Understanding Digital Potentiometer Resistor Variations

1 2007 Microchip Technology 1AN1080 INTRODUCTIONAll semiconductor devices have Variations overprocess. In the case of Digital Potentiometer devices,this process variation affects the device resistiveelements (RAB -> RS and RW). These resistiveelements also have Variations with respect to voltageand temperature, which will also be application note will discuss how process , voltage,and temperature affect the Resistor Network scharacteristics and specifications. Also, applicationtechniques will be covered that can assist in optimizingthe operation of the device to improve performance inthe process technology used also affects theoperational characteristics.

2 We will focus on thecharacteristics for devices fabricated in assist with the discussions in this application note,the following terminology will be used. Figure 1illustrates several of these - The number of unique wiper positionsthat can be selected between Terminal B and Value - The value in the wiper register whichselects the one wiper switch to close so that the WiperTerminal is connected to the Resistor - The total resistance between the A Terminal andthe B Terminal. RS - The Step resistance. This is the change in resis-tance that occurs between two adjacent wiper registervalues. It is also the RAB resistance divided by the num-ber of RS resistors (resolution) in the Resistor - The resistance of the analog switch that connectsthe Wiper Terminal to the Resistor Ladder.

3 Each analogswitch will have slightly different resistive Ladder - Is the serial string of RS resistorsbetween Terminal B and Terminal A. The total resis-tance of this string equals RAB. Resistor Network - Is the combination of RS resistorsand RW Resistor that create the voltage levels and cur-rent paths between the A Terminal, B Terminal, andWiper - The total resistance from Terminal B to the WiperTerminal. This resistance equals: RS * (Wiper Register value) + RW. RAW - The total resistance from Terminal A to the WiperTerminal. This resistance equals: RS * (Full Scale value - Wiper Register value) + RW. Full Scale - When the Wiper is connected to theclosest tap point to Terminal Scale - When the Wiper is connected to theclosest tap point to Terminal 1:8-Bit Resistor :Mark Palmer Microchip Technology = 0n = 1n = 2n = 254n = 255n = 256(Zero (Full Scale)RSRABRBWRAWRWRSRSRSRWRWRWRWRW Scale) Understanding Digital Potentiometer Resistor VariationsAN1080DS01080A-page 2 2007 Microchip Technology RAB RESISTANCE The RAB resistance is the total resistance betweenTerminal A and Terminal B.

4 The RAB resistance is reallya Resistor string of RS resistors. The RS resistors aredesigned to be uniform, so they have minimal variationwith respect to each other. The RS resistors, and theRAB resistance, will track each other over voltage, tem-perature, and manufacturers specify the devices RAB resis-tance to be 20% from the targeted (typical) value. Thisspecification is to indicate that from device-to-device the resistance could range 20% from the typical specification is NOT meant that a given devicesresistance will vary 20% over voltage andtemperature. So, when the RAB resistance is +10% from the typicalvalue, then each RS Resistor is also +10% from thetypical device-to-device RAB resistance could be off byup to 40% of the typical value.

5 This occurs if one devicehas a resistance (RAB) that is -20% and the otherdevice is +20%.FIGURE 2:RAB , naturally the RAB resistance may have some effectin a Potentiometer configuration (voltage divider), butthis variation can have a real effect in a Rheostatconfiguration (variable Resistor ). In the Potentiometer configuration, if the A and Bterminals are connected to a fixed voltage, then thisvariation should not effect the system. But, if either (orboth) the A or/and B terminals are connected throughresistors to the fixed voltage source, then the change inRAB value could effect the voltage at the W terminal (fora given wiper code value).In the Rheostat configuration, the RBW resistance valuewill vary as RS varies.

6 So, at full scale RBWapproximately equals RAB, and will have the same 20% from the typical Step Resistance (RS)Microchip offers Digital Potentiometer devices withtypical RAB resistances of k , 5 k , 10 k , 50 k and 100 k . These devices will either offer 6-bits or 8-bits of resolution. The step resistance (RS) is the RABresistances divided by the number of wiper step resistance is important to understand whenyou are using the device in a rheostat mode, or thepotentiometer is being windowed by resistors on theTerminal A and/or on the Terminal B. Ta b l e 1 shows thestep resistances available for the different RAB valuesavailable. TABLE 1:STEP RESISTANCEOn a semiconductor device, a Resistor can be madewith metal/poly/contact components.

7 Designing astructure from these components can be used to forma resistive element (RS). Repeating this resistiveelement into a string of resistors (RS) creates the RABresistance. The node between each RS Resistor is acontact point (source or drain) for the wiper (TYP) = 10 k RAB(MAX) = 12 k RAB(MIN) = 8 k -20% +20% 40% RAB Resistance (k - typ.)Step Resistance (RS) ( - typ.)Comment6-Bit Device(63 RS)8-Bit Device(256 RS) Smallest Step resistance trade off between cost and Step Resis-tance (resolution) trade off between cost and Step Resis-tance (resolution) RAB resistance 2007 Microchip Technology 3AN1080 Devices with Multiple PotentiometersSome devices are offered that have two or moreindependent potentiometers.

8 Each Potentiometer willexhibit similar characteristics given similar conditions(terminal voltages, wiper settings, ..).The RAB variation between potentiometers on thesame silicon is relatively small. In dual potentiometerdevices, the variation is typically specified as amaximum variation (RAB1-RAB2/RAB1 or RAB1-RAB2/RAB2) of 1%. This is true even though from device-to-device, the RAB variation can be 20% over RAB of both potentiometers (and therefore theRSs) will track each other as the device conditionschange. It is assumed that the terminals of eachpotentiometer are at the same voltages (and wipervalue). If not, then they may not track each other to thesame vs.

9 RBW Resistance The RAB resistance is constant in that it is indepen-dent of the value in the wiper register. While the RBW(or RAW) resistance is directly related to the value in thewiper register. When the wiper register is loaded withit s maximum value, the RBW resistance is close to RABresistance. The closeness depends on the ResistorNetwork implementation (see Figure 4), the RS resis-tance, and the wiper resistance (RW). THE RW RESISTANCE Figure 4 show the common way to illustrate the blockdiagram. In this figure, the wiper resistance isrepresented as a Resistor . In actuality, the wiper is con-nected to each RS node with an analog switch (seeFigure 3). Each of these analog switches has a resis-tive property to them and will vary from switch to , the resistive nature of these analog switches ismore susceptible to process Variations , voltage, andtemperature than the step resistors (RS) in the 3:RW characteristics of the analog switch depends onthe voltages on the switch nodes (source, drain, andgate).

10 The characterization graphs shown in Figure 10through Figure 13 had Terminal B to VSS and TerminalA to VDD. Within a voltage range, the change in resistance will belinear relative to the device voltage. At some point asthe voltage decreases, the resistive characteristics ofthe switches will become non-linear at increaseexponentially. This is related to the operational charac-teristics of the switch devices at the lower the wiper switches will start to increase non-linearlyat about the same also effects the resistive nature of thewiper switches greater than the RAB (RS) wiper resistance increases as the voltage deltabetween the Resistor network node and the voltage onthe analog mux switch becomes small , so that theswitch is not fully turned on.