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AD8233ACBZ-R7 数据表(PDF) 24 Page - Analog Devices |
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AD8233ACBZ-R7 数据表(HTML) 24 Page - Analog Devices |
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24 / 30 page  Data Sheet AD8233 Rev. 0 | Page 23 of 29 Additional High-Pass Filtering Options In addition to the topologies explained in the previous sections, an additional pole may be added to the dc blocking circuit for the rejection of low frequency signals. This configuration is shown in Figure 64. 10kΩ IAOUT HPSENSE HPDRIVE S1 +IN –IN HPA SW 10kΩ S2 D4 REFOUT C3 TO NEXT STAGE = REFOUT B4 C5 A5 A4 B5 C1 R1 R2 RCOMP C2 Figure 64. Schematic for an Alternative Two-Pole, High-Pass Filter An extra benefit of this circuit topology is that it allows a lower cutoff frequency with lower R and C values. The resistor, RCOMP, can also be used to control the quality factor (Q) of the filter to achieve narrow band-pass filters (for heart rate detection) or maximum pass-band flatness (for cardiac monitoring). With this circuit topology, the filter attenuation reverts to a single-pole roll-off at very low frequencies. Because the initial roll-off is 40 dB per decade, this reversion to 20 dB per decade has little impact on the ability of the filter to reject out of band low frequency signals. The designer may choose different values to achieve the desired filter performance. To simplify the design process, use the following recommendations as a starting point for component value selection. R1 = R2 ≥ 100 kΩ C1 = C2 RCOMP = 0.14 × R1 The cutoff frequency is located at fC = C2 R2 C1 R1 2 10 The selection of RCOMP to be 0.14 times the value of the other two resistors optimizes the filter for a maximally flat pass band. Reduce the value of RCOMP to increase the Q and, consequently, the peaking of the filter. Note that a very low RCOMP value may result in an unstable circuit. The selection of values based on these criteria results in a transfer function similar to what is shown in Figure 65. When additional low frequency rejection is desired, a high-order, high-pass filter can be implemented by adding an ac coupling network at the output of the instrumentation amplifier, as shown in Figure 65. The SW terminal is connected to the ac coupling network to obtain the best settling time response when fast restore engages. 10kΩ IAOUT HPSENSE HPDRIVE S1 +IN –IN HPA SW 10kΩ S2 D4 REFOUT C3 TO NEXT STAGE = REFOUT B4 C5 A5 A4 B5 C1 C3 R1 R2 RCOMP C2 R3 Figure 65. Schematic for a Three-Pole, High-Pass Filter 60 40 20 0 –20 –40 –60 0.01 100 10 1 0.1 FREQUENCY (Hz) THREE-POLE FILTER TWO-POLE FILTER 40dB PER DECADE 40dB PER DECADE 20dB PER DECADE 60dB PER DECADE Figure 66. Frequency Response of the Circuits Shown in Figure 64 and Figure 65 Careful analysis and adjustment of all of the component values in practice is recommended to optimize the filter characteristics. To reduce the value of RCOMP, increase the peaking of the active filter to overcome the additional roll-off introduced by the ac coupling network. Proper adjustment yields the best pass-band flatness. |
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