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MCP41010E/P Datasheet(PDF) 16 Page - Microchip Technology |
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MCP41010E/P Datasheet(HTML) 16 Page - Microchip Technology |
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16 / 32 page  MCP41XXX/42XXX DS11195C-page 16 2003 Microchip Technology Inc. FIGURE 4-7: Single Supply programmable differential amplifier using digital potentiometers. 4.2.3 PROGRAMMABLE OFFSET TRIM For applications requiring only a programmable voltage reference, the circuit in Figure 4-8 can be used. This circuit shows the device used in the potentiometer mode along with two resistors and a buffered output. This creates a circuit with a linear relationship between voltage-out and programmed code. Resistors R1 and R2 can be used to increase or decrease the output volt- age step size. The potentiometer in this mode is stable over temperature. The operation of this circuit over temperature is shown in Figure 2-3. The worst perfor- mance over temperature will occur at the lower codes due to the dominating wiper resistance. R1 and R2 can also be used to affect the boundary voltages, thereby eliminating the use of these lower codes. FIGURE 4-8: By changing the values of R1 and R2, the voltage output resolution of this programmable voltage reference circuit is affected. 4.3 Calculating Resistances When programming the digital potentiometer settings, the following equations can be used to calculate the resistances. Programming code 00h effectively brings the wiper to the B terminal, leaving only the wiper resis- tance. Programming higher codes will bring the wiper closer to the A terminal of the potentiometer. The equa- tions in Figure 4-9 can be used to calculate the terminal resistances. Figure 4-10 shows an example calculation using a 10 k Ω potentiometer. FIGURE 4-9: Potentiometer resistances are a function of code. It should be noted that, when using these equations for most feedback amplifier circuits (see Figure 4-4 and Figure 4-5), the wiper resistance can be omitted due to the high impedance input of the amplifier. FIGURE 4-10: Example Resistance calculations. MCP601 VB VSS VDD -IN +IN VOUT AB A B (SIG -) MCP42010 1/2 VREF NOTE: Potentiometer values must be equal + - Where: RA RAB 256 Dn – () 256 --------------------------------------- = RB RABDn 256 ------------------ = RAB Total Resistance of pot = Dn Wiper setting forDn 0 to 255 = = VOUT VA VB – () RB RA ------- = VA (SIG +) MCP606 OUT VSS VDD -IN +IN VDD VSS R1 R2 B A 0.1 uF + - PA PB PW Where: PA is the A terminal PB is the B terminal PW is the wiper terminal RWA is resistance between Terminal A and wiper RWB is resistance between Terminal B and Wiper RAB is overall resistance for pot (10 kΩ, 50 kΩ or 100 kΩ) RW is wiper resistance Dn is 8-bit value in data register for pot number n RWA Dn () RAB () 256 Dn – () 256 -------------------------------------------- RW + = RWB Dn () RAB () Dn () 256 ---------------------------- RW + = Example: Code = C0h = 192d R = 10 k Ω Note: All values shown are typical and actual results will vary. PA PB PW 10 k Ω RWA Dn () RAB () 256 Dn – () 256 -------------------------------------------- RW + = RWB C0h () 10k Ω () 192 () 256 ----------------------------------- 52 Ω + = RWA C0h () 10k Ω () 256 192 – () 256 --------------------------------------------------- 52 Ω + = RWA C0h () 2552 Ω = RWB Dn () RAB () Dn () 256 ---------------------------- RW + = RWB C0h () 7552 Ω = |
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