Patent Application
20090212855
Continuous time filters may be used in a variety of analog circuit applications. For example, many communications systems employ continuous time filters to filter out signal components above or below a frequency of interest, or otherwise modify the amplitude of a signal at a particular frequency or frequency range. DC components and high frequency noise may be filtered out, for example.
A general example of a traditional bandpass filter 20 utilizing a differential amplifier is shown in
A schematic graph of the frequency characteristics of the filter 20 is shown in
Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without various of these particular details. Also, in some instances, well-known circuits, control signals, timing protocols, system blocks and software operations have not been shown in detail in order to avoid unnecessarily obscuring the described embodiments of the invention.
A resistor string 260 is coupled between the differential output signals 250 and 251. The resistor string 260 includes a plurality of resistive elements 261, 265, 267, and 263. Although four resistive elements are shown in
The filter 200 employs feedback connected in a different manner than the feedback described above with reference to
By connecting the feedback resistor 240 between the input node 227 and an intermediate node of the resistor string 260, a smaller feedback capacitor 235 and input capacitor 225 may be used. Accordingly, the feedback capacitor 235 and input capacitor 225 are shown as having capacitance Cfb/x and Cin/x, respectively, in
Note that the transfer function above is the same result as a corresponding analysis for a filter having capacitances Cfb and Cin, but having the resistance Rfb coupled directly to VON. Accordingly, by coupling the feedback resistor Rfb to a lower voltage node (VON/x in this example), for the same feedback resistor value Rfb, the feedback capacitor Cfb may also be reduced by a factor of x while preserving an effective Rfb*Cfb time constant for the filter. Similarly, the capacitance of the input capacitor 225 may also be reduced by a factor of x, while preserving the transfer function and ratio of the input capacitance to the feedback capacitance, a ratio that is related to the gain of the filter.
In an analogous manner for feedback between a second differential input node 228 and output node 254, the capacitances of input capacitor 226 and feedback capacitor 236 coupled to a differential input node 228 may be reduced. A feedback resistor 241 is coupled between an intermediate node 264 of the common mode feedback resistor string 260. The intermediate node 264 is coupled to the midpoint node 266 by a resistance R, and to a differential output signal VOP 251 by a resistance R(x−1). Accordingly, the voltage at the node 264 may be equal to VOP/x.
Although the resistor string 260 of
For a given resistor value, and thus resistor size, the ability to reduce the feedback capacitance, input capacitance, or both, as described above, may have a variety of advantages. Generally, for filters such as the filter 200, reducing power consumption, area, or both, of the filter is desirable in that more die may be fabricated per semiconductor wafer, yielding a cheaper product in a less expensive package. Further, lower power consumption may be desired in products where power consumption is a factor in evaluating competing designs.
Capacitive loading at the output of the amplifier 230 affects the achievable bandwidth of the filter 200. To obtain desired bandwidth performance, power may need to be increased to drive a capacitive load at the output nodes 253 and 254. By reducing the capacitance of the feedback capacitors 235 and 236, capacitive loading at the output nodes may be reduced, and less power may be required to achieve desired bandwidth of the filter 200. However, the bandwidth is also affected by the product of the feedback capacitance Cfb and the feedback resistance Rfb. The Cfb*Rfb product affects the pole and zero locations for the filter response. As described in examples above, however, capacitive loading, in terms of the capacitance of the feedback and input capacitors, may be reduced while maintaining a same effective R*C product when a feedback resistor is coupled to an intermediate node of a resistor string tied between differential amplifier outputs instead of coupling the feedback resistor directly to an output node of the differential amplifier.
Another advantage may be gained in some examples by reducing a size of the input capacitors 225 and 226 in
Not all of the advantages described herein may be achieved in each example or implementation of feedback described herein. The advantages described are not intended to limit the applications or examples of filters, devices, or feedback implementations achievable. Rather, the advantages are provided to allow those skilled in the art to appreciate some of the performance variables that may be manipulated using examples described.
In some examples, disadvantages may occur. For example, input-referred offset and output-referred noise, metrics that may affect the dynamic range and fidelity of a signal receive path, may be adversely affected in some examples. However, benefits attained from advantages described may outweigh the adverse affects from input-referred offset and output-referred noise occurring when a feedback resistor is coupled to an intermediate node in a resistor string tied between differential outputs. Those skilled in the art will appreciate these design trade-offs in selecting an implementation suitable for desired performance specifications.
Another example of a filter 300 is shown in
The filter 300 accordingly employs the resistor string 260 to provide common mode feedback, to provide a reduced feedback voltage to the feedback resistors 240 and 241, and to provide dynamic offset cancellation. In other examples, it should be understood that any combination of these features may be provided by the resistor string 260. In some examples, the feedback resistors 240 and 241 may be coupled to the output nodes 253 and 254, respectively, instead of intermediate nodes of the resistive string 260, and the resistive string 260 used to provide common mode feedback and dynamic offset cancellation. In this manner, some of the capacitance reductions in the feedback and input capacitances may not be achieved as described above, but output loading may be reduced by use of the resistor string 260 to provide dynamic offset cancellation.
The system 400 shown in
In full duplex operation, a locally generated transmit signal, as well as a received signal from a link partner, may be superimposed on a same physical medium, such as a CAT6 cable 430. The cable 430 is coupled to an interface 432 and a transformer 434 couples the superimposed signal onto a chip for coupling to a hybrid block 440. A line driver 450 generates the local transmit signal, and couples the local transmit signal to the transformer 434 for coupling to the interface 432. Accordingly, a superimposed signal containing both a locally generated transmit signal and a received signal, may be present at the input to the hybrid block 440. The locally generated transmit signal may generally be stronger than the received signal, which may have passed through a noisy medium, or traveled over a lossy path prior to receipt at the interface 432.
The hybrid block 440 cancels out the locally generated transmit signal from the superimposed signal at the input of the hybrid block 440. In this manner, substantially only the received signal may be applied to the receive signal path 410. The power requirements for the hybrid block 440 may be reduced through use of techniques described above that may reduce capacitance sizes and power requirements of the filter 200. That is, by reducing power requirements of the filter 200, the power consumption of the hybrid block 440 may be reduced.
The system 400 may be supplied in any of a variety of communications devices for processing received signals, transmitted signals, or both. Devices employing examples of the system 400 may include, but are not limited to, laptop computers, desktop computers, cellular telephones, and other mobile devices.
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.
This application claims the benefit of the filing date of U.S. Provisional Application 61/066,641, entitled “Novel feedback method to reduce area and power in continuous-time filters,” filed Feb. 22, 2008, which application is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61066641 | Feb 2008 | US |
