The filter limits the input signal bandwidth according to the following relationship: ) 2 ( π 2 1 C D DIFF C C R uency FilterFreq C CM RC uency FilterFreq π 2 1 where CD 10 CC. VOUT R1 R2 +IN +VS –VS CC 330pF 5% CC 330pF 5% 10µF 10µF CD 3300pF 0.1µF 0.1µF R 20kΩ 1% R 20kΩ 1% AD8420 –IN Figure 66. Suggested RFI Suppression Filter CD affects the differential signal and CC affects the common-mode signal. Values of R and CC are chosen to minimize out of band RFI at the expense of reduced signal bandwidth. Mismatch between the R × CC at the positive input and the R × CC at the negative input degrades the CMRR of the AD8420. By using a value of CD that is at least one magnitude larger than CC, the effect of the mismatch is reduced and performance is improved. |
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