AN1080 Understanding Digital Potentiometer Resistor Variations Author: Mark Palmer Microchip Technology Inc. INTRODUCTION All semiconductor devices have variations over process. In the case of digital potentiometer devices, this process variation affects the device resistive elements (RAB -> RS and RW). These resistive elements also have variations with respect to voltage and temperature, which will also be discussed. This application note will discuss how process, voltage, and temperature affect the Resistor Network’s characteristics and specifications. Also, application techniques will be covered that can assist in optimizing the operation of the device to improve performance in the application. The process technology used also affects the operational characteristics. We will focus on the characteristics for devices fabricated in CMOS. RBW - The total resistance from Terminal B to the Wiper Terminal. This resistance equals: RS * (Wiper Register value) + RW. RAW - The total resistance from Terminal A to the Wiper Terminal. This resistance equals: RS * (Full Scale value - Wiper Register value) + RW. Full Scale - When the Wiper is connected to the closest tap point to Terminal A. Zero Scale - When the Wiper is connected to the closest tap point to Terminal B. A n = 256 (Full Scale) RW RS TERMINOLOGY n = 255 To assist with the discussions in this application note, the following terminology will be used. Figure 1 illustrates several of these terms. Resolution - The number of unique wiper positions that can be selected between Terminal B and Terminal A. RW RS W n=2 RAB - The total resistance between the A Terminal and the B Terminal. Resistor Network - Is the combination of RS resistors and RW resistor that create the voltage levels and current paths between the A Terminal, B Terminal, and Wiper Terminal. © 2007 Microchip Technology Inc. RW RBW RS RS - The Step resistance. This is the change in resistance that occurs between two adjacent wiper register values. It is also the RAB resistance divided by the number of RS resistors (resolution) in the Resistor Ladder. Resistor Ladder - Is the serial string of RS resistors between Terminal B and Terminal A. The total resistance of this string equals RAB. RW RAB Wiper Value - The value in the wiper register which selects the one wiper switch to close so that the Wiper Terminal is connected to the Resistor Network. RW - The resistance of the analog switch that connects the Wiper Terminal to the Resistor Ladder. Each analog switch will have slightly different resistive characteristics. RAW n = 254 n=1 RW RS n=0 (Zero Scale) RW B FIGURE 1: 8-Bit Resistor Network. DS01080A-page 1 AN1080 THE RAB RESISTANCE The Step Resistance (RS) The RAB resistance is the total resistance between Terminal A and Terminal B. The RAB resistance is really a resistor string of RS resistors. The RS resistors are designed to be uniform, so they have minimal variation with respect to each other. The RS resistors, and the RAB resistance, will track each other over voltage, temperature, and process. Microchip offers Digital Potentiometer devices with typical RAB resistances of 2.1 kΩ, 5 kΩ, 10 kΩ, 50 kΩ and 100 kΩ. These devices will either offer 6-bits or 8bits of resolution. The step resistance (RS) is the RAB resistances divided by the number of wiper steps. Many manufacturers specify the devices RAB resistance to be ±20% from the targeted (typical) value. This specification is to indicate that from “device-to-device” the resistance could range ±20% from the typical value. This specification is NOT meant that a given devices resistance will vary ±20% over voltage and temperature. The step resistance is important to understand when you are using the device in a rheostat mode, or the potentiometer is being windowed by resistors on the Terminal A and/or on the Terminal B. Table 1 shows the step resistances available for the different RAB values available. TABLE 1: STEP RESISTANCE Step Resistance (RS) (Ω - typ.) So, when the RAB resistance is +10% from the typical value, then each RS resistor is also +10% from the typical value. RAB Resistance (kΩ - typ.) The “device-to-device” RAB resistance could be off by up to 40% of the typical value. This occurs if one device has a resistance (RAB) that is -20% and the other device is +20%. 2.1 33.33 — 5.0 79.37 — 10.0 158.73 39.06 Can trade off between cost and Step Resistance (resolution) 50.0 793.65 195.31 Can trade off between cost and Step Resistance (resolution) 100.0 — 390.63 Largest RAB resistance 6-Bit Device (63 RS) Comment 8-Bit Device (256 RS) RAB(MAX) = 12 kΩ +20% Δ40% RAB(TYP) = 10 kΩ -20% RAB(MIN) = 8 kΩ FIGURE 2: RAB Variations. So, naturally the RAB resistance may have some effect in a Potentiometer configuration (voltage divider), but this variation can have a real effect in a Rheostat configuration (variable resistor). In the Potentiometer configuration, if the A and B terminals are connected to a fixed voltage, then this variation should not effect the system. But, if either (or both) the A or/and B terminals are connected through resistors to the fixed voltage source, then the change in RAB value could effect the voltage at the W terminal (for a given wiper code value). Smallest Step resistance available On a semiconductor device, a resistor can be made with metal/poly/contact components. Designing a structure from these components can be used to form a resistive element (RS). Repeating this resistive element into a string of resistors (RS) creates the RAB resistance. The node between each RS resistor is a contact point (source or drain) for the wiper switch. In the Rheostat configuration, the RBW resistance value will vary as RS varies. So, at full scale RBW approximately equals RAB, and will have the same ±20% from the typical value. DS01080A-page 2 © 2007 Microchip Technology Inc. AN1080 Devices with Multiple Potentiometers THE RW RESISTANCE Some devices are offered that have two or more independent potentiometers. Each potentiometer will exhibit similar characteristics given similar conditions (terminal voltages, wiper settings, …). Figure 4 show the common way to illustrate the block diagram. In this figure, the wiper resistance is represented as a resistor. In actuality, the wiper is connected to each RS node with an analog switch (see Figure 3). Each of these analog switches has a resistive property to them and will vary from switch to switch. Also, the resistive nature of these analog switches is more susceptible to process variations, voltage, and temperature than the step resistors (RS) in the resistor ladder. The RAB variation between potentiometers on the same silicon is relatively small. In dual potentiometer devices, the variation is typically specified as a maximum variation (RAB1-RAB2/RAB1 or RAB1-RAB2/ RAB2) of 1%. This is true even though from device-todevice, the RAB variation can be ±20% over process. The RAB of both potentiometers (and therefore the RSs) will track each other as the device conditions change. It is assumed that the terminals of each potentiometer are at the same voltages (and wiper value). If not, then they may not track each other to the same degree. A RW N = 255 (FFh) RW RS RAB vs. RBW Resistance The RAB resistance is “constant” in that it is independent of the value in the wiper register. While the RBW (or RAW) resistance is directly related to the value in the wiper register. When the wiper register is loaded with it’s maximum value, the RBW resistance is close to RAB resistance. The “closeness” depends on the Resistor Network implementation (see Figure 4), the RS resistance, and the wiper resistance (RW). N = 256 (100h) RS W RS B FIGURE 3: N=1 (01h) RW N=0 (00h) RW Analog Mux RW Implementation. The characteristics of the analog switch depends on the voltages on the switch nodes (source, drain, and gate). The characterization graphs shown in Figure 10 through Figure 13 had Terminal B to VSS and Terminal A to VDD. Within a voltage range, the change in resistance will be linear relative to the device voltage. At some point as the voltage decreases, the resistive characteristics of the switches will become non-linear at increase exponentially. This is related to the operational characteristics of the switch devices at the lower voltage. All the wiper switches will start to increase non-linearly at about the same voltage. Temperature also effects the resistive nature of the wiper switches greater than the RAB (RS) resistance. The wiper resistance increases as the voltage delta between the resistor network node and the voltage on the analog mux switch becomes “small”, so that the switch is not fully turned on. The wiper resistance curve would look different if Terminal A was at VDD/2 while Terminal B is at VSS. In this case, the higher value wiper codes would have the higher wiper resistance (RW). © 2007 Microchip Technology Inc. DS01080A-page 3 AN1080 Implementation B has 255 steps (28 - 1 steps) but 256 Step Resistors (RS). This allows the wiper register to be 8-bits wide, but now the Wiper (W) can no longer connect to Terminal A, since there is one RS resistor between the maximum wiper tap position and the Terminal A connection. The Resistor Network Figure 4 shows three possible Resistor Network implementations for an 8-bit resistor network. Each has an advantage and a disadvantage. The system designer needs to understand which implementation the device uses to ensure the circuit meets the system requirements. Implementation C has 255 steps (28 - 1 steps) and 255 Step Resistors (RS). This allows the wiper register to be 8-bits wide, and to allow the selection of N = 255 (Full Scale). Implementation A has 256 steps (28 steps) and 256 Step Resistors (RS), but the wiper register must be 9bits wide to allow the selection of N = 256 (Full Scale). This increases the complexity of the wiper decode logic (increases cost), but this implementation allows the Wiper (W) to be connected to Terminal A. TABLE 2: Note: The possible nodes that the wiper can connect to on the resistor ladder will depend on the digital potentiometer device. IMPLEMENTATION DIFFERENCES Implementation “True” Wiper Full Register Scale RAB = RBW = Comment A Yes 9-bits 256 RS 256 RS + RW Wiper can connect to the full range of taps from Terminal A and Terminal B, but firmware must take into account the extra addressing bit. The increased complexity of the addressing decode adds cost to the device. B No 8-bits 256 RS 255 RS + RW Wiper can not connect to the Terminal A tap. The application design or the controller firmware may be required to take this into account. C Yes 8-bits 255 RS 255 RS + RW Wiper can connect to the full range of taps from Terminal A and Terminal B, but the controller firmware would need to ensure it addressed that there are 255 RS resistors and not 256 RS resistors. Implementation A A A N = 256 (100h) Implementation B A Implementation C RW RS RS N = 255 (FFh) RS N = 255 (FFh) RW RS N = 255 (FFh) RW RS W W N=1 (01h) RS N=0 (00h) B FIGURE 4: DS01080A-page 4 RW RW N=1 (01h) RS N=0 (00h) Analog Mux RW B RW RW W N=1 (01h) RS N=0 (00h) Analog Mux B RW RW Analog Mux Possible 8-Bit Resistor Network Implementations. © 2007 Microchip Technology Inc. AN1080 THE RBW OR RAW RESISTANCE The Floating Terminal, What to do? When using a Digital Potentiometer device in a Rheostat configuration, should the variable resistor be created from the Wiper to Terminal B (RBW) or from the Wiper to Terminal A (RAW)? When the Digital Potentiometer device is used in a Rheostat configuration, the third terminal (let’s say Terminal A) is “floating”. So what should be done with it? This question really depends on which Terminal (A or B) that the Wiper connects to when the wiper register is loaded with 0h (Zero Scale). For this discussion, we will assume that the Wiper will connect to Terminal B. 1. 2. “Tie” it to the W Terminal. Leave it floating. Method 1 Method 2 A A W W RW RW B FIGURE 5: RBW1 RAW1 RBW1 RAW1 In either case, you can load the wiper register to get the desired resistance value, but if you recall Terminal B is at Zero-Scale. So, that means when using the RBW configuration, as the wiper register is incremented, the resistance increases. Conversely, when using the RAW configuration, as the wiper register is incremented, the resistance decreases. Which configuration is used depends more on any advantages that may occur in the applications firmware algorithm for the control of the resistance. There are two possibilities: B Rheostat Configurations. Method 1: “Tie” it to the W Terminal In this case, the effective resistance of the wiper resistance (RWEFF) will be RW || RAB1. This resistance will always be less than RW, but it will vary over the selected tap position. The RWEFF resistance can be calibrated out of the system, but it becomes a much more complicated controller firmware task. Method 2: Leave it floating This way, the wiper resistance remains “constant” over the selected tap position. This becomes much easier for the controller firmware to calibrate out of the system. © 2007 Microchip Technology Inc. DS01080A-page 5 AN1080 There are two variations that occur over voltage and temperature that we will look at. These are the variations of the RAB resistance and the RW resistance. The characterization graphs also show how these variations effect the INL and DNL error of the device. RAB Variation 4875 VDD = 5.5V 4850 4825 VDD = 2.7V Table 3 shows the RAB data from the MCP402X Data Sheet (DS21945D) Characterization Graphs at 5.5V and 2.7V, and over temperature (@ -40°C, +25°C and +125°C). The minimum and maximum resistance values are also captured. This data was then analyzed over this characterization range. The RS value can be calculated by: RAB / (# RS resistors in RAB) AB) 2080 2060 AB) AB) 10250 10230 10210 10190 10170 10150 10130 10110 10090 10070 10050 2040 0 20 40 60 80 100 120 Ambient Temperature (°C) VDD = 5.5V VDD = 2.7V -40 -20 0 20 40 60 80 100 120 Ambient Temperature (°C) FIGURE 8: MCP402X 10 kΩ – Nominal Resistance (Ω) vs. Ambient Temperature and VDD. 49800 49600 49400 49200 49000 48800 48600 48400 48200 48000 VDD = 5.5V VDD = 2.7V -40 VDD = 5.5V -20 0 20 40 60 80 100 120 Ambient Temperature (°C) FIGURE 9: MCP402X 50 kΩ – Nominal Resistance (Ω) vs. Ambient Temperature and VDD. 2020 VDD = 2.7V 2000 0 40 80 Ambient Temperature (°C) Nominal Resistance (R (Ohms) Depending on the silicon implementation of the RS resistors will determine the characteristic shape of the resistance over temperature. For these devices, the RS resistor was designed so that one part of the resistor has a negative temperature coefficient and another part of the resistor has a positive coefficient. That is the reason why the resistance bows over the temperature range. This is done to minimize the end-to-end change in resistance, and in effect reduces the worst-case delta resistance over temperature. -20 FIGURE 7: MCP402X 5 kΩ – Nominal Resistance (Ω) vs. Ambient Temperature and VDD. AB) 2: For this characterization, Terminal A = VDD and Terminal B = VSS. Nominal Resistance (R (Ohms) 4900 -40 Nominal Resistance (R (Ohms) Note 1: The MCP401X and MCP402X devices have 6-bits of resolution (RAB = 63 RS). -40 2.7V Vdd 5.5V Vdd 4925 4800 For this discussion, we will look at the characterization graphs from the MCP402X Data Sheet (DS21945D). These graphs are shown in Figure 6 through Figure 9. These graphs are used to illustrate several points, but the general characteristics will be seen on all digital potentiometers. Note: 4950 Nominal Resistance (R (Ohms) VARIATIONS OVER VOLTAGE AND TEMPERATURE 120 FIGURE 6: MCP402X 2.1 kΩ – Nominal Resistance (Ω) vs. Ambient Temperature and VDD. DS01080A-page 6 © 2007 Microchip Technology Inc. AN1080 From the analysis, it can be determined that the smaller the RAB resistance, the greater the effect that voltage and temperature has as a percentage of the target resistance. It is interesting to note that depending on the devices target RAB value, either limiting the voltage of operation or limiting the temperature range will lead to minimizing the variation. In the case of the 2.1 kΩ device, if the voltage is held constant, the variation is about 1%, while the variation over temperature is about 2.2%. On the 5.0 kΩ device, variation over temperature is about the same as the variation over voltage. for the 10.0 kΩ and 50.0 kΩ devices, the variation over voltage is much larger than the variation over temperature. Also, if the application is operating at a narrower voltage or temperature window, the RAB variation will be less than across the entire voltage/temperature range. TABLE 3: RAB VALUES AND VARIATION OVER VOLTAGE AND TEMPERATURE Max. 2053 2075 2052 2075 22 0.95% 2030 2010 2018 2007 2030 23 1.10% Delta Resistance over Voltage 35 43 57 48 45 % (of Target Resistance: 2.1 kΩ) 1.67% 2.05% 2.71% 2.29% 2.14% 5.0 kΩ 5.5V 4895 4873 4920 4873 4920 47 0.94% 2.7V 4860 4825 4860 4824 4860 36 0.72% Delta Resistance over Voltage 35 48 60 49 60 % (of Target Resistance: 5.0 kΩ) 0.70% 0.96% 1.20% 0.98% 1.20% 10.0 kΩ 5.5V 10223 10113 10152 10092 10223 131 1.31% 2.7V 10200 10073 10102 10050 10200 150 1.50% Delta Resistance over Voltage 23 50 40 42 23 0.23% 0.50% 0.40% 0.42% 0.23% 5.5V 49590 48880 49220 48810 49590 780 1.56% 2.7V 49510 48880 49080 48790 49510 720 1.44% % (of Target Resistance: 10.0 kΩ) 50.0 kΩ Delta Resistance over Voltage % (of Target Resistance: 50.0 kΩ) 80 0 140 20 80 0.16% 0.00% 0.28% 0.04% 0.16% % (of Target Resistance) Min. 2065 Lowest Min.(1) to Highest Max.(1) +125°C % (of Target Resistance) +25°C 5.5V 2.7V Delta -40°C 2.1 kΩ Voltage Device RAB Characterization RAB Value 68 3.24% 96 1.92% 173 1.73% 800 1.6% Note 1: The lowest Minimum is typically found at 2.7V and the highest Maximum is typically found at 5.5V. See shaded cells. © 2007 Microchip Technology Inc. DS01080A-page 7 AN1080 This change in wiper resistance (RW) effects the INL of the device much greater for devices with the smaller RAB (and therefore RS) resistance value. This can be seen in comparing the wiper resistance and INL error in the graphs of Figure 11 and Figure 13. Wiper Resistance (Rw)(ohms) 100 25C Rw 25C INL 25C DNL 85C Rw 85C INL 85C DNL 125C Rw 125C INL 125C DNL 80 60 RW 20 0 0 8 16 24 32 40 48 Wiper Setting (decimal) 8 -2 16 24 32 40 48 Wiper Setting (decimal) 56 -40C Rw -40C INL -40C DNL 150 25C Rw 25C INL 25C DNL 85C Rw 85C INL 85C DNL 125C Rw 125C INL 125C DNL 0.15 0.1 INL 0.05 100 0 RW 50 -0.05 DNL 0 -0.1 0 8 16 24 32 40 48 56 Wiper Setting (decimal) FIGURE 12: MCP402X 50 kΩ Rheo Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper Setting and Ambient Temperature (VDD = 5.5V). -40C Rw -40C INL -40C DNL 25C Rw 25C INL 25C DNL 85C Rw 85C INL 85C DNL 125C Rw 125C INL 125C DNL 1.5 1 RW 400 0.5 INL 300 0 DNL 200 -0.5 -0.2 100 -1 -0.4 0 56 FIGURE 10: MCP402X 2.1 kΩ Rheo Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper Setting and Ambient Temperature (VDD = 5.5V). DS01080A-page 8 0 DNL RW 200 500 0 DNL 2 100 0.6 0.2 40 4 200 600 0.4 6 300 0.8 INL 8 FIGURE 11: MCP402X 2.1 kΩ Rheo Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper Setting and Ambient Temperature (VDD = 2.7V). Wiper Resistance (Rw)(ohms) -40C Rw -40C INL -40C DNL Error (LSb) 120 10 125C Rw 125C INL 125C DNL Error (LSb) Depending on the configuration of the digital potentiometer in the application (VDD, VA, VB, and wiper code), the wiper resistance may show waveform over wiper code. 85C Rw 85C INL 85C DNL INL 0 Wiper Resistance (Rw)(ohms) The variation of the wiper resistance is also influenced by the wiper code selected and the voltages on Terminal A and Terminal B. 400 25C Rw 25C INL 25C DNL 0 2: For this characterization, Terminal A = VDD and Terminal B = VSS. When the device is at 5.5V, the wiper resistance is relatively stable over the wiper code settings. As the device voltage drops, the wiper resistance increases. Then, at some threshold voltage, the middle codes of the wiper will start to have the highest resistance (see Figure 11). This is due to the resistive characteristics of the analog switch with respect to the voltages on the switch nodes (source, drain, and gate). -40C Rw -40C INL -40C DNL Error (LSb) Note 1: The MCP401X and MCP402X devices have 6-bits of resolution (RAB = 63 RS). 500 Wiper Resistance (Rw)(ohms) For this discussion, we will look at the characterization graphs from the MCP402X Data Sheet (DS21945D). These graphs are shown in Figure 10 through Figure 13. These graphs are used to illustrate several points, but the general characteristics will be seen on all digital potentiometers. Error (LSb) RW Variation -1.5 0 8 16 24 32 40 48 56 Wiper Setting (decimal) FIGURE 13: MCP402X 50 kΩ Rheo Mode – RW (Ω), INL (LSb), DNL (LSb) vs. Wiper Setting and Ambient Temperature (VDD = 2.7V). © 2007 Microchip Technology Inc. AN1080 Table 4 shows the relationship of the Step resistance (RS) to the Wiper Resistance. This is important to understand when the resistor network is being used in a Rheostat configuration, since the variation of the wiper resistance (RW) has a direct effect on the RBW (or RAW) resistance. The system can be designed to calibrate these variations as long as the system is capable of measuring the digital potentiometer device voltage and the system temperature. TABLE 4: TYPICAL STEP RESISTANCES AND RELATIONSHIP TO WIPER RESISTANCE RW / RS (%) (1) Resistance (Ω) RW = Typical RW = Max @ 5.5V RW = Max @ 2.7V RW = Typical RW = Max @ 5.5V RW = Max @ 2.7V Wiper (RW) (3) Typical 225.0% 375.0% 975.0% 3.57% 5.95% 15.48% 325 94.5% 157.5% 409.5% 1.5% 2.50% 6.50% 325 47.25% 78.75% 204.75% 0.75% 1.25% 3.25% 125 192.0% 256.0% 320.0% 0.75% 1.0% 1.25% 0.65% 8-bit Device (256 resistors) 33.33 — 75 125 325 5000 79.37 — 75 125 10000 158.73 — 75 125 — 39.06 75 100 2100 50000 100000 (4) 6-bit Device (63 resistors) Step (RS) Total (RAB) Typical RW / RAB (%) (2) Max @ Max @ 5.5V 2.7V 793.65 — 75 125 325 9.45% 15.75% 40.95% 0.15% 0.25% — 195.31 75 100 125 38.4% 51.2% 64.0% 0.15% 0.20% 0.25% — 390.63 75 100 125 19.2% 25.6%% 32.0% 0.08% 0.10% 0.13% Note 1: RS is the typical value. The variation of this resistance is minimal over voltage. 2: RAB is the typical value. The variation of this resistance is minimal over voltage. 3: RW values are taken from the MCP402X Data Sheet (6-bit devices) and the MCP41XXX/MCP42XXX Data Sheet (8-bit devices). 4: MCP41XXX and MCP42XXX devices. © 2007 Microchip Technology Inc. DS01080A-page 9 AN1080 THE A AND B TERMINALS TABLE 5: The voltage on the A and B terminals (VA and VB) can be any voltage within the devices power supply rails (VSS and VDD). Lets call the voltages at these nodes, VA and VB. This allows a less precise (lower cost) device to be used for more precise circuit tuning over a narrower voltage range. Table 5 shows the effective resolution of the digital potentiometer relative to the system voltage and the VA - VB voltage. V1 R1 6-bit Device (63 RS) 8-bit Device (256 RS) VAB (V) 5.0 79.4 19.5 6-bits 8-bits VAB = VDD 2.5 39.7 9.8 7-bits 9-bits VDD = 5.0V, VAB = VDD/2 1.25 1.98 4.9 8-bits 10-bits VDD = 5.0V, VAB = VDD/4 W POT1 (RAB) Some devices support a “shutdown” mode. The purpose of this mode is to reduce system current. A common implementation is to disconnect either Terminal A or Terminal B from the internal resistor ladder. This creates an open circuit and eliminates the current from Terminal A (or Terminal B) through the RS resistors to Terminal B (or Terminal A). The current to/ from the wiper depends on what the device does with the W Terminal in shutdown. The MCP42XXX device forces the W Terminal to connect to Terminal B (Zero Scale). A SHDN B VB R2 V2 FIGURE 14: N = 256 (100h) RW N = 255 (FFh) RW RS RS W Windowed Trimming. There is no requirement for a voltage polarity between Terminal A and Terminal B. This means that VA can be higher or lower then VB. Comment Shutdown Mode VA A Effective Resolution 8-bit Device (256 RS) This means that the potentiometer can be used to trim a voltage set point within a defined voltage window (see Figure 14). So, if the digital potentiometer is 8-bits (256 steps) and the delta voltage between VA and VB is 1V, then each step of the digital potentiometer results in a change of 1/256 V, or 3.9 mV. If the device needed to have this resolution over an entire 5V range, then the digital potentiometer would require 1280 steps, which is over 10-bits of accuracy. Step Voltage (VS) (mV) 6-bit Device (63 RS) The voltage across the resistor RAB (VAB) is | VA - VB |. In the circuit shown in Figure 14, as the VAB voltage becomes smaller relative to the voltage range, the effective resolution of the device increase, though the resolution is limited to between the VA and VB voltages. HOW THE VAB VOLTAGE EFFECTS THE EFFECTIVE RESOLUTION RS SHDN B N=1 (01h) RW N=0 (00h) RW Analog Mux FIGURE 15: Disconnecting Terminal A (or Terminal B) from the Resistor Ladder. DS01080A-page 10 © 2007 Microchip Technology Inc. AN1080 IMPLEMENTING A MORE PRECISE RHEOSTAT 12 kΩ The RAB (RS) value of a typical digital potentiometer can vary as much as ±20% from device to device. This variation can have a great effect on a circuit that is using the RBW resistance for tuning and this variation for the rheostat value may not be desirable. 8 kΩ If you want to make your variable resistor more precise for system calibration and tuning, the following technique may be useful. To create a circuit with greater accuracy, the system needs to be able to calibrate the digital potentiometer to make a precise rheostat. This is at a cost of the resolution of the digital potentiometer. At the system manufacturing test, a method needs to be present to measure the resistance of the RAB value. This could be done by measuring the current through RAB. This value (RAB(CAL)) would be saved on the embedded systems non-volatile memory. The embedded systems controller could use this information to calibrate the rheostat value (RBW), where: RBW = ((RAB(CAL)/Resolution) * Wiper Value) + RW For this discussion, we will use a digital potentiometer with a typical RAB resistance of 10 kΩ. That means that the RAB resistance could be as small as 8 kΩ (RAB(MIN)) or as large as 12 kΩ (RAB(MAX)). Figure 16 a graphic representation of the variations of RAB resistance by showing the minimum and maximum resistances verses the wiper code value. Table 6 shows the actual calculations for each step for the typical RAB resistance (10 kΩ) and worst-case RAB resistances (8 kΩ and 12 kΩ). When the RAB (RBW) resistance is 12 kΩ, the RBW = 8 kΩ crossover occurs at wiper value 171 (decimal). Very few devices will actually be the 8 kΩ value, but every device will have a wiper register value that will be close to this 8 kΩ resistance. The circuit should assume that the resistance is the minimum. That is because all devices can have a wiper value which “creates” this resistance value. The embedded systems controller firmware would take the calibration value and ensure that the digital potentiometer wiper value did not exceed the desired resistance (8 kΩ). For a system that had a “typical” device (10 kΩ), that would mean the wiper value would not exceed 205 (decimal), while for a “+20% “device (12 kΩ) the wiper value would not exceed 171 (decimal). These values give the closest resistance value to the desired rheostat target value of 8 kΩ. The calibration information could be represented as the maximum wiper value code or as the actual RAB or RS value. The embedded systems controller firmware then would calculate the appropriate wiper values for the desired RBW resistance. Voltage and temperature calibration information could also be stored. © 2007 Microchip Technology Inc. Zero Scale (0) FIGURE 16: RBW Resistance 10 kΩ Full Scale (256) 171 205 Wiper Code RAB Variation. Here we have designed the application circuit where this rheostat only operates from 0Ω to 8 kΩ and all digital potentiometer devices (over process) will meet this requirement. This means that we have reduced the resolution of the digital potentiometer since we no longer have the full 256 steps. Looking at the worstcase resistance (+20%), there are a maximum of 171 steps. This means that the worst-case step accuracy is 1/171 (~0.58%). This represents a resolution of approximately 7.4-bits. We have a trade-off between a precise variable resistor and the resolution (number of steps) that the variable resistor can support. The error from the 8 kΩ target will be no greater than ±RS(MAX)/2 (or ± 23.5Ω) which is ≤ 0.29%. Where: RS(MAX) = RAB(MAX)/Resolution = 12000/256 = 46.875Ω. Any RBW resistance ≤ 8 kΩ can be selected for the variable resistor range. Choosing a lower resistance does not necessarily affect the accuracy, but does affect the number of steps available for the resistor. Let’s say that we select a 5 kΩ resistance, the wiper values would range from 107 (+20%) to 160 (-20%). The worst-case (minimum) number of steps is 107, which gives an step accuracy of 1/107 (~0.93%) and an error from target resistance ≤ 0.47%. This is still in line with systems designed using 1% resistors, but still requires a fixed voltage and temperature. Additional calibration values can be used to correct for the change of the wiper resistance (RW) over temperature and voltage. Additional embedded systems controller firmware calibration can be done to take into account the change in RS and RW resistance over temperature and voltage. DS01080A-page 11 AN1080 Referring to Table 3, for the 10 kΩ (typical) device, the RAB variation over the specified voltage range is ~0.4%. The RAB variation for a given device over temperature is ~1.4%. Other system techniques could be used to calibrate out the effect of these variations. A precise variable resistor can be implemented in a system, if each system’s digital potentiometer is calibrated. Table 6 shows the calculations for a 10 kΩ device, over process. The calculation is based on an 8-bit device that has 256 step resistors (RS) and 257 steps. When the Wiper code value is “01”, that shows the step resistance (RS). TABLE 6: Min. (-20%) Typical 0.00 0.00 31.25 39.0625 46.875 02 62.50 78.125 93.75 0.00 : : : 106 3312.50 4140.625 4868.75 107 3343.75 4179.6875 5015.625 108 3375.00 4218.75 5062.50 : : : 159 4968.75 6210.9375 7453.125 160 5000.00 6250.00 7500.00 161 5031.25 6289.0625 7546.875 : : : 170 5312.50 6640.625 7968.75 171 5343.75 6679.6875 8015.625 172 5375.00 6718.75 8062.50 : : : 204 6375.00 7968.75 9562.50 205 6406.25 8007.8125 9609.375 206 6437.50 8046.875 9656.25 : : : : : Comment Max (+20%) 01 : Using some of these calibration techniques, it was shown how a precise rheostat (variable resistor) can be implemented in a system. RBW RESISTANCE AT WIPER CODE - 10 kΩ (TYPICAL) 8-BIT (256 RS’S) DEVICE 00 : We have discussed how the components of the resistor network (RAB, RS, and RW) can vary over process, voltage, temperature, and wiper code. Understanding these variations allows you to understand the implications in your application and if required use techniques to compensate or calibrate for these variations to optimize the application operation. RBW Resistance (Ω) (1) Wiper Code : SUMMARY 254 7937.50 9921.875 11906.25 255 7968.75 9960.9375 11953.125 256 8000.00 10000.00 12000.00 This indicates the RS resistance value This Wiper Code makes a +20% device have the closest resistance to the 5 kΩ target. This Wiper Code makes a -20% device have the closest resistance to the 5 kΩ target. This Wiper Code makes a +20% device have the closest resistance to the 8 kΩ target. This Wiper Code makes a typical device have the closest resistance to the 8 kΩ target. 8 kΩ resistance is the maximum resistance that is supported by ALL 10 kΩ (typical) devices (over process) Note 1: RBW resistance assume a wiper resistance (RW) of 0Ω. DS01080A-page 12 © 2007 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • Microchip products meet the specification contained in their particular Microchip Data Sheet. • Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. • There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. • Microchip is willing to work with the customer who is concerned about the integrity of their code. • Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. 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Trademarks The Microchip name and logo, the Microchip logo, Accuron, dsPIC, KEELOQ, KEELOQ logo, microID, MPLAB, PIC, PICmicro, PICSTART, PRO MATE, PowerSmart, rfPIC, and SmartShunt are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. AmpLab, FilterLab, Linear Active Thermistor, Migratable Memory, MXDEV, MXLAB, PS logo, SEEVAL, SmartSensor and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Analog-for-the-Digital Age, Application Maestro, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, PICkit, PICDEM, PICDEM.net, PICLAB, PICtail, PowerCal, PowerInfo, PowerMate, PowerTool, REAL ICE, rfLAB, rfPICDEM, Select Mode, Smart Serial, SmartTel, Total Endurance, UNI/O, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2007, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. 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