AND8413/D Former Catalyst Document Number AN7 Programmable Analog Functions Abstract http://onsemi.com Analog circuits are made programmable by using digital potentiometers (POTs) to vary the key circuit parameters. This application note provides the analog design engineer with basic reference designs and circuit ideas for controlling the key parameters of analog circuits using digital POTs connected to a computer bus or microcontroller. APPLICATION NOTE Bringing Digital to the Analog World Programmable Voltage Amplifier Programmable Instrumentation Amplifier Programmable Dual-Gain Amplifiers Programmable Square Wave Oscillator (555) Programmable Duty Cycle Programmable Bandpass Filter Programmable Voltage Regulator Programmable Current Source/Sink Programmable Schmitt Triggers Programmable Transimpedance Amplifier BRINGING DIGITAL TO THE ANALOG WORLD The potentiometer is a three-terminal, variable, resistive divider analog device. It is used primarily to control, regulate, or adjust a characteristic or parameter of an analog circuit. The changing of the potentiometer’s wiper’s position along the resistive divider provides the variability in a circuit. As opposed to the mechanical potentiometer where the control of the mechanical potentiometer’s wiper is physical, the control of the electronic potentiometer’s wiper is digital. The digital control signals of the electronic potentiometer are commonly connected to a computer bus where the computer’s power of high speed programming, control, and computation are brought to bear in the analog circuit. The potentiometer varies an analog function; digital signals vary the potentiometer; thus, computer generated digital signals vary the analog function. Figure 1 illustrates the concept of computerizing an analog function in block diagram form. Any analog circuit whose parameters depend on a resistance is a candidate for application of a digital POT. The potentiometer can be configured as a two-terminal variable resistance or a three-terminal resistance divider and is used to control voltage, current, resistance, inductance, capacitance, power, frequency, time, duty cycle, gain, Q, bandwidth, etc. The mixed signal digital POT provides the design engineer with a superior analog component that is digitally controlled, programmable, flexible, small size, and low weight. The digital POT provides the manufacturing engineer with a cost effective component that has a short programming time, is reliable, compatible with automated assembly test techniques, and has low field service costs. Semiconductor Components Industries, LLC, 2013 July, 2013 − Rev. 1 VIN ANALOG FUNCTION VOUT Serial Bus Figure 1. Programmable Analog Functions The number of basic functions in the analog domain are limited; they are amplification, oscillation, regulation, filtering, and conversion and the circuits which implement these functions are amplifiers, oscillators, regulators, filters, and converters. The most important of these circuits is, of course, the amplifier. These fundamental circuits are the basic building blocks in analog systems. Typical examples of each of these circuits will illustrate the computerization of the analog functions. Hence, they become programmable analog circuits. While the number of basic analog circuits are limited, they are repeatedly and uniquely used as building blocks to implement more advanced analog functions at the system or end product level. The designs feature the use of the 32-tap CAT5114 and CAT5112 digital POTs with an increment/decrement interface in single supply applications. 1 Publication Order Number: AND8413/D AND8413/D PROGRAMMABLE VOLTAGE AMPLIFIER The circuit in Figure 2 is a noninverting, single-supply, voltage amplifier circuit whose voltage gain is programmed using a digital POT. The voltage gain is established by a traditional noninverting operational amplifier circuit and the circuit’s lower cutoff frequency is established by a first-order, R3C low pass filter implementing the bias network. Variability through the movement of the wiper and programmability through the pot’s instruction set are added to the circuit by the ON Semiconductor digital POT. The ac voltage gain for the circuit has the form of Vo Vs + where p reflects the proportionate position of the wiper from one end of the pot (0) to the other end of the pot (1). The gain versus p or wiper position, shown in Figure 3, is logarithmic-like and is programmable from 1 to 31 for a 32-tap pot. The gain of this noninverting amplifier is relatively stable with respect to temperature because the gain depends on the wiper position and not RPOT. The temperature dependence of RPOT is relatively high and is not guaranteed. The amplifier’s lower cutoff frequency fC is established by the input R3-C network: G o jw fc + jw ) w c wc 2P + 1 2PǒR 3ń2ǓC G0 is the programmable closed-loop passband gain G0 + 1 ) R2 R1 +1) (1 * p)R POT pR POT + 1 p 0 v p v 1, 1 v G v 31 R3 R3 100 kW 100 kW +5 V VS + C 0.1 mF − VO +5 V CAT5114 (1−p) RPOT R2 p RPOT R1 CLGE Figure 2. Programmable Voltage Amplifier Voltage Gain 1......P (32......Tap) Figure 3. Gain versus Wiper Setting http://onsemi.com 2 + 32 Hz AND8413/D PROGRAMMABLE INSTRUMENTATION AMPLIFIER The circuit in Figure 4 is a three-amplifier implementation of an instrumentation amplifier in a single-supply circuit. The implementation, in essence, consists of two noninverting amplifiers (A1 and A2) and a differential amplifier (A3). The voltage gain of the circuit is programmed by a digital POT R1 and is given as Vo V2 * V1 +1) R1 is configured as a variable resistance. The practical gain for the amplifier with a 32 tap digital POT varies from 2 to about 25. For higher resolution and higher gain applications, the 100 tap version of the digital POT should be used. The instrumentation amplifier is ideally suited for bridge, sensor, and data acquisition amplifier applications where high gain, high common mode rejection ratio and high input resistance are required. 10 kW R1 3 V1 (−) 2 + A1 R3 1 R4 − +5 V Digital POT +5 V U/D INC CS R1 9 R2 10 R2 6 V2 (+) 5 − A2 R3 7 − 4 + A3 11 VO 8 R4 + +2.5 V A1 = A2 = A3 = 1/4 LM 6064 R2 = R3 = R4 = 5 kW RPOT = 10 kW Figure 4. Programmable Instrumentation Amplifier PROGRAMMABLE DUAL-GAIN AMPLIFIERS The circuit in Figure 5 is an inverting amplifier and, for a given potentiometer setting, has two values of gain; one for positive input voltages (VO1/VS) and the other for negative input voltages (VO2/VS). The steering diodes D1 and D2 of this circuit establish the polarity of the output voltages and select the gain channel. For single supply circuits, the use of Schottky diodes extends the useable output voltage range. The diodes are within the closed loop and do not impact the value of the gain. The circuit gains in terms of the programming of the digital POT 1 are V O1 VS + G1 + * (1 * p)R POT R where p varies from 0 to 1 and represents the relative pot setting and RPOT represents the end to end resistance. For the values shown in the schematic of this single supply circuit, the gains are individually programmable from 0 to −10. They are however complementary, i.e., there sum equals −10. The transfer characteristic (VO versus VS) in Figure 6 illustrates graphically the programmable dual-gain aspect of the circuit. Figure 7 shows another version of the dual gain amplifier. This version uses a second potentiometer and provides independent values of gain for each polarity of the input voltage. for V S u 0 and V O2 VS + G2 + * (p)R POT R for V S t 0 http://onsemi.com 3 AND8413/D +5 V VO2 } } SERIAL BUS p RPOT (1−p) RPOT D2 +5 V 2 VS R 1 kW 3 − + A1 4 1 11 D1 +2.5 V VO1 A1 = 1/4 LM6064 Figure 5. Dual Gain Amplifier VO increasing p G2 VS G1 increasing p Figure 6. Transfer Characteristic VS VO1 VO2 − + Figure 7. Dual Independent Gain Amplifier http://onsemi.com 4 AND8413/D PROGRAMMABLE SQUARE WAVE OSCILLATOR (555) For R1 = R2 = 0, RPOT = 10 kW, and C = 0.01 mF, The circuit in Figure 8 is a square wave oscillator with programmable control of frequency (fOSC) and duty cycle (DC). The circuit uses the ubiquitous 555 timer IC connected in the astable mode and ON Semiconductor’s CAT5114 digital POT. For this traditional configuration, resistances RA and RB and capacitor C establish the frequency of oscillation and duty cycle. They are given as f OSC + 1.44 ǒR A ) 2R BǓC , DC + 7.20 kHz v f OSC v 14.4 kHz and 0.5 v DC v 1. The nominal value of fOSC is 10.8 kHz with about a 33% adjustment range. The duty cycle is fully adjustable. If R1 = R2 = RPOT = 10 kW and C = 0.003 mF, 9.66 kHz v f OSC v 12.0 kHz and 0.6 v DC v 0.75 RA ) RB The nominal value of fOSC is again 10.8 kHz but with a 20% adjustment range. The adjustment of the duty cycle is limited in this case from 0.6 to 0.75. The desired programming resolution for frequency of oscillation and duty cycle can be achieved either through the potentiometer’s number of taps (programming steps) or the use of R1 and R2. The circuit uses ON Semiconductor’s CAT5114 32-tap digital POT. This circuit can be used as a general purpose, programmable oscillator or the oscillator circuit with its simple, asynchronous increment/decrement interface can be used as a building block in real time, closed-loop feedback type applications. R A ) 2R B If RA and RB are replaced by R1, R2, and the potentiometer resistances pRPOT (wiper resistance to one end) and (1−p) RPOT (wiper resistance to the other end), DC + R 1 ) R 2 ) R POT R 1 ) 2R 2 ) R POT (2 * P) The factor p is a dimensionless number that reflects the relative position of the wiper from one end (0) of the potentiometer to the other end (1) and RPOT is the pot’s end-to-end resistance. The role of resistors R1 and R2 is to control the range over which the frequency and duty cycle can be adjusted. The larger R1 and R2 are relative to the RPOT, the greater the ‘effective’ resolution but the narrower the range of adjustment. The greatest range of adjustment occurs for R1 = R2 = 0. For this case, f OSC + 1.44 1 , DC + (2 * p) R POT(2 * p)C +5 V +5 V 8 U/D INC CS 2 1 7 RA 4 R1 6 3 RB k RPOT 5 7 4 8 3 (1−k) RPOT R2 555 6 5 2 C 0.01 mF, 0.003 mF 1 0.01 mF Figure 8. Dual Independent Gain Amplifier http://onsemi.com 5 AND8413/D PROGRAMMABLE DUTY CYCLE The circuit in Figure 9 is a modification of the traditional one-comparator implementation of a square wave oscillator. The positive feedback network establishes the upper (+3.75 V) and lower voltage (+1.25 V) limits for the comparator while the negative feedback network provides a rising and falling exponential waveform. The digital POT 1 and the steering diodes D1 and D2 establish independent charge and discharge paths for the capacitor C. Hence, where p represents the pot’s relative (0−1) wiper setting and RPOT represents the pot’s end to end resistance. The pot setting programs the duty cycle for the output signal. The total period of the square wave oscillator (Tdisch + Tcharge) output is T + R POT C ln 3 The oscillator’s period, and thus frequency, is constant. For the values shown, the frequency is 910 Hz and the duty cycle can be varied from less than 15% to more than 92%. T disch + pR POT C ln 3 T charge + (1 * p)R POTC ln 3 D1 (1−p) RPOT D2 p RPOT +5 V 2 Digital POT 1 CAT5114 10 kW − 3 C 0.1 mF 360 W + VO 1 20 kW A1 20 kW A1 = 1/2 LM393 +2.5 V Figure 9. Programmable Duty Cycle PROGRAMMABLE BANDPASS FILTER The circuit in Figure 10 is a second-order, bandpass filter whose center frequency is varied using a digital POT. The filter is a member of the Infinite Gain Multiple Feedback (IGMF) class which is characterized by a fixed five-component (three RS and two CS) configuration. The three key parameters for the bandpass filter are characteristic or center frequency wO, passband gain AO, and the figure of merit Q. The gain expression or transfer function for the circuit is VO VS + + Q+ A Osǒw 0ńQǓ 2 s 2 ) sǒw 0ńQǓ ) w 0 Ǹ ǒR 1 ) R 2Ǔ R 1R 2R 3 + 1 2PC Ǹ R3 2R 1 Ǹ , ǒR 1 ) R 2ǓR 3 f0 Q + R 1R 2R 3 + 1 2 Ǹ ǒR 1 ) pR POTǓR 3 R 1pR POTR 3 1 PR 3C The relative position of the wiper is mathematically modeled as a dimensionless number p and varies from 0 (one end of the pot) to 1 (the other end of the pot). A useable range for p in this application is from 0.05 to 1.0. R2 is independent of gain (A0) and bandwidth (BW) and can be used to independently vary the center frequency f0 (and Q). Using digital POTs to vary key parameters in filters (and other analog circuits) saves the cost of using expensive, precision resistors and capacitors to guarantee a circuit’s performance. ON Semiconductor’s CAT5114 potentiometer has a increment/decrement asynchronous s 2 ) ǒ2ńR 3CǓs ) ǒR 1 ) R 3ǓńR 1R 2R 3C 2 1 2PC 1 2 BW + * sńR 1C where f0 + A0 + * ǒR 1 ) pR POTǓ R 1pR POTR 3 http://onsemi.com 6 AND8413/D For the circuit values shown, the passband gain is minus one, the bandwidth is 3.2 kHz, and the center frequency can be varied from 5.5 kHz to 22 kHz. For the wiper set half way, p = 0.5, the filter’s center frequency is 7.5 kHz. interface which lends itself to closed-loop, feedback type applications. For example, the center frequency of the tunable bandpass filter could be varied in a closed loop configuration to match the frequency of an incoming sinusoidal signal in a signal processing system. 1 mF R1 50 kW R3 100 kW C1 0.001 F +5 V C2 VS 0.001 F +5 V 2 3 − + 7 6 4 VO A1 R2 10 kW +2.5 V CAT5114 Figure 10. Programmable Bandpass Filter PROGRAMMABLE VOLTAGE REGULATOR resistors, R1 – R3, as shown in Figure 6. The voltage at the feedback (FB) pin is the regulator’s internal reference of 1.235 V. The internal reference is within the 5 V limitation of the analog potentiometer which does not see the high regulator output voltage in this position. The output voltage for this circuit is given as VO(reg) = 1.235 V{1 + R1/(R2 + R3)}. For VIN(unreg) of 15 V, the regulator output voltage of Figure 6 will vary from 2.5 V to 13.5 V. Discrete resistors R1 and R2 and the potentiometer’s end-to-end resistance RPOT are used to control the range over which the potentiometer will program the output voltage. These resistors and the number of taps of the digital POT are used to define the resolution. The digital POT shown in ON Semiconductor’s low cost, 32 tap CAT5114. This potentiometer has an increment/decrement interface which lends itself to a closed-loop, automated calibration procedure which saves production test time and provides additional security. The circuit can be used as a bias supply or programmable voltage source. In mixed signal systems, the compatibility of the voltages of the analog, digital, and mixed signal devices can be an issue when they meet in the same circuit. For example, the LP2952 in Figure 11 is one of a series of voltage regulators designed for analog output voltages from 1.2 V to 30 V. The regulator also has digital controls in the form of an input digital signal for shutting down to conserve power and an output digital signal indicating an output undervoltage condition. The digitizing of the voltage regulator in a processor based system can be completed by programming the output voltage using the mixed signal, digital POT. The digital POT brings variability to the regulator output voltage through its analog potentiometer and programmability through its digital control signals and serial bus. With the 2952 type regulator, system designers can use the basic device to provide voltage regulation and use digital signals to address power and low voltage concerns and program the output voltage. One problem is most digital POTs are designed for +5 V or 5 V analog systems. This problem can be overcome by placing the variable resistance of the digital POT in the ground leg of the regulator’s programming http://onsemi.com 7 AND8413/D 100 kW VOUT ERROR VO (REG) R1 11 kW 2952 VIN (UNREG) SHUTDOWN 1 mF R2 820 W +5 V 8 U/D INC CS 2 1 7 3 R3 10 kW 5 Control and Memory POR 6.8 mF 1.23 V FB SD GND 0.1 F 6 CAT5114 4 Figure 11. Programmable Voltage Regulator PROGRAMMABLE CURRENT SOURCE/SINK The circuit in Figure 12 is a voltage-controlled, constant current source whose value is programmed using the ON Semiconductor 5112 digital POT. The output current IS = − VS / R where VS is the buffered wiper output voltage of the digital POT. The circuit can either source (VS < 2.50 V) or sink current (VS > 2.50 V). The source is implemented using a summing/ differential amplifier (A1) and a voltage follower (A2). The constant voltage VS across the constant resistor R establishes the constant current. The A2 circuit raises the output voltage of A1 by the amount of the load voltage thus maintaining a constant voltage VS across R. For the circuit values shown, the output current can be programmed from 0 to 1 mA in 64 mA steps R1 100 kW CAT5112 +5 V +5 V − Serial Bus VS R1 100 kW + +5 V 2 3 +2.5 V − 4 R 2.5 kW 1 + IS 11 R1 100 kW R1 100 kW + 7 A2 − 5 6 +2.5 V A1 = A2 = 1/4 LMC6064A Figure 12. Programmable Current Source/Sink http://onsemi.com 8 AND8413/D PROGRAMMABLE SCHMITT TRIGGERS The circuit in Figure 13 is a single-supply version of a programmable Schmitt Trigger or a comparator with hysterisis. The function is implemented using a comparator A1 and a digital POT. The lower (VLL) and upper (VUL) limits of the hysterisis characteristic are a function of the relative setting of the potentiometer’s wiper and are given as the hysterisis curve and the lower and upper limits. The two limits are complementary, i.e. their values sum to 5 V, and hence are not independently programmable. The lower and upper limits of the characteristic can be independently programmed by adding steering diodes D1 and D2 and a second potentiometer as shown in Figure 15. For this circuit, V LL + (1 * p)2.5 V and V UL + 2.5 V ) (p)2.5 V. V LL + ǒ1 * p 2Ǔ2.5 V and V UL + 2.5 V ) ǒp 1Ǔ2.5 V. where p is a dimensionless number from 0 to 1 and represents the potentiometer wiper’s position from one end of the pot (0) to the other end (1). The characteristic’s lower limit can be programmed from 0 V to 2.5 V and the upper limit can be programmed from 2.5 V to 5 V. The circuit’s transfer characteristic (VO versus VS), Figure 14, illustrates With ON Semiconductor’s 32 tap digital POTs, the lower limit can be programmed from 0 to 2.5 V in 81 mV increments and the upper limit can be independently programmed from 2.5 V to 5 V with the same resolution. VO +5 V 5V A1 LM393 VS 2 +5 V 3 360 W − 8 + 4 VO Serial Bus 0 +2.5 V Digital POT 1 CAT5114 2.5 V 5V VS R = RPOT = 10 kW pR (1−p)R VLL Figure 13. Programmable Schmitt Trigger VUL Figure 14. Transfer Characteristic +5 V A1 LM393 VS 360 kW − VO 500 kW +5 V Serial Bus +2.5 V + D1 Digital POT 2 D2 +5 V P2R (1−P2)R Digital POT 1 P1R (1−P1)R CAT5114 (2) Figure 15. Schmitt Trigger with Independently Programmable Limits http://onsemi.com 9 AND8413/D PROGRAMMABLE TRANSIMPEDANCE AMPLIFIER avoids the use of high value resistors in measuring low values of current. For this circuit, The circuit in Figure 16 is an input current (I) to output voltage (V) convertor, or transimpedance amplifier, with programmable gain. The circuit is a current to voltage convertor circuit (A1) cascaded with an inverting amplifier circuit (A2) with a common digital POT varying the gains of the two. Transducers like photodiodes and photovoltaic cells, with their high output impedance, are best modeled as current sources. Their output current is typically converted to a voltage for further signal processing. The potentiometer resistances are modeled as pR (= pRPOT) and (1−p)R where p is a number that varies from 0 to 1 and reflects the proportionate position of the wiper from one end (0) of the potentiometer to the other end (1). The programming of the location of the wiper changes the scale factor between the input current and output voltage without changing the values of any of the resistances and VO Is + * NJ330 W ) pRNj 1 MW 330 W ) (1 * p)10 kW The gain of the transimpedance amplifier (VO/IS) is pseudo-logarithmic and varies from 31kW to 31MW as p is varied from 0 to 1. With these resistances, over four decades of current can be measured from less than 6 nA to over 60 mA. The circuit usWs the low cost, 32 tap ON Semiconductor CAT5114 with a increment/decrement serial interface. This simple, asynchronous interface can be computer controlled or driven by logic and lends itself to automated calibration and test-and-measurement procedures. +5 V 8 2 U/D CAT5114 1 INC 7 CS 4 pR 330 W 6 5 (1−p)R 330 W 1 MW 3 +5 V 2 3 − + 7 4 +5 V 10 kW 6 2 3 A1 − 7 + 4 IS LT1097 +2.5 V Figure 16. Programmable I to V Convertor http://onsemi.com 10 6 A2 VO AND8413/D REFERENCES [1] “Op Amp Network Design” by John R. Hufault, John Wiley and Sons, New York, 1986, pp. 102, 103 [2] “Operational Amplifiers” by Charles F. Wojslaw and Evangelos A. Moustakas, John Wiley and Sons, New York, 1986 [3] “IC Op Amp Cookbook” by Walter G. Jung, Howard Sams, Indianapolis, 1979 [4] “Operational Amplifiers” by G.B. Clayton, Butterworth Scientific, London, 1982 [5] “Linear Potentiometer Implements Logarithmic Gain” by W. Stephen Woodward, EDN, October, 23, 1997 [6] “PWM Circuit uses One Op Amp” by Ferran Bayes, EDN, July 6, 2000 [7] “Decompensated Op Amp Gain is Adjustable from Zero to Open-Loop” by Stephen Woodward, ED, June 4, 2001 [8] “Bipolar Single-supply Schmitt Trigger Extends DPM Range” by Andrea Sosso, ED, October 16, 2000 [9] “Building the Analog Tool Kit”, Signal Conditioning Seminar, Texas Instruments Publication, 2001 [10] “Linear Applications Handbook, Volume III”, Linear Technology Publication, 1997 [11] “Digital Potentiometer Controls AGC Circuit” by Tavares, Tavares, and Piedade, EDN, August 3, 2000 [12] “RTD Auto-nulling Delta-T Thermometer” by Stephen Woodward, ED, June 28, 1999 [13] “IC Timer Cookbook” by Walter G. Jung, Howard Sams and Co. [14] LP2952/53 Voltage Regulator Data Sheets, National Semiconductor Publication ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC reserves the right to make changes without further notice to any products herein. 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