A calibration circuit, method and system for an r2r dac

Abstract

The application discloses a kind of calibration circuit, method and system of R2R DAC, the calibration circuit includes additional LSB unit, comparator, reference DAC, calibration DAC and controller;Additional LSB unit is connected in parallel with the LSB unit of the main DAC of R2R DAC;The input end of comparator is connected with the output end of reference DAC and the output end of R2R DAC;Controller controls additional LSB unit, main DAC and the calibration DAC of R2R DAC, and adjusts the register value of reference DAC and calibration DAC according to the output of comparator.The application discards the physical level trim, additional LSB unit, comparator, reference DAC, calibration DAC and controller are added, and the register value of calibration DAC is adjusted to realize the calibration of R2R DAC, equivalent to inject trim current in parallel node in R2R resistance network, equivalent to change resistance size, improve the matching degree of resistance.

Description

Technical Field

[0001] This invention relates to a calibration circuit, method, and system for an R2R DAC, belonging to the field of integrated circuit technology. Background Technology

[0002] The primary requirement for high-precision R2R ladder resistor network DACs (Digital to Analog Converters) is resistor matching. However, due to manufacturing process variations, the resistors cannot achieve perfect consistency, necessitating finished product adjustments. Currently, common adjustments include mask adjustments and fuse adjustments. These are physical-level adjustments, which cannot be altered once made, and are also costly and complex. Summary of the Invention

[0003] This invention provides a calibration circuit, method, and system for an R2R DAC, which solves the problems disclosed in the background art.

[0004] According to one aspect of this application, a calibration circuit for an R2R DAC is provided, including an additional LSB unit, a comparator, a reference DAC, and a controller; The additional LSB unit is connected in parallel with the LSB unit of the main DAC in the R2R DAC; The input of the comparator is connected to the output of the reference DAC and the output of the R2R DAC; The controller controls the additional LSB unit, the reference DAC, the main DAC, and the calibration DAC of the R2R DAC, and adjusts the register values ​​of the reference DAC and the calibration DAC based on the comparator output.

[0005] Furthermore, the controller calculates the INL value corresponding to the R2R DAC input code value based on the temperature characteristics of the resistor, and adjusts the calibrating DAC setting value according to the INL value.

[0006] Furthermore, the connection method and resistance of the additional LSB unit are consistent with those of the main DAC's LSB unit.

[0007] Furthermore, additional LSB units are integrated into the main DAC of the R2R DAC.

[0008] According to another aspect of this application, a calibration method for an R2R DAC is provided, which uses the aforementioned R2R DAC calibration circuit for calibration, the calibration method comprising: The main DAC input code value is controlled to be calibrated, the additional LSB unit is enabled, and the calibration DAC is disabled. Adjust the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC; Increment the input code value of the main DAC by 1, disable the additional LSB unit, and enable the calibration DAC; Adjust the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0009] Furthermore, the calibration method also includes: calculating the INL value corresponding to the R2R DAC input code value based on the temperature characteristics of the resistor, and adjusting the setting value of the calibration DAC based on the INL value.

[0010] Furthermore, the INL value corresponding to the R2R DAC input code value is calculated using the following formula: INL=k(-D+3D 2 -2D 3 ); In the formula, D is the R2R DAC code value, and k is the resistance parameter. θ A For the thermal resistance per unit area, T CR Let ρ be the first-order temperature coefficient of the resistive element, ρ be the resistivity of the resistive element, and L be the length of the 2R resistive element.

[0011] According to another aspect of this application, a calibration system for an R2R DAC is provided, the calibration system being installed within a controller of the calibration circuit of the aforementioned R2R DAC, the calibration system comprising: The first control module controls the input code value of the main DAC to be calibrated, controls the additional LSB unit to be turned on, and controls the calibration DAC to be turned off. The first adjustment module adjusts the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC. The second control module controls the main DAC input code value to increase by 1, controls the additional LSB unit to turn off, and controls the calibration DAC to turn on. The second adjustment module adjusts the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0012] The beneficial effects achieved by this invention are as follows: This invention abandons physical-level adjustment, adds an extra LSB unit, comparator, reference DAC and controller, and achieves R2R DAC calibration by adjusting the register value of the calibration DAC. This is equivalent to injecting adjustment current into the parallel node of the R2R resistor network, which effectively changes the resistance value and improves the resistance matching degree. Attached Figure Description

[0013] Figure 1 This is a block diagram of the calibration circuit for an R2R DAC. Figure 2The circuit schematic for calibrating the DAC; Figure 3 The circuit schematic of the main DAC after adding an additional LSB unit; Figure 4 A flowchart illustrating the calibration method for an R2R DAC; Figure 5 This is a schematic diagram of the self-heating effect of resistance; Figure 6 This is a block diagram of the calibration method system for an R2R DAC. Detailed Implementation

[0014] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0015] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application.

[0016] At the same time, it should be understood that, for ease of description, the dimensions of the various parts shown in the accompanying drawings are not drawn according to actual scale.

[0017] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0018] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0019] It should be noted that similar symbols and letters in the accompanying drawings represent similar items; therefore, once an item is defined in one accompanying drawing, it does not need to be discussed further in subsequent accompanying drawings.

[0020] To address the problems associated with physical-level adjustments, this application provides a calibration circuit for an R2R DAC that eliminates physical-level adjustments. By adjusting the register value, it is equivalent to injecting adjustment current into the parallel nodes of the R2R resistor network, effectively changing the resistance value and thus achieving R2R DAC calibration.

[0021] See Figure 1 , Figure 1 The present application provides a structural block diagram of a calibration circuit for an R2R DAC. The calibration circuit may include at least an additional LSB unit, a comparator, a reference DAC, and a controller. The additional LSB (Least Significant Bit) unit is connected in parallel with the LSB unit of the main DAC of the R2R DAC. The input of the comparator is connected to the output of the reference DAC and the output of the R2R DAC. The controller controls the additional LSB unit, the reference DAC, the main DAC, and the calibration DAC of the R2R DAC, and adjusts the register values ​​of the reference DAC and the calibration DAC according to the output of the comparator.

[0022] It should be noted that the calibration DAC in the R2R DAC is a 10-bit current-mode DAC. In order to improve the matching accuracy of the current of each channel, a segmented approach is adopted. The high 2 bits are encoded with thermometer and the low 8 bits are encoded with binary. In order to ensure working accuracy, the step size of the calibration DAC is 1 / 32 of the least significant bit of the main DAC, and the calibration range is (-32+1 / 32) LSB to (32-1 / 32) LSB.

[0023] For the specific calibration DAC structure, please refer to Figure 2The design employs a PMOS (P-channel Metal Oxide Semiconductor) current mirror combined with an NMOS (N-channel Metal Oxide Semiconductor) current mirror, allowing adjustment of the output current's polarity and magnitude. Internally, the NMOS current mirror has four MSB (Most Significant Bit) current sources, while the lower 8-bit LSB module uses one of these MSB current sources as its tail current source, distributing the current according to a binary ratio. The PMOS current mirror consists of two mirrors: the left mirror receives the copied current, and the right mirror receives the copied current. Each NMOS current mirror in the circuit has two switches: one connected to the left PMOS current mirror, and the other connected to the right PMOS current mirror. When the NMOS current connected to the left PMOS current mirror is greater than the NMOS current connected to the right PMOS current mirror, the current value replicated by the right PMOS current mirror is greater than the connected NMOS current value. Therefore, the excess current flows into the main DAC through the output port. Conversely, when the NMOS current connected to the left PMOS current mirror is less than the NMOS current connected to the right PMOS current mirror, the current value replicated by the right PMOS current mirror is less than the connected NMOS current value. Therefore, the missing current is drawn from the main DAC through the output port. The controller controls the calibration DAC by controlling the switch of the calibration DAC (i.e., the current mirror). Specifically, it adjusts the current flowing into or out of the main DAC by controlling the specific value of the current mirror input in the running memory. Figure 1 The calibration current in the middle.

[0024] It's important to note that both the most significant bit (LSB) current and the least significant bit (LSB) current affect the INL, and these two effects are independent. This means that for any given DAC code value, independent calibration coefficients can be set for the LSB and LSB respectively, and then digitally added together to obtain the total calibration code. The S-parameters are calibration coefficients used to eliminate temperature effects. Different calibration current settings are required for different DAC input code values, and the running memory performs write and read operations on these settings.

[0025] It should be noted that the main DAC uses an R2R network. The calibration circuit in this application adds an extra LSB unit for calibration to the existing R2R network, namely the aforementioned extra LSB unit. In some embodiments, this extra LSB unit can be directly integrated into the main DAC of the R2R DAC. The connection method and resistance of the extra LSB unit are the same as those of the main DAC's LSB unit, but it is necessary to ensure that the current of the extra LSB unit is the same as that of the main DAC's LSB unit. The structure can be found in [reference needed]. Figure 3 An additional LSB cell is connected in parallel with the existing LSB cell. Specifically, a resistor is connected to VREFP (Voltage Reference Positive) via a switch. Figure 3 The reference voltage at terminal P) and VREFN (Voltage Reference Negative, i.e.) Figure 3 (Reference voltage N terminal).

[0026] It should be noted that during the calibration process, automatic and manual calibration can be selected via the data selector. Manual calibration requires writing control signals to the register, setting the calibration value, and writing it to the latch. The controller can input the input code value of the main DAC through the latch, initially setting it to the code to be calibrated. After adjusting the reference DAC register value, the input code value is incremented by 1 to adjust the calibration DAC register value. During automatic calibration, the DAC input code value is used, and then the calibration current is adjusted through the calibration program (similar to manual calibration). The calibrated value is stored in the running memory. When the DAC is working normally, the corresponding calibration current setting value is automatically retrieved from the running memory based on the input code value.

[0027] It should be noted that the comparator, reference DAC, and controller are all external components of the R2R DAC. The comparator's accuracy needs to meet the minimum step size requirements of both the reference DAC and the calibration DAC; a low offset voltage comparator can be used. The reference DAC needs to reference the range of the main DAC to meet the calibration range requirements; that is, to ensure sampling accuracy, the accuracy of the reference DAC must be higher than that of the main DAC. The controller can be a common field-programmable gate array, with an input start calibration signal and an output calibration end signal.

[0028] It should be noted that, in order to eliminate nonlinear deviations caused by temperature characteristics, the controller also calculates the INL value corresponding to the R2R DAC input code value based on the temperature characteristics of the resistor, and adjusts the calibrating DAC setting value according to the INL value.

[0029] This application abandons physical-level adjustments and adds an extra LSB unit, comparator, reference DAC, calibration DAC and controller. The calibration of the R2R DAC is achieved by adjusting the register value, which is equivalent to injecting adjustment current into the parallel node of the R2R resistor network, effectively changing the resistance value and improving the resistance matching degree. Unlike mask adjustment and fuse adjustment, which cannot be changed once adjustment is made, this application reduces the risk of adjustment failure.

[0030] Unlike mask adjustment and fuse adjustment, which require more equipment and are more complex to operate, this application does not set up a complex calibration circuit in the R2RDAC. It only uses simple components, which is simple to operate and low in cost. Furthermore, the comparator and reference DAC are located outside the R2R DAC and are only used during calibration, which saves the R2R DAC area and manufacturing cost.

[0031] Based on the circuit described above, this application also provides a calibration method for an R2R DAC, specifically using the above calibration procedure. The calibration method can be executed by a controller and may include at least the following steps: Step 1: Control the input code value of the main DAC to be calibrated, control the additional LSB unit to be turned on, and control the calibration DAC to be turned off.

[0032] Step 2: Adjust the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0033] Step 3: Increase the input code value of the main DAC by 1, turn off the additional LSB unit, and turn on the calibration DAC.

[0034] Step 4: Adjust the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0035] It should be noted that the calibration method involves repeating the calibration steps; for details, please refer to [link to documentation]. Figure 4First (step 1), the main DAC is set to the low-order input code value to be calibrated, for example, 0xIFF. The additional LSB unit is enabled, and the calibration DAC is disabled. At this point, the output of the R2R DAC is the sum of the output voltage of the additional LSB unit and the output voltage of the main DAC (i.e., the main DAC without an additional LSB unit). The output of the R2R DAC is considered the "ideal voltage" of the next code (i.e., 0x200). A comparator compares this "ideal voltage" with the output voltage of the reference DAC. By adjusting the register value of the reference DAC, its output voltage is made equal to the "ideal voltage." Next (step 2), the code of the main DAC is incremented by 1 to obtain the "carry" signal to be corrected. The additional LSB unit is disabled, and the calibration DAC is enabled. At this point, the adjustment current of the calibration DAC is connected to the R2R network node. The output of the R2R DAC is the sum of the output voltages of the main DAC (i.e., the main DAC without an additional LSB unit) and the calibration DAC. Similarly, a comparator compares this voltage with the output voltage of the reference DAC, adjusting the register value of the calibration DAC until the R2R... Once the output voltage of the DAC is equal to that of the reference DAC, the register value of the calibration DAC is recorded and stored in a one-time programmable memory. The next bit code to be calibrated repeats the above process, thereby achieving the calibration of the R2R DAC.

[0036] In the aforementioned R2R DAC calibration process, the R2R DAC offset is calibrated in the first step by sampling the "ideal voltage," while in the second step, the calibration is completed by resetting the offset to 0. By adjusting the register value of the calibration DAC, a trimming current is injected into the parallel node of the R2R resistor network, which is equivalent to changing the resistance value. This improves the situation where the physical size, doping concentration, and other parameters of the internal integrated devices deviate from the design value during the manufacturing process, and improves the resistance matching degree.

[0037] The above calibration process reuses the same comparator in the sampling process of the reference DAC and the adjustment current of the calibration DAC. The analog circuit structure is simple, and the connection positions of the reference voltage and the working voltage remain unchanged. The same comparator deviation is maintained between each working voltage, which can reduce the deviation caused by comparator deviation.

[0038] It should be noted that temperature changes can cause parameter drift in the internal resistor and current units of the R2R DAC, leading to deviations in the characteristics of originally matched components. This directly causes INL (Integral Nonlinearity) degradation, causing the output to deviate from the ideal linear curve and fail to meet the accuracy requirements of high-precision systems. Therefore, in some embodiments, the calibration method also includes: calculating the INL value corresponding to the input code value of the R2R DAC based on the temperature characteristics of the resistor, and adjusting the setting value of the calibration DAC based on the INL value.

[0039] The formula for calculating the INL value corresponding to the input code value of the R2R DAC can be expressed as: INL=k(-D+3D 2 -2D 3 ); In the formula, D is the R2R DAC code value, expressed as a fraction of full scale, and k is the resistance parameter. θ A For the thermal resistance per unit area, T CR Let ρ be the first-order temperature coefficient of the resistive element, ρ be the resistivity of the resistive element, and L be the length of the 2R resistive element.

[0040] Further calculate the INL deviation corresponding to different input code values, specifically how much least significant bit current is involved, and then supplement the corresponding calibration current when the DAC inputs this code value.

[0041] See also Figure 5 , Figure 5 The solid "S" line in the middle is calculated based on the k value and the corresponding INL value, which is calculated according to the basic characteristics of the resistor. In order to eliminate the INL deviation caused by temperature characteristics, it can be corrected by adjusting the adjustment current. Specifically, the deviation caused by the self-heating effect of the resistor at a certain code value is calculated. On the basis of the calibration adjustment current, the temperature effect adjustment current is added to achieve the effect of eliminating the nonlinear deviation caused by the temperature effect. Figure 5 The "S"-shaped dashed line in the figure is a correction curve that neutralizes the nonlinear deviation caused by temperature effects, improves the linearity of the resistance, and can adapt to more complex external working environments.

[0042] This application enables online tuning based on calibration circuits and methods, which can greatly improve tuning efficiency, increase R2RDAC yield, and further optimize R2R DAC performance. The entire process can be implemented using timing programming of the test system, making it completely independent and controllable.

[0043] See Figure 6 , Figure 6This is a block diagram of an R2R DAC calibration system provided in an embodiment of this application. The device is a virtual device loaded within the controller of the calibration circuit. The device may include at least a first control module, a first adjustment module, a second control module, a second adjustment module, and an INL module. When executing the above calibration method, it can: The first control module controls the input code value of the main DAC to be calibrated, controls the additional LSB unit to be turned on, and controls the calibration DAC to be turned off.

[0044] The first adjustment module adjusts the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0045] The second control module controls the main DAC to increase by 1, controls the additional LSB unit to turn off, and controls the calibration DAC to turn on.

[0046] The second adjustment module adjusts the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

[0047] The self-heating effect adjustment module calculates the INL value corresponding to the R2R DAC input code value based on the temperature characteristics of the resistor, and adjusts the calibrating DAC setting value according to the INL value.

[0048] The above system adjusts the register value of the calibration DAC and injects adjustment current into the parallel node of the R2R resistor network, which effectively changes the resistance value. This improves the situation where the physical size, doping concentration and other parameters of the internal integrated devices deviate from the design value during the manufacturing process, and improves the resistance matching degree.

[0049] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A calibration circuit for an R2R DAC, characterized in that, Includes an additional LSB unit, comparator, reference DAC, and controller; The additional LSB unit is connected in parallel with the LSB unit of the main DAC in the R2R DAC; The input of the comparator is connected to the output of the reference DAC and the output of the R2R DAC; The controller controls the additional LSB unit, the reference DAC, the main DAC, and the calibration DAC of the R2R DAC, and adjusts the register values ​​of the reference DAC and the calibration DAC based on the comparator output.

2. The circuit according to claim 1, characterized in that, The controller also calculates the INL value corresponding to the R2RDAC input code value based on the temperature characteristics of the resistor, and adjusts the calibrating DAC setting value according to the INL value.

3. The circuit according to claim 1, characterized in that, The connection method and resistance of the additional LSB unit are the same as those of the main DAC's LSB unit.

4. The circuit according to claim 1 or 3, characterized in that, The additional LSB unit is integrated into the main DAC of the R2R DAC.

5. A calibration method for an R2R DAC, characterized in that, The calibration is performed using the circuit described in any one of claims 1 to 4, and the calibration method includes: The main DAC input code value is controlled to be calibrated, the additional LSB unit is enabled, and the calibration DAC is disabled. Adjust the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC; Increment the input code value of the main DAC by 1, disable the additional LSB unit, and enable the calibration DAC; Adjust the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2R DAC.

6. The method according to claim 5, characterized in that, If the circuit described in claim 2 is used for calibration, the calibration method further includes: Based on the temperature characteristics of the resistor, calculate the INL value corresponding to the R2R DAC input code value, and adjust the DAC setting value according to the INL value.

7. The method according to claim 6, characterized in that, The formula for calculating the INL value corresponding to the input code value of the R2R DAC is as follows: INL=k(-D+3D 2 -2D 3 ); In the formula, D is the R2R DAC code value, and k is the resistance parameter. θ A For the thermal resistance per unit area, T CR Let ρ be the first-order temperature coefficient of the resistive element, ρ be the resistivity of the resistive element, and L be the length of the 2R resistive element.

8. A calibration system for an R2R DAC, characterized in that, The calibration system is installed within the controller of the circuit according to any one of claims 1 to 4, and the calibration system comprises: The first control module controls the input code value of the main DAC to be calibrated, controls the additional LSB unit to be turned on, and controls the calibration DAC to be turned off. The first adjustment module adjusts the register value of the reference DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2RDAC. The second control module controls the main DAC input code value to increase by 1, controls the additional LSB unit to turn off, and controls the calibration DAC to turn on. The second adjustment module adjusts the register values ​​of the calibration DAC until the comparator determines that the output of the reference DAC is the same as the output of the R2RDAC.

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