View AN133_2958669.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

application note AN133-1 AN133 putting analog on the bus chuck wojslaw the objective of this application note is to (1) illustrate the idea of computerizing analog functions and (2) provide the design engineer with reference designs for controlling the key parameters of analog circuits using digitally controlled potentiometers connected to a computer bus. basic ideas the computer has extended the capabilities of electronics products and systems by adding the power of high speed control, computation, and programmability at a very low cost. there are, however, applications where the compact, continuous signal, analog circuits are still the preferred design approach. we can, however, have the best of both worlds if we combine the advantages of the computer with the advantages of the analog circuits. this can be done through a xicor digitally controlled potentiometer (xdcp); a system level control device performing a component level function. figure 1 is a caricature depicting the idea of putting analog on the bus. figure 2 illustrates the idea of computerizing an analog function in block diagram form. if parameters of an analog function are varied using a potentiometer and if the potentiometer is computer controlled, then the parameters of the analog function are controlled using the computers bus. the idea of putting analog on the bus refers to varying one or more of an analog circuits parameters through the use of the digitally controlled potentiometer connected to the computer bus. any analog circuit whose parameters depend on a resistance is a candidate for application of a digitally controlled potentiometer. the potentiometer can be con?gured as a two-terminal variable resistance or a three-terminal voltage divider. the mixed signal xdcp provides variability through its analog potentiometer and programmability through its computer interface and bus . figure 1. caricature of the idea of putting analog on the bus clk data address amplifiers filters oscillators, etc.
xicor application note AN133-2 AN133 figure 2. block diagram of computerizing an analog function analog function v+ v+ v s v out clock data address control and memory clock data address r h digitally controlled potentiometer r w r l if a computer controls a potentiometer and if a potentiometer controls an analog circuit then a computer controls the analog circuit the key analog functions are ampli?cation, regulation, ?ltering, oscillation, and conversion and the circuits which implement these functions are ampli?ers, regulators, ?lters, oscillators, and converters. representative examples of each of these circuits will illustrate the computerization of the analog functions. amplifiers: customized potentiometers advance ampli?er design regulators: programmable system-level voltage regulator filters: tunable bandpass (igmf) filter oscillators: digitally controlled potentiometer programs the 555, phase shift sinusoidal oscillator convertors: i to v convertor these basic analog circuits are used as universal building blocks in the design of analog systems and they also serve as models for more specialized analog functions. the following collection of independent articles and circuits are examples of putting analog on the bus. all of them have been breadboarded and tested. customized potentiometers advance amplifier design the circuit in figure 3 is a model of an ampli?er circuit whose cutoff frequency and gain are changed using variable resistors. the cutoff frequency is established by a ?rst-order, rc low pass ?lter and the gain is established by a traditional noninverting operational ampli?er circuit. variability and programmability are added to the circuit if digitally controlled potentiometers (xdcps) are used to implement the variable resistors. the circuit in figure 4 shows the implementation of the frequency and gain controls. the potentiometer r is con?gured as a pseudo-tee network and along with capacitor c establishes the upper cutoff frequency f c . potentiometer r 2 is used as a three-terminal device and establishes the voltage gain g 0 . the voltage gain for the circuit has the form of g 0 is the programmable closed-loop passband gain v o v s ------ g o w c j ww c + ------------------ - = g 0 r 1 r 2 + r 1 k 2 r 2 + ------------------------ - = 0k 2 1
xicor application note AN133-3 AN133 figure 3. model of amplifier circuit figure 4. gain and frequency control 1 f c = 2 p rc r 2 g = 1 + r 1 r 1 r 2 r c low pass filter non-inverting amplifier v o v s + r 1 + r 2 g = f c = r 1 +k 2 r 2 k 2 r 2 + (1? 2 ) r 2 = r 2 control and memory sda addr scl 1 2 p (k 1 r)c x9418w r 1 k 2 r 2 (1-k 2 )r 2 k 1 r (1-k 1 )r +5v +5v ?v ?v v o v s c l w w h r r2 l h + controls cutoff frequency controls passband gain
xicor application note AN133-4 AN133 where k 2 re?ects the proportionate position of the wiper from one end of the pot (0) to the other end of the pot (1). the gain is programmable from 1 to (r 1 +r 2 )/r 1 . the ?xed resistor r 1 limits the circuits maximum voltage gain, a condition usually necessary for accuracy and bandwidth purposes. the upper cutoff frequency f c is established by the input r-c network where k 1 like k 2 re?ects the proportionate position of the wiper from one end of the pot (0) to the other end of the pot (1). the dual versions of the xdcps use the same serial bus with, of course, different addresses for the individual potentiometers. for high frequency ampli?ers, the high end-to-end resistance of 10k w ( r total ) of the x9418 creates time constants limiting the bandwidth of the circuit. the effective end-to-end resistance of the potentiometers can be reduced using two techniques shown in the high frequency ampli?er circuit of figure 5. if the wiper of the potentiometer is connected to a high impedance, shunting r total directly with an external resistor reduces the effective end-to-end resistance. resistor r 3 changes the effective end-to-end resistance of potentiometer r 2 from 10k w to .909k w . if the wiper of the potentiometer is not connected to a high impedance, the effective end-to-end resistance can be reduced by adding external, equal valued resistors (r 4 and r 5 ) from the wiper to the high terminal and from the wiper to the low terminal. this technique, however, creates a potentiometer whose taper is pseudo-linear and whose end-to-end resistance varies with wiper position by about 20%. for most applications, these conditions are acceptable. the gain of the ampli?er circuit in figure 5 is programmable from 1 to 2 and the cutoff frequency is programmable from 130khz to over 1mhz. the principle of computerizing an ampli?er is shown in figure 6 where the ampli?er is a functional block and the gain and cutoff frequency are controlled through a serial bus connected to a digitally controlled potentiometer. the design of high frequency circuits requires low value and sometimes odd value resistances. the two techniques shown in this circuit allow the designer to customize the values of a potentiometers end-to-end resistance to ?t the design requirements. f c w c 2 p ------- 1 2 p k 1 r () c -------------------------- - == 0k 1 1 figure 5. high frequency amplifier control and memory sda addr scl x9418w r1 = 910 r3 = 1k r2 = 10k 10k r4 = 5k 130khz fc 1mhz 1 g o 2 r5 = 5k 300pf +5v lt1220 +5v ?v ?v v o v s +
xicor application note AN133-5 AN133 figure 6. putting an amplifier on the bus voltage amplifier computer bus controls bandwidth and gain v+ v v s v o programmable system-level voltage regulator power consumption and failing batteries are key issues in battery operated applications. features in the new generation of series voltage regulators address these concerns. for example, national semiconductors lp2951 is a micropower voltage regulator which provides a logic- level output signal detecting a low regulated output voltage and has a logic-level input to shutdown the regulator to conserve power. these signals are digital and hence compatible with processor based systems. with these type regulators, system designers can now use the basic device to provide voltage regulation and use the digital signals to address the power and low voltage concerns. but what about the programming or control of the regulators output voltage? surely a mechanical potentiometer or resistor selection is not the solution. we can complete the so-called computerization or digitizing of the voltage regulator by adding a digitally controlled potentiometer to program the regulators output voltage. the circuit in figure 7 is wide-range, computer controlled voltage regulator whose nominal output voltage varies from 1.235v to 14.8v. the regulator is programmed using a xicor digitally controlled potentiometer (xdcp) which, with its 100 taps or steps, can program the regulator output voltage with a resolution of .136v per step. the output voltage is given as v o = 1.235v (1 + kr/910 w ) where k is a number that varies from 0 to 1 and re?ects the proportionate position of the wiper from one end of the pot (0) to the other end of the pot (1). r is the end to end resistance of the potentiometer and is also called r total . the xdcp is programmed using a 3-wire bus and the potentiometer is con?gured as a two-terminal variable resistor. the regulator output signal /error warns of a low output voltage and may be used as a power-on reset. the logic compatible shutdown input signal enables the regulator to be switched on and off to conserve power. these signals along with the ones required to program the xdcp are typically connected to the i/o port of the processor or controller. the potentiometer adds variability to the regulator circuit and its digital controls attached to a computer controlled bus provides programmability. an automated closed-loop calibration procedure to program the regulator saves manufacturing test time and provides enhanced performance and security. the circuit can be used as a bias supply, voltage reference, or as a programmable, high output current, voltage source in test and measurement applications. the stability of the regulator makes it a potential candidate for applications as a voltage reference. the low-power circuit in figure 8 is a programmable voltage reference whose output voltage varies from 4.73v to 5.10v in 3.7mv steps. the output voltage is given as v o = 1.235v [1 + (kr +100k w )/ 34k w )]
xicor application note AN133-6 AN133 figure 7. programmable voltage regulator figure 8. programmable voltage reference 1 + 4 4 100k +5v x9312w lp2951 3.3 f v o (reg) v o (reg) = 1.235v (1 + kr/910 ) 1.235v v o (reg) 14.8v resolution = .136v/step r (10k ) 910 7 7 8 8 2 3 5 1 .01 u/d shutdown error vin (unreg) [ 3 (v o + iv)] cs inc controls output voltage 1 4 4 100k +5v x9312w lp2951 3.3 f v o (reg) = 5.00v r 10k 100k 34k 7 7 8 8 2 3 5 1 .01 u/d shutdown error vin (unreg) ( 3 6.1v) cs inc
xicor application note AN133-7 AN133 the electronic potentiometer adds variability to the regulator circuit and its digital controls, through its computer controlled bus, provides programmability. an automated closed-loop calibration procedure to program the regulator saves test time and provides enhanced performance and security. ( to be published in edn ) tunable bandpass (igmf) filter the circuit in figure 9 is a tunable bandpass ?lter that falls in the in?nite gain multiple feedback (igmf) class. igmf type ?lters are characterized by a ?xed ?ve-component con?guration. for the circuit in figure 9, the gain expression is v o v s ------ sr 1 c C s 2 2 ( r 3 c ) sr 1 r 2 + () r 1 r 2 r 3 c 2 + + --------------------------------------------------------------------------------------------------- a o s w o q () s 2 s w o q ()w o 2 ++ ---------------------------------------------- - = = figure 9. tunable band pass (igmf) filter (r 1 +r 2 ) f 0 = , 2.5khz f 0 12.5khz a 0 = = ? = 796 hz r 1 +r 2 r 3 control and memory u/d cs incr r 3 2r 1 1 2c bw = 1 p r 3 c x9313z 1/2 lm 6142 c 1 =c 2 =c=.01 f r 2 1k 20k r 1 r 3 40k c 1 c 2 +5v +5v ?v v o 1 4 8 2 3 2 v s + controls center frequency
xicor application note AN133-8 AN133 where a o , w o , and q represent the passband gain, characteristic frequency, and ?gure of merit respectively. from the gain expression, and a problem common to most second order, active ?lters is the dependence of each of the ?lters parameters on the values of all or most circuit components. ideally, of course we would like to have one component control one parameter only. the equations above show that f o can be varied by changing r 2 without changing the gain a o . the ?lters bandwidth bw is also independent of r 2 . hence varying r 2 will change the center frequency f o but not the gain nor the bandwidth of the ?lter. if r 2 is a xicor digitally controlled potentiometer (xdcp) connected as a variable resistor, the bandpass ?lter is digitally tunable and programmable. for the circuit values shown, the passband gain is minus one, the bandwidth is 796hz, and the center frequency can be varied from 2.5khz to 12.5khz. the circuit can be modi?ed for self-tuning operation. digitally-controlled potentiometer programs the 555 the use of the digitally-controlled potentiometer in the circuit of figure 10 represents a new twist to controlling the heavily used 555 timer ic. the replacement of discrete resistors r a and r b in the traditional oscillator application of the 555 circuit with a programmable solid-state potentiometer introduces a degree of freedom that allows the computer control of both the frequency of oscillation and the duty cycle. the potentiometer adds variability to the analog circuit and its digital controls, through its computer-controlled serial bus, provides programmability. for discrete values of r a and r b , the frequency and duty cycle are given as if r a and r b are replaced by kr (wiper resistance to one end) and (1-k)r (wiper resistance to the other end) of the potentiometer, the frequency and duty cycle are given as k is a number that varies from 0 to 1 and re?ects the proportionate position of the wiper from one end (0) of the potentiometer to the other end (1). as the wiper is programmed from one end of the potentiometer to the other, the frequency of oscillation will vary from 1.44/2rc < f o < 1.44/rc and the duty cycle will vary from .5 < dc < 1 for the circuit values shown, the frequency will vary 725hz to 1450hz. the circuit uses xicors x9315w which is a 10k w potentiometer with 32 taps and a three- wire interface. the number of taps represents the number of programmable wiper positions or frequency and duty cycle values and r (or r total ) is equivalent to r a + r b . for the xicor digitally-controlled (xdcp) potentiometers, the number of taps vary from 32 to 256, r total varies from 1k w to 1m w , and potentiometers are also available with spi and i 2 c interfaces. a wiper setting can be stored in the xdcps nonvolatile memory permitting the circuit to return to a predetermined frequency and duty cycle value on power-up. the circuit can be used in control, test and measurement, and signal processing applications. a 0 r 3 2r 1 --------- , C = f 0 1 2 p c ----------- - r 1 r 2 + () r 1 r 2 r 3 ----------------------- - , = q12 () r 1 r 2 + () r 3 r 1 r 2 ------------------------------ - = bw f o q ---- 1 p r 3 c --------------- == f o 1.44 r a 2r b + () c --------------------------------- = and dc r a r b + r a 2r b + ------------------------ = f o 1.44 2k C () rc ------------------------- = and dc 1 2k C ----------- - =
xicor application note AN133-9 AN133 figure 10. 555 rectangular oscillator f 0 = , 725 hz f 0 1450 hz u/d cs inc 555 control and memory 1.44 (2-k)rc dc = , .5 dc 1 1 2-k x9315w kr (r a ) (1-k)r (r b ) +5v .01 c .1 f 1 4 7 8 out 2 3 5 6 controls frequency and duty cycle phase shift sinusoidal oscillator the sinusoidal oscillator in figure 11 is implemented with two all pass ?lters, a 2 and a 3 , and inverting ampli?er a 1 . the magnitude of the gain for each of the circuits a 1 , a 2 and a 3 (r 3 =r 2 ) is ideally one. hence the magnitude of the loop gain is ideally one. the phase shift of the loop gain is dependent on the phase shift through the all pass ?lters and it is a function of r, r 1 , and c. the frequency where the phase shift of the loop gain is 360 (or 0) satis?es the condition for oscillation and hence is the frequency of oscillation. an advantage of this con?guration is that the frequency of oscillation can be directly controlled by resistances r or r 1 and no gang tuning of elements is necessary like in many popular oscillator circuits. the ideal loop gain for this con?guration is given by whose magnitude is and whose phase shift is the frequency of oscillation can be found by solving for the value that results in 360 phase shift or the closed loop gain of a 1 (r 3 >r 2 ) is set to greater than one to compensate for the ?nite values of open loop gain of the ampli?ers and to ensure startup. r or r 1 or both resistors can be used to vary the frequency of oscillation. for the values shown, the programmable frequency range is from 1.5khz to 9khz. zener or reference diodes d 1 and d 2 control the amplitude of the signal. af g a1 g a2 g a3 r 3 r 2 ------ C ? ? ? ?? 1sr 1 c C 1sr 1 c + ---------------------- ? ? ? ?? 1 src C 1 src + ------------------- - ? ? ?? == af r 3 r 2 ------ = f 2tan 1 C C w rc = w o 2 p f o 1 c --- 1 rr 1 ---------- 12 / ==
xicor application note AN133-10 AN133 figure 11. phase shift sinusoidal oscillator 1 1 1 4 + + + 4 4 7 7 10k 10k d2 d1 +5v +5v +5v ?v ?v ?v c c c = .01 f 30pf a2 lm308a a1 lm308a 1/2 x9410w a3 lm308a 30pf 30pf 10k 9.1k 10k r 1 (10k ) r 3 10k r 2 r (10k ) 7 8 8 8 2 2 2 3 3 3 6 6 6 so si sck control and memory +5v ?v addr i to v convertor the circuit in figure 12 is an input current (i) to output voltage (v) convertor. the feedback portion of the circuit is a tee network of resistive elements consisting of a digitally controlled potentiometer and a ?xed resistor r 1 (=r). the inputCoutput relationship for the i to v convertor is where k is a number that varies from 0 to 1 and re?ects 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 avoids the use of high value resistors in measuring low values of current. as k goes from 1 to 0, the scale factor goes from Cr (1) to a theoretical Cr( ). the high impedance output of many transducers, like photodiodes and photovoltaic cells, is modeled as a current source. the programming of the 100 tap potentiometer provides for a two-decade change in effective resistance thus allowing the circuit to measure at least four decades of current. for the values shown, the circuit can measure current from 1a to 1ma. v o i s ------ r C 1kk 2 C + () k ---------------------------- =
xicor application note AN133-11 AN133 figure 12. i to v convertor inc u/d cs v o i s + lt1097 = ? 1 a < i s < 1ma v o i s tee network x9c102 (1k ) +5v +5v ?v control & memory kr (1-k)r h w l r (1k ) 1 + k ?k 2 k (1 ?k)r + kr = r = 1k

▲Up To Search▲

Price & Availability of AN133

	To Download AN133 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .