This application claims, under 35 U.S.C. §119, priority to, and the benefit of, Indian provisional application number 4522/CHE/2014, entitled “A NOVEL IMPLEMENTATION OF A RESISTOR ATTENUATOR USING CROSS CONNECTION”, and filed in India on Sep. 17, 2014, the entirety of which is hereby incorporated by reference.
The present disclosure relates to attenuators, and more particularly programmable step attenuator circuits.
Attenuator circuits are used in a variety of situations to control the amplitude of an analog signal provided to the input of a subsequent circuit. For example, analog to digital converters (ADCs) convert received analog signals within a defined signal range, and analog front end signal conditioning circuits with attenuators may be used to adapt an input signal to best use the conversion range of the ADC. Automatic gain control (AGC) circuits may be used in connection with adjustable attenuator circuits to dynamically set an attenuation value to optimize the usage of the ADC input range for varying input signal amplitudes. Step attenuators provide for attenuation adjustment in steps or increments, allowing digital implementation of programmable attenuators and AGC circuits. Digital step resistor attenuators are generally of two forms, including series and parallel architectures.
Described examples include an attenuator circuit including first and second input nodes to receive an input signal, as well as first and second output nodes to provide an output signal. The attenuator circuit also includes a plurality of attenuator impedance components, and a switching circuit to connect at least a first attenuator impedance component between the first input node and the second output node, to connect at least a second attenuator impedance component between the second input node and the first output node, to connect a third attenuator impedance component between the first input node and the first output node, and to connect a fourth attenuator impedance component between the second input node and the second output node. The switching circuit in one example includes an output impedance component coupled between the first and second output nodes. The output and attenuator impedance components can be resistors or capacitors in certain examples.
In the drawings, like reference numerals refer to like elements throughout, and the various features are not necessarily drawn to scale. In the following discussion and in the claims, the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are intended to be inclusive in a manner similar to the term “comprising”, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to include indirect or direct electrical connection or combinations thereof. For example, if a first device couples to or is coupled with a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via one or more intervening devices and connections.
The example IC 101 includes an analog to digital converter (ADC) 106 that converts the differential output signal VO and provides a corresponding digital value 107 (labeled VAL in the drawing). In one example, the IC 101 includes output terminals (not shown) to provide the converted digital value 107 to an external system component, such as a host processor (not shown). In another example, the IC 101 includes processing circuits (not shown) that receive the digital value 107 from the ADC 106. The IC 101 also includes an automatic gain control (AGC) circuit 108 that generates switching control signals 118 to set the attenuation value α of the attenuator circuit 110. In one example, the attenuation α is represented as the ratio of the amplitude of the output signal VO divided by the amplitude of the input signal VIN. The attenuation α may also be expressed in decibels as 20log10 VO/VIN. In one example, the ADC 106 provides a feedback signal or control signal to the AGC 108 indicating the amplitude of the converted output signal VO. The AGC 108 generates the control signals 118 in a closed loop fashion to adjust the attenuation α of the attenuator circuit 110 by selectively increasing the attenuation α to prevent exceeding the input voltage range of the ADC 106, and by selectively decreasing the attenuation α to preferentially use as much of the input voltage range of the ADC 106.
The attenuator circuit 110 includes a plurality of attenuator impedance components ZC. In one example, the impedance components ZC form a first impedance circuit 114A and a second impedance circuit 114B. The first impedance circuit 114A in the example of
Any suitable attenuation and output impedance components ZC, ZP can be used which provide an electrical impedance between two terminals or nodes. In one example, the attenuator impedance components ZC have generally equal impedance values. In another example, the attenuator impedance components ZC of the impedance circuits 114 can have different impedance values. The output impedance component or components ZP connected across the output circuit 120 have a generally equal impedance value to the attenuator impedance components ZC in one example. In other examples, the output impedance component or components ZP can have a different impedance value from the attenuator impedance components ZC.
In addition, any suitable type of impedance component can be used, or multiple impedance types can be used for the attenuation impedance components ZC and/or the output impedance component or components ZP. For example, resistor components can be used as shown below in
The attenuator circuit 110 also includes a plurality of switching circuits S1 and S2 forming switching circuitry 116. The individual switching circuits S1 in one example are formed into a first switching circuit 116A coupled between the first impedance circuit 114A and the output circuit 120. A second switching circuit 116B includes switching circuits S2 coupled between the second impedance circuit 114A and the output circuit 120. The first switching circuit 116A includes an integer number “K” switches S1-1, S1-2, . . . , S1-K operated according to corresponding switching control signals 118-1-1, 118-1-2, . . . , 118-1-K from the gain control circuit 108, respectively. Similarly, the second switching circuit 116B includes “K” switches S2-1, S2-2, . . . , S2-K operated according to corresponding switching control signals 118-2-1, 118-2-2, . . . , 118-2-K. The switching circuits 116 allow selective direct connection of one or more of the attenuator impedance components ZC between the upper input and output nodes 112A, 120B, as well as direct connection of one or more attenuator impedance components ZC between the lower input node 112B and the lower output node 120B.
The configuration of the switching circuits 116A and 116B provides for cross-coupling of one or more of the attenuator impedance components ZC between the upper input node 112A and the lower output node 120B, and also allows cross coupling of one or more components ZC between the lower input node 112B and the upper output node 120A. The individual switching circuits S1, S2 in one example provide two selectable states. In a first state, the corresponding impedance component ZC is directly coupled. In a second state, the corresponding attenuation impedance component ZC is cross-coupled between the connected input node 112A or 112B and the complementary or opposite output node 120B or 120A. This individual switching circuit functionality can be implemented in any suitable fashion, for example, using a pair of switches 301, 302 as illustrated and described below in connection with
In one example, the gain control circuit 108 provides the control signals 118 to selectively connect at least a first one of the first attenuator impedance components ZC1 between the first input node 112A and the second output node 120B. In this manner, a programmable impedance is coupled between the “plus” input node 112A and the “minus” output node 120B. In addition, the gain control circuit 108 provides the control signals 118 in one example to selectively connect at least a second one of the attenuator impedance components ZC2 between the second input node 112B and the first output node 120A. This cross-coupling from plus to minus and/or from minus to plus allows adjustment of the attenuation value α of the attenuation circuit 110, while mitigating or avoiding one or more of the shortcomings of the series attenuator 700 shown in
In one example, the switching circuit 116 is also operative according to the control signals 118 from the gain control circuit 108 to selectively connect at least a third one of the attenuator impedance components ZC1 between the “plus” first input node 112A and the “plus” first output node 120A, and to selectively connect at least a fourth one of the attenuator impedance components ZC2 between the “minus” second input node 112B and the “minus” second output node 120B. In this manner, the control signals 118 can be used to provide a variety of different direct impedance (e.g., plus to plus and/or minus to minus) in combination with one or more cross-coupled attenuator impedances ZC (e.g., plus to minus and/or minus to plus).
In the example of
When the first switch 301 is closed (e.g., low impedance) and the second switch 302 is open (e.g., high impedance) according to the signals 118-1-1A and 118-1-1B, the switch circuit S1-1 is in a first state. In the first state, the corresponding resistive impedance component RC1-1 is connected between the “plus” input node 112A and the “plus” output node 120A. A second switching circuit state is provided when the first switch 301 is open according to the signal 118-1-1A and the second switch 302 is closed according to the signal 118-1-1B. In the second state, the impedance RC1-1 is cross-coupled between the “plus” input node 112A and the “minus” output node 120B. In one example, the other switching circuits S1 and S2 in
Referring now to
The following Table 1 shows attenuation operation of an example circuit for K=6 with six upper switch circuits S1 and six lower switch circuits S2. In this example, RC1 is the effective direct coupling resistance, and RC2 is the effective cross coupling resistance. The switch states for the upper switch circuits S1 are shown in the table having a value “HI” indicating the switch connects the corresponding impedance to the upper output 120A, and “LO” indicating that the switch connects the corresponding impedance to the lower output terminal 120B, where the lower switch circuits S2 are operated using corresponding switch settings to achieve the resulting attenuations α:
The attenuation value α of the attenuator circuit 110A in
For a simple case of two resistors in the circuit 110A of
The circuit attenuation value α in this example is given according to the following equation (1):
where gs=1/RS, gc1=1/RC1, gc2=1/RC2, and gp=1/RP.
The port voltage reflection coefficient scattering parameter S11 (dB) for the circuit 110A in
In one implementation, gc1+gc2=gc=+1/RC=a constant, a source resistance RS=50 ohms, an output resistance value RP=70 ohms, and RC=28 ohms. The value of RC1 is increased by cross-coupling part of RC to the opposite side to get more attenuation (e.g., plus to minus or minus to plus or both, identified hereinafter as ΔRC1, which is the incremental amount of resistance that gets transferred from direct connection to cross connection when the attenuation settings are successively changed). Table 2 shows attenuation α and S11 values in dB for various example values of RC1 and ΔRC1 corresponding to the curves 402 and 404 in
As seen in Table 2 and
In contrast, Table 2 below shows attenuation α and S11 values for different parallel resistance values in the parallel attenuator 800 in
As seen in Table 3 and
The disclosed attenuator circuit examples 110, 110A and 110B and the ICs 101 facilitate better frequency response in terms of bandwidth due to smaller off state switch parasitics than the parallel resistive attenuator circuit 800 in
The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. In addition, although a particular feature of the disclosure may have been disclosed with respect to only one of multiple implementations, such feature may be combined with one or more other features of other embodiments as may be desired and advantageous for any given or particular application. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.
Number | Date | Country | Kind |
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4522/CHE/2014 | Sep 2014 | IN | national |