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AD8556ACPZ-R2 数据表(PDF) 26 Page - Analog Devices |
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AD8556ACPZ-R2 数据表(HTML) 26 Page - Analog Devices |
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26 / 28 page  Data Sheet AD8556 EMI/RFI PERFORMANCE Real-world applications must work with ever increasing radio/magnetic frequency interference (RFI and EMI). In situations where signal strength is low and transmission lines are long, instrumentation amplifiers, such as the AD8556, are needed to extract weak, small differential signals riding on common-mode noise and interference. Additionally, wires and PCB traces act as antennas and pick up high frequency EMI signals. The longer the wire, the larger the voltage it picks up. The amount of voltages picked up is dependent on the impedances at the wires, as well as the EMI frequency. These high frequency voltages are then passed into the in-amp through its pins. All instrumentation amplifiers can rectify high frequency out-of- band signals. Unfortunately, the EMI/RFI rectification occurs because amplifiers do not have any significant common-mode rejection above 100 kHz. Once these high frequency signals are rectified, they appear as dc offset errors at the output. The AD8556 features internal EMI filters on the VNEG, VPOS, FILT, and VCLAMP pins. These built-in filters on the pins limit the interference bandwidth and provide good RFI suppression without reducing performance within the pass-band of the instrumentation amplifier. A functional diagram of the AD8556 along with its EMI/RFI filters is shown in Figure 51. The AD8556 has built-in filters on its inputs, VCLAMP, and filter pins. The first-order, low-pass filters inside the AD8556 are useful to reject high frequency EMI signals picked up by wires and PCB traces outside the AD8556. The most sensitive pin of any amplifier to RFI/EMI signal is the noninverting pin. Signals present at this pin appear as common-mode signals and create problems. The filters built at the input of the AD8556 have two different bandwidths: common mode and differential mode. The common- mode bandwidth defines what a common-mode RF signal sees between the two inputs tied together and ground. The EMI filters placed on the input pins of the AD8556 reject EMI/RFI suppressions that appear as common-mode signals. VDD VSS 1 2 3 +IN –IN OUT A3 VDD VSS 1 2 3 +IN –IN OUT A4 VOUT VDD VSS 1 2 3 +IN –IN OUT A2 VDD VSS 1 2 3 +IN –IN OUT A1 VSS 1 2 3 +IN –IN OUT A5 R7 P4 R5 VDD DIGIN VCLAMP R3 P2 P1 R1 R2 R4 R5 P3 VNEG VPOS DAC LOGIC EMI FILTER EMI FILTER EMI FILTER RF EMI FILTER FILT/DIGOUT AD8556 VSS EMI FILTER Figure 51. Block Diagram Showing EMI/RFI Built-In Filters Rev. B | Page 25 of 27 |
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