Thermometer digital / analog converter

Abstract

Thermometer-coded n-bit digital-to-analog converter (DAC) comprising: 'm' banks of resistors connected in 'p' chains, where an m-th resistor of a first chain is connected to an m-th resistor of a second chain, a 1-th resistor of the second chain is connected to a 1-th resistor of a third chain, and connections between remaining chains continue in a similar pattern up to a p-th chain; a set of 'p' switches is connected to each resistor in each bank; and an up / down counter (550) is configured to control the set of 'p' switches, with a DAC code (600) divided into low-order bits and high-order bits.

Description

Background area

[0001] The disclosure generally relates to resistor-chain-based digital-to-analog converters (DACs) and, in particular, to thermometer DACs whose output signal is switched at high speeds. Description

[0002] A thermometer-coded DAC contains the same resistor or current source segment for each possible value of the DAC output signal. This configuration results in a high-accuracy architecture, but with the disadvantage of high cost of silicon "real estate" (i.e., space required on an integrated circuit) for the control logic. In general, a thermometer-coded n-bit DAC can support up to 2 n Switches. A high degree of control / decoding logic is required to control each switch separately. US Patent No. 5,680,132 describes a digital-to-analog converter with a series of resistors. US 5 812 595 A describes a waveform shaping circuit that provides a shaped variable pulse width modulation signal for an SAE (Society of Automotive Engineers) J1850 compliant bus. US 9 734 904 B1 discloses a digital low-dropout regulator and a resistive memory device using the same.

[0003] In Fig. Figure 1 shows a typical thermometer-coded DAC 100. It consists of six chains of 16 resistor banks (banksels) and 96 switches (16×6), requiring 96 dedicated control signals to turn the switches on and off. The resistor chains are supplied with the reference voltage Vref, and each of the control signals can be used to determine the DAC output voltage Vout.

[0004] In the state of the art known to the inventor, the amount of control / decoding logic can be optimized. Fig. Figure 2 shows a simplified prior art configuration 200. This configuration includes six chains of 16 resistor banks (Banksels) and 96 switches (16×6). Each of the six chains is connected to a Muxsel switch controlled by a single control signal, so only 6+16 control signals are required for this implementation. One of the six Muxsel switches is selected using the high-order 3 bits of the DAC decoding logic, and one of the 16 resistor banks is selected using the low-order 4 bits of the DAC decoding logic. If the low-order 4 bits of the Banksel code are 0, Bank0 of the resistor banks is selected, and if the Banksel code is 15, Bank15 of the resistor banks is selected. The resistor banks are supplied with the reference voltage Vref and the control signals are used to determine the DAC output voltage Vout.

[0005] In the thermometer DAC configuration of Fig. 2, 16+6 control signals are required, saving routing area and space for control logic, while the configuration in Fig. 1 96 separate control signals are required. However, at certain times, when the Muxsel switches of Fig. 2, the output voltage spike becomes very large. This is due to the fact that the low-order 4 bits of the DAC code for Banksel selection increase up to code 15 and then decrease to code 0. In this case, the next Banksel control signal is at the furthest position from the previous time, both for rising and falling voltages, creating a large charge injection in the DAC output signal.

[0006] In Fig. 3 300 are the Banksel selection and the DAC output voltage of Fig. 2. When the high-order 3 bits of the code are 0, Muxsel 0 is selected, and when the high-order 3 bits of the code are 1, Muxsel 1 is selected. As the original 7-bit DAC code goes from 15 to 16, the Banksel code goes from 15 to 0 and the Muxsel code goes from 0 to 1, causing the Banksel voltage output to be farthest from the target output voltage Vout 310 just before the selection, resulting in a large output voltage spike (overshoot or undershoot) at Vout. Summary

[0007] It is an object of the disclosure to reduce an output voltage spike overshoot or undershoot during an output transition of a resistor chain-based DAC, in particular an n-bit thermometer digital-to-analog converter (n-bit thermometer DAC).

[0008] Another task of the disclosure is to eliminate a code jump when the most significant bit of the DAC code changes.

[0009] Yet another objective of the disclosure is to reduce the area occupied by a thermometer DAC by reducing the number of required control signals.

[0010] To achieve the above-described and other objects, an n-bit thermometer digital-to-analog converter (n-bit thermometer DAC) is disclosed, comprising 'm' banks of resistors connected in 'p' chains, where an m-th resistor of a first chain is connected to an m-th resistor of a second chain, a 1-th resistor of the second chain is connected to a 1-th resistor of a third chain, and connections between remaining chains continue in a similar pattern up to a p-th chain. The n-bit thermometer DAC further consists of a set of 'p' switches connected to each resistor in each bank. An up / down counter is configured to control the set of 'p' switches.

[0011] The above-described and other objects are further achieved by a method for a thermometer-encoded digital-to-analog converter (DAC). The steps include providing 'm' banks of resistors connected in 'p' chains. The steps also include connecting an m-th resistor of a first chain to an m-th resistor of a second chain, connecting a first resistor of the second chain to a first resistor of a third chain, and connecting remaining chains in a similar pattern up to a p-th chain. The steps also include connecting a set of 'p' switches to each resistor in each bank. The steps also include controlling the set of 'p' switches with an up / down counter.

[0012] In various embodiments, the thermometer-coded n-bit DAC can be implemented using any combination of Muxsel and Banksel switches of up to 2n switches can be realized. Short description of the drawings Fig. Figure 1 represents a conventional thermometer-coded digital-to-analog converter (DAC) of the state of the art. Fig. Figure 2 shows a simplified configuration of a state-of-the-art thermometer-coded digital-to-analog converter (DAC). Fig. Figure 3 illustrates the Banksel selection and DAC output voltage of a simplified configuration of a state-of-the-art thermometer-coded DAC. Fig. Figure 4 shows the Banksel selection and the DAC output voltage, which embodies the principles of disclosure. Fig. Figure 5a represents the configuration of connections between 6 resistance chains that embody the principles of disclosure. Fig. Figure 5b illustrates the up / down counter used in the bank selection disclosure. Fig. Figure 6 shows the behavior of the low-order 4 bits of a DAC code embodying the principles of disclosure. Fig. Figure 7a represents a simulation comparison between the prior art and the present disclosure. Fig. Figure 7b shows the configuration of the connections between p resistor chains in the disclosure. Fig. Figure 8 shows a method for a thermometer-encoded digital-to-analog converter embodying the principles of the disclosure. Detailed description

[0013] A thermometer-coded digital-to-analog converter (DAC) is described whose output signal changes at a high speed with reduced output voltage spikes (overshoot or undershoot). Unlike previously known designs, the thermometer-coded DAC has additional select switches and an up / down counter. The DAC code is divided into high-order bits (for muxsel or chain selection) and low-order bits (for banksel or bank selection). The low-order bits increase up to a maximum code and then decrease. This configuration reduces output voltage spikes, particularly at the DAC code change point.

[0014] There are advantages to reducing overshoot or undershoot output voltage spikes. If the voltage spikes are large enough, a monotonically rising or falling output signal cannot be maintained. Additionally, the DAC output voltage can be used as a reference voltage, and the block using this reference voltage will follow the voltage spike, resulting in additional power consumption. Furthermore, high-frequency noise generated by overshoot or undershoot output voltage spikes can cause the system in which the DAC is located to fail.

[0015] For example, when the DAC code changes from 0111 to 1000, the elimination of a code jump occurs when the most significant bit changes. Just before the Muxsel switch, which controls the selection of 1 of 6 resistor chains, changes, the future Banksel switch, which controls the selection of 1 of 16 resistors in the chain, has already moved to the nearest location for the next DAC code.

[0016] In the present disclosure, a new resistor arrangement helps achieve this. When the DAC code changes, some of the resistor Banksel output signals change in a manner opposite to the others. The next Banksel switch output signal is set to the closest position to the previous time for both rising and falling voltages.

[0017] Fig. 4400 shows the Banksel selection and the DAC output voltage, embodying the principles of disclosure. Now, some of the unselected Banksel output signals change in such a way as to cancel the charge injection effect of other Banksel output signals. The Banksel output signals corresponding to Muxsel switch inputs 1, 3, and 5 change in an opposite direction (decreasing instead of increasing) to the Banksel output signals corresponding to Muxsel switch inputs 0, 2, and 4. This causes the Banksel output voltage to be closest to the target output voltage Vout 410 just before the selection.

[0018] There are two key features of the present disclosure. One is the connections between the six resistor chains. Another is the behavior of the low-order four bits of the DAC code, which controls the selection of the 16 resistors in each chain.

[0019] Fig. Figure 5a shows the configuration 500 of the connections between the 6 resistor chains. Similar to Fig. 2, there are 6 resistor chains and 16 switches in each chain. Each of the 6 resistor chains is selected with a Muxsel switch using the high-order 3 bits of the DAC decoding logic, and each of the 16 resistors in the chain is selected with a Banksel switch using the low-order 4 bits of the DAC decoding logic and an up / down counter.

[0020] In Fig. 2 of the prior art, Bank15 resistance of chain0 is connected to the Bank0 resistance of chain1, whereas in Fig. 5 of the present disclosure, the Bank15 resistor of Chain0 is connected to the Bank15 resistor of Chain1. In the prior art, the Bank15 resistor of Chain1 is connected to the Bank0 resistor of Chain2, and in the present disclosure, the Bank0 resistor of Chain1 is connected to the Bank0 resistor of Chain2. The configuration of the present disclosure reduces any voltage spike in the output voltage Vout because the Bank15 switch connection is moved to the switch closest to the location of the next DAC code.

[0021] Fig. Figure 5b illustrates the up / down counter 550 used in the disclosure for bank selection. The up / down counter, or bidirectional counter, receives a control signal that determines the counting direction, as well as a clock signal. The counter's output signal is used for resistor bank selection in Fig. 5a and Fig. 7b is used.

[0022] In Fig. Figure 6 shows the behavior of the lower 4 bits of DAC code 600. In the prior art, the DAC code is a 1-step increment code. In the present disclosure, the lower 4 bits of the DAC code increase from 0 to 15 and then decrease from 15 to 0, repeating this pattern and controlling the selection of the 16 resistors in each of the 6 resistor chains. Although the lower bits of the DAC code are used differently between the prior art and the present disclosure, the DAC output still increases with a 7-bit code.

[0023] In Fig. Figure 7a shows a simulation comparison 700 between the prior art and the present disclosure. In the prior art 720, the overshoot occurs by the low-order 4 bits of the DAC code increasing up to code 15, then decreasing back to code 0, creating a large charge injection into the DAC output voltage. In the present disclosure 710, the Banksel switch connections are moved to the nearest location of the next DAC code. Increasing the low-order 4 bits of the DAC code from 0 to 15 and then decreasing the low-order 4 bits of the DAC code from 15 to 0 reduces any voltage spike in the output voltage.

[0024] For example, the description above only shows the use of 6 resistor chains, each controlled by a Muxsel switch, and 16 resistors in each chain, each controlled by a Banksel switch. The number of resistor chains and banks can, of course, vary. The present disclosure further includes a DAC code of 3 high-order bits for Muxsel control and 4 low-order bits for Banksel control. It should be noted that a combination of Muxsel and Banksel switches for an n-bit DAC code for up to 2 n Switch can be configured similarly.

[0025] Fig. Figure 7b illustrates the configuration 750 of the connections between p resistor chains in the disclosure. Here, there are m banks, denoted Bank0, Bank1, and Bankm. In general, the number of Muxsel switches will correspond to the number of chains. Likewise, the number of Banksel switches will correspond to the number of resistor banks. Each of the p resistor chains is selected using the high-order bits of the DAC decode logic, and each of the m resistors in the chain is selected using the low-order bits of the DAC decode logic and an up / down counter.

[0026] Fig.8 is a flowchart 800 of a method for a thermometer-encoded digital-to-analog converter (DAC). Step 810 provides for 'm' banks of resistors connected in 'p' chains. Step 820 connects an m-th resistor of a first chain to an m-th resistor of a second chain, connects a first resistor of the second chain to a first resistor of a third chain, and connects remaining chains in a similar pattern up to a p-th chain. Step 830 connects a set of 'p' switches to each of the resistors in each bank. Step 840 controls the set of 'p' switches with an up / down counter.

[0027] The advantages of one or more embodiments of the present disclosure include providing a thermometer-coded digital-to-analog converter (DAC) with a smaller area and reduced control signals. This configuration improves resistor-string-based DAC performance by minimizing output overshoot / undershoot when the most significant bits change, in the situation where the DAC input code is continuously ramping up or down.

[0028] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

Claims

[1] Thermometer-coded n-bit digital-to-analog converter (DAC) comprising: 'm' banks of resistors connected in 'p' chains, where an m-th resistor of a first chain is connected to an m-th resistor of a second chain, a 1-th resistor of the second chain is connected to a 1-th resistor of a third chain, and connections between remaining chains continue in a similar pattern up to a p-th chain; a set of 'p' switches is connected to each resistor in each bank; and an up / down counter (550) is configured to control the set of 'p' switches, with a DAC code (600) divided into low-order bits and high-order bits. [2] A thermometer-coded DAC according to claim 1, configured such that when a voltage from the DAC is to be connected to an output voltage terminal (410), all bank select switches for a selected bank are switched together. [3] A thermometer-coded DAC according to claim 2, configured such that a chain from which the voltage is to be read is selected by closing a switch in a muxsel set of switches. [4] A thermometer coded DAC according to claim 3, wherein the DAC code is divided into the low-order bits for Banksel switch selection and the high-order bits for Muxsel switch selection. [5] The thermometer coded DAC of claim 4, wherein the up / down counter (550) is configured to increment and then decrement the low order bits of the DAC code up to a maximum code. [6] Thermometer-coded DAC according to claim 4, which is for a total of 2n Banksel switch and Muxsel switch are configured. [7] The thermometer coded DAC of claim 4, wherein the DAC code is configured to change to eliminate a code jump. [8] A thermometer coded DAC according to claim 2, wherein the Banksel switches are connected at a location for a next DAC code from a previous time for both rising and falling voltages. [9] The thermometer-coded DAC of claim 2, wherein the Banksel switches are configured to change to cancel a charge injection effect of further Banksel switches. [10] The thermometer-coded DAC of claim 2, wherein the voltage from the DAC is configured to be close to a desired output voltage. [11] The thermometer coded DAC of claim 2, wherein the n bits comprise a 7-bit code. [12] Method for a thermometer-coded digital-to-analog converter (DAC), comprising: Providing 'm' banks of resistors connected in 'p' chains; Connecting an m-th resistor of a first chain to an m-th resistor of a second chain, connecting a 1-th resistor of the second chain to a 1-th resistor of a third chain, and connecting remaining chains in a similar pattern up to a p-th chain; Connecting a set of 'p' switches to each resistor in each bank; and Controlling the set of 'p' switches with an up / down counter (550) and with a DAC code (600) divided into low-order bits and high-order bits. [13] The method of claim 12, further comprising interconnecting all switches in the set of 'p' switches or banksel switches for a selected bank when a voltage from the DAC is to be connected to an output voltage terminal (410). [14] The method of claim 13, further comprising selecting a chain for the voltage by closing a switch in a muxsel set of switches. [15] The method of claim 14, further comprising dividing the DAC code into the low-order bits for Banksel switch selection and the high-order bits for Muxsel switch selection. [16] The method of claim 12, further comprising incrementing the up / down counter (550) to a maximum code and then decrementing the up / down counter (550). [17] The method of claim 15, wherein the DAC code is modified to eliminate a code jump. [18] The method of claim 13, further comprising connecting the Banksel switches at a location for a next DAC code from a previous time for both rising and falling voltages. [19] The method of claim 13, wherein changing the Banksel switches cancels a charge injection effect of further Banksel switches. [20] The method of claim 13, further comprising configuring the voltage from the DAC close to a desired output voltage. [21] The method of claim 13, wherein the n bits comprise a 7-bit code.

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