Patent Publication Number: US-7710179-B2

Title: Programmable gain attenuator for track and hold amplifiers

Description:
BACKGROUND OF THE INVENTION 
   The invention pertains to the general field of track-and-hold amplifiers. In particular this invention pertains to the development of programmable gain attenuators, so called PGAs, for track and hold amplifiers. 
   The general purpose of track-and-hold amplifiers is to accurately track an analog input signal and, at specified times, to accurately hold at its output for a certain length of time the instantaneous value of the input signal. This form of signal conditioning is particularly important ahead of analog-to-digital converters, where the quality of conversion is improved by maintaining a substantially constant value at the input of the analog-to-digital converter during the conversion process. 
   Programmable gain attenuators are used in various analog signal-processing applications where an electrical signal of varying amplitude must be either amplified or attenuated before subsequent signal processing. Various gain and/or attenuation settings are required to accommodate the wide dynamic range of the electrical signal. 
   Programmable gain attenuators for track-and-hold amplifiers are known from the state of the art. However as circuits become faster and faster there is a need for providing an improved PGA that can be used in these high frequency circuits. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention is aimed at providing an improved programmable gain attenuator for track-and-hold amplifiers, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The above and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings, wherein: 
       FIG. 1  shows a gain attenuator of conventional art; 
       FIG. 2  shows another gain attenuator of conventional art; 
       FIG. 3  shows a first embodiment of a programmable gain attenuator (PGA) according to the invention; 
       FIG. 4  shows a second embodiment of a PGA according to the invention; 
       FIG. 5  shows another embodiment of a PGA according to the invention; 
       FIG. 6  shows a graph of the input impedance of a PGA according to the invention; 
       FIG. 7  shows the attenuation of the PGA according to the invention; and 
       FIG. 8  shows a graph of possible attenuation values of the PGA. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a programmable gain attenuator (PGA)  101 , which is in the negative feedback path of an operational amplifier  102 . This circuitry is a straightforward approach of a PGA, which is known from prior art. 
   The positive connector  103  of the amplifier  102  is connected to the analog signal, which is to be tracked. 
   The output connector  105  of the operational amplifier  102  provides the output signal of the circuitry, which is the modified input signal. 
   The input connector Vin  106  of the PGA  101  is connected to the output connector THout  105  of the operational amplifier  102 , so that the output signal of the operational amplifier  102  is fed into the PGA  101 . 
   The output connector Vout  107  of the PGA  101  is connected to the negative input connector  104  of the operational amplifier  102  by means of a switch  108 . Thus, if the switch  108  is closed, the PGA  101  forms a negative feedback line to the operational amplifier  102 . 
   The PGA comprises a voltage divider consisting of a first resistive device Rt  109  and further resistive devices Rs  110 . The further resistive devices  110  are in series, the last of which is connected to the input connector Vin  106  of the PGA  101 . The first resistive device  109  is at one end connected to a reference voltage Vcm  111  and to the next of the line of the further resistive devices Rs  110 . Each of the taps between the resistive devices of the voltage divider can be connected to the output Vout  107  of the PGA via one of the tap switches  112 . As the voltage supplied to the negative input connector  104  of the operational amplifier is defined by one of the tap switches  112  of the PGA, the gain of the operational amplifier  102  is defined by said setting of the tap switch  112 , so that the amplitude of the output signal of the amplifier  102  stays within a (small) required range for different amplitude values of the input signal. 
   Tap switches  112  thus implement the feature of the PGA  101  of being programmable. 
   The PGA furthermore comprises a select input line  113  for receiving information for setting the tap switches  112 . 
   The operational amplifier  102  has an internal switch  114  for switching between the two states of the track-and-hold amplifier, namely the track-state and the hold-state. Usually the internal switch  114  is set via a clock signal CLK  115 , which also triggers the switch SWFB  108  connecting the PGA  101  to negative input connector  104  of the operational amplifier  102 , so that each of the states lasts half the clock cycle. 
   If the internal switch  114  and the connecting switch  108  and at least one of the tap switches  112  are closed and thus in conducting state then the circuit is in track mode, so that the output signal THout follows the input signal Thin. 
   During the hold mode, the internal switch TS  114  separates the amplifier stages and the switch SWFB  108  separates the negative input connector  104  from the PGA  101 . This supports the input stage of the operational amplifier  102  to slew faster to the actual amplitude value of the input signal THin at the beginning of the next track phase. 
     FIG. 2  shows a different circuitry of a PGA  200 , which is also known from prior art. This PGA  200  is also planned to be in the feedback around an operational amplifier similar to the PGA of  FIG. 1 . The operational amplifier is not shown in this figure. 
   The PGA  200  comprises multiple PGA sections  203 . As shown in the dotted rectangle a PGA section comprises a voltage divider consisting of first resistive device R 1   204  and a second R 2   205 . One end of the first resistive device R 1   204  is connected to the input connector Vin  201 . One end of second resistive device R 2   205  is connected to a voltage supply Vcm  206 . Similar to the PGA of  FIG. 1  the tap between the two resistive devices R 1   204  and R 2   205  is connected to the output connector Vout  202  by means of a tap switch  207 . A multiplicity of these PGA sections is cascaded, whereby one end of the first resistive devices R 1   204  of each subsequent PGA section is connected to the tap between the resistive devices of the preceding PGA section. 
   The transfer function between the input Vin  201  and the output Vout  202  and thus the gain of the amplifier can be defined by the setting of tap switches  207 . 
   Both the PGA circuits of  FIG. 1  and  FIG. 2  use one switch for one gain setting. These switches introduce two parasitic effects into the feedback path around the operational amplifier. As each of the switches is implemented by a transistor, usually a MOS transistor, each switch introduces a capacity and a resistance into the path containing the switch. This capacitance along with the resistance acts as an RC-filter and thus works towards lowering the frequency of the parasitic pole in the feedback path. If that frequency is lowered too much, the pole in the feedback path reduces the phase margin of the loop, hence affecting the stability of the feedback amplifier. 
   Besides the switches in the PGA the switch SWFB  108  used to couple the PGA to the negative input of the operational amplifier contributes its own share of parasitics, both capacitive and resistive. 
   Thus there is a need for reducing the parasitic effects in the feedback path. 
   To minimize the parasitic capacitance coupled to the negative input of the operational amplifier, the total size of the tap switches and of the switch used to connect the PGA to the negative input of the operational amplifier has to be reduced. This can be accomplished by reducing either the number or the width of the tap switches. Both these options are in contrast with the need for a lower resistance when the switches are in state ON, which calls for a larger size of the tap switches and also a larger size of the switch connecting the PGA to the negative input of the operational amplifier. Alternatively the functionality of the switch connecting the PGA to the operational amplifier can be implemented by the tap switches, so that the tap switches are clocked tap switches. 
   Furthermore topology changes of the circuitry should account for the requirement that any changes made to increase the bandwidth of the PGA should not affect the number of possible gain settings that can be implemented. 
   Turning now to  FIG. 3 , a programmable gain attenuator PGA  300  according to the invention is disclosed. The PGA  300  has an input connector Vin  301  and an output connector Vout  302 . The PGA  300  is provided to be in the feedback path around an operational amplifier used as a track-and-hold amplifier—which is not shown here—namely by connecting the input connector Vin  301  to the output connector of an operational amplifier and the output connector Vout  302  to the negative input of the operational amplifier. 
   The PGA  300  has at least one PGA section  303  as defined by the dotted line  304 . Each PGA section  303  comprises a voltage divider. The voltage divider comprises a single first resistive device R 1   305 , at least two resistive devices R 2   307  and R 3   308  serving as second resistive devices in the voltage divider and a tap  306  between the first and second resistive devices. 
   The input of the voltage divider of first PGA section  303 , namely the first resistive device R 1   305 , is connected to the input connector Vin  301  of the PGA  300  with its one end. The other end of the voltage divider, namely the individual second resistive device R 2   307  and R 3   308  of the voltage divider, is connected to a reference voltage Vcm  309  by means of a switch SW  310 . The switches SW  310  can be set individually with respect to the neighboring switches  310  of the same PGA section  303  or another adjacent, cascaded PGA section  313 . 
   The tap  306  is connected via a tap switch  311  to the output connector Vout  302  of the PGA  300 . If the tap switch  311  of at least one PGA section  303  is closed, then the feedback line around the operational amplifier—not shown here—is closed. 
   The voltage between input connector Vin  301  and the output connector Vout  302  of the PGA  300  can be changed by closing the tap switch  311  of a PGA section  303  and by closing at least one of the switches  310 . For example, if the switch SW  310 , which is connected to the resistive device R 2   307  is closed and all other switches SW  310  are left open, then the voltage divider consisting of the resistive devices R 1   305  and R 2   307  defines the voltage of the tap  306 , which is connected by means of a tap switch  311  to the output connector Vout  302 . 
   The resistive value of the resistive device located between the tap and the reference voltage Vcm  309  is defined by setting the switches SW  310 . Thus the voltage of the tap  306  can be altered by closing one or more of the switches SW  310 . 
   Alternatively, the switch SW  310  being connected to the resistive device R 2   307  is open and the switch SW  310  connected to the parallel resistive device R 3   308  may be closed. In this case the voltage of the tap  306  is defined by the voltage divider consisting of the resistive devices R 1   305  and R 3   308 , namely by the ratio of their resistive values. 
   In a preferred embodiment, the values of the second resistive devices are different from each other, so that a maximum of different settings is possible by closing one or more switches SW  310  at a time. 
   Hence another variation of setting the switches SW  310  and thus of defining the voltage at the tap  306  is to close both switches SW  310 , so that the resistive devices R 2   307  and R 3   308  are parallel and thus the combination of these makes up the second resistive device in the voltage divider. 
   In this way the voltage of the tap  306 , which is the voltage of the output connector Vout  302  of the PGA  300  and thus the gain of this gain attenuator can be defined by setting the tap switch  311  and the switches SW  310 . 
   The disclosed gain attenuator as shown in this embodiment introduces one tap switch  311  into the feedback line and allows three different gain settings. The switches SW  310  do not contribute to the,parasitic and thus undesirable capacitance and resistive value as they are not in the feedback path around the operational amplifier. Therefore this circuitry reduces the amount of parasitic effects, which stick to each switch/transistor introduced into the feedback line that is the line from the input connector Vin  301  to the output connector Vout  302  of the PGA  300 . 
   The PGA  300  may also comprise further PGA sections, which is indicated by the dotted line  312  connecting the first PGA section  303  and another PGA section  313 , which is surrounded by the dotted line  314 . 
   Further PGA sections in this PGA  300  are designed similar to the first PGA section  303 . For example PGA section  313  comprises a voltage divider having a first resistive device R 1   315 . The second resistive devices R 2   317  and R 3   318  are connected to a tap  316  with their one end and to the reference voltage  309  by means of switches  319 . Also the tap  316  is connected to the output connector Vout  302  via a tap switch  320 . 
   The first resistive device R 1   315  is connected to the tap  316  of this PGA section  313  and to the tap  306  of the preceding PGA section. The PGA sections are thus cascaded. 
   The setting of the gain of the PGA  300  with more than one PGA section is similar to setting the gain of a PGA  300  with only one PGA section, except that an additional PGA section increases the count of possible gain settings. Different gain values can be set by any combination of the switches SW  310 ,  319  and the tap switches  311 ,  320 . In the example shown, there are four switches SW  310 ,  319  and two tap switches  311 ,  320 , that is six switches in total. So there are 2 6−1 =2 5 =32 possible gain values, if the values of the resistive devices are not identical. 
   The gain attenuator further has at least one select input line Sel  321  for receiving information for setting the switches. The information received via this line may be analog or digital or there may be one select input line  321  for each switch. The switches SW  310 ,  319  as well as the tap switches  311 ,  320  may be set by information received via the select input line Sel  321 . 
     FIG. 4  shows a second embodiment of a PGA  400  according to the invention. Similar to the PGA of  FIG. 3  this embodiment has an input connector Vin  401 , an output connector  402  and at least one PGA section  403 , as indicated by the dotted line  404 . 
   The PGA section  403  is similar in design to the PGA sections in  FIG. 3 , except there are several, at least more than two, resistive devices serving as second resistive devices in the voltage divider of that PGA section. The voltage divider has a first resistive device R 1   405 . The second resistive devices R 2   407 , R 3   408  and Rn  409  are connected to the first resistive device R 1   405 , the other end of each second resistive device is connected via a switch SW  410  to a reference voltage Vcm  411 . The dotted line  412  indicates that there can be even more resistive devices serving as second resistive devices in the voltage divider than the three examples shown in the figure, each of these creates—in combination with the first resistive device R 1   405 —another rate of the voltage divider, provided that the values of the second resistive devices of that PGA section are different. 
   The tap  406  is connected to the output connector Vout  402  via a tap switch  413 . 
   Furthermore there may be several cascaded PGA sections in the PGA as is indicated by the dotted line  414 . Thus there may be a multiplicity of cascaded PGA sections wherein each PGA section has a voltage divider with a multiplicity of adjustments. 
   For a PGA with a given number of n PGA sections, wherein each PGA section comprises a given number of m switches SW and thus voltage divider adjustments, the total number of switches in the PGA can be determined by n·m+n. Thus the possible number of gain settings in such a generalized PGA is 2 n·m+n −1, if the values of the resistive devices within one PGA section are different. 
   Similar to the embodiment of  FIG. 3  the gain attenuator  400  comprises at least one input line  421  for receiving information for setting the switches SW  410  as indicated by the line touching the switches SW  410 . Although not indicated by a crossing line, in a preferred embodiment the information for setting the tap switches  413  can also be received via this input line  421 . 
     FIG. 5  shows a further improved embodiment of a programmable gain attenuator  500 , having an input connector Vin  501 , an output connector  502  and at least one PGA section, wherein the PGA section is designed as described in the previous  FIGS. 3 and 4 . 
   The PGA  500  furthermore has an input line  521  for receiving information for setting the switches SW  510 . Although not indicated in the drawing the information for setting the tap switches  513  may also be delivered to the PGA via this input line  521 , so that the information received via input line  521  comprises the information on how to set all switches in the PGA. 
   The information received via input line  521  is fed into the encoder  522 , which is connected to each of the switches SW  510  via lines  523 . The encoder  522  interprets the received information and sets the switches SW  510  accordingly by signaling a corresponding signal via the lines  523 , wherein one or more than one switch SW  510  can be set to the conducting state at the same time. 
   Preferably the encoder receives the information for setting the switches SW  510  as digital information, for instance as a binary word consisting of eight bits. The encoder reads and interprets the information and translates the information into control bits for the switches SW  510  and the tap switches  513 , wherein the eight bits of the information word are not necessarily assigned each to a particular switch. Instead the information can be rearranged so that more than eight switches can be controlled. For example the information can be a decimal value of a requested gain value. The encoder knows the settings for the requested gain and thus sets the switches SW  510  and the tap switches  513  accordingly. 
     FIG. 6  shows a graph of the input impedance of a PGA according to  FIG. 4 , wherein the PGA consists of 4 PGA sections each with two voltage dividers and thus two switches SW, so that there is a total of 8 switches SW in the PGA. The resistive devices are ohmic resistors having values of R 1 =2.4 kΩ/64 for the first resistor, R 2 =2.4 kΩ for one and R 3 =2.4 kΩ/8 for the other second resistor. 
   The reference voltage Vcm in the PGA may be a common-mode voltage source for a depicted differential application or simply a ground for a single-ended application. In this example of a PGA, the reference voltage Vcm is a ground. 
   The eight switches SW can map 256 different states, so that the PGA can be set to 256 different attenuation values. 
   The vertical y-axis of the graph shows the input impedance of the PGA in a range between 100Ω and 10,000Ω in an exponential presentation. On the horizontal x-axis the settings of the switches are plotted, whereby the states of the eight switches are assigned numerical values. 
   The input impedance for the status where all switches are open, which corresponds to the numerical value of 0, is not shown. In this case the input impedance of the PGA is infinite. 
   As can be seen from the graph of  FIG. 6  the input impedance of the PGA.ranges from 250Ω to 5 kΩ depending on the settings of the switches. Although the range between the maximum and the minimum value of the input impedance is big, most of the impedance values are close to the minimum value. This property of the PGA can be used in the system design to draw the performance specifications for the circuit driving the PGA or to choose the values of the resistors, so that for example a minimum impedance may be guaranteed. 
   The new PGA does not show a monotonic behavior of the input impedance with increasing values of the selection word neither as a function of the selected tap switch. 
     FIG. 7  shows four graphs displaying the attenuation of the PGA depending from the setting of the switches SW. The PGA is the one as in  FIG. 6 , thus the PGA comprises four PGA sections each having two secondary resistive devices. 
   Each x-axis shows the numerical value of the select word, which controls the setting of the switches SW. The y-axis shows the corresponding attenuation of the PGA in dB. 
   The graphs each show the attenuation for all 255 different settings, wherein in each graph only one of the four tap switches is closed. In the left graph only the tap switch of the first PGA section, that is the one being closest to the input connector, is closed. The adjacent graph shows the curve of the attenuation if only the tap switch of the PGA section being adjacent to the first PGA section is closed. Similarly the next graph shows the attenuation of the PGA with the tap switch of the next cascaded PGA section being closed. Lastly the right graph in  FIG. 7  shows the attenuation of the PGA with the tap switch of the last cascaded PGA section being closed, which is closest to the output connector Vout of the PGA. 
   As is obvious from the graphs the attenuation is not a monotonous falling curve with increasing numerical values of the selection word. However they cover a big range of attenuation values. 
   As there are four tap switches and eight switches SW in the described PGA as described above the number of available attenuation settings is  1024 . Although most of the attenuation values are fairly spaced apart a big range of attenuation values is covered. 
     FIG. 8  shows all possible  1024  attenuation of the PGA as a function of the select word. The graph on the left side shows the attenuation values as a function of the numerical value of the select word. It is obvious that a given attenuation value can be selected by different values of the select word. 
   The graph on the right hand side of  FIG. 8  shows a pseudo-monotonous curve, wherein the attenuation value of the PGA increases with increasing values of the select word. However the gradient of the curve is fairly constant. 
   This behavior of the PGA can be achieved by using a PGA according to  FIG. 5 . The information on how to set the switches SW  510  is received for example as a digital value, the select word, by the encoder  522 . The encoder encodes the numerical value of the select word into corresponding settings of the switches SW  510 , so that for a small numerical value of the select word the attenuation of the PGA is small and increases with increasing values of the select word. In this way the information received via the input Sel  521  does not control the switches SW  510  directly but is interpreted by the encoder, which generates the control signals—bits—for the switches corresponding to the value of the received information. 
   The tap switches  513  can be controlled by also using the input line Sel  521 , whereby the encoder determines which of the tap switches  513  is to be closed for the desired attenuation value. Alternatively a second input line can be used for controlling the tap switches  513 . 
     FIG. 8  shows on the left side an attenuation curve of a PGA, wherein the PGA has been designed to implement a desired attenuation curve. The information determining the desired attenuation is encoded in order to produce switch settings according to an attenuation curve being linear-in-dB, wherein the information about the desired attenuation uses 100 steps for attenuation between 0 and −8.5 dB. In this example the encoder drives both the tap switches and the switches SW. The resulting attenuation curve has an attenuation step size of 0.09 dB. 
   The graph on the right hand side of  FIG. 8  shows the error of the desired linear-in-dB attenuation curve in dependency from the encoded Sel value. 
   As can be seen from this graph, the biggest error values occur at the beginning and at the end of the attenuation values. However the maximum error value is around 0.04 dB for any selected step, whereas the average error is much smaller. 
   While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.