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rev. c information furnished by analog devices is believed to be accurate and reliable. however, no responsibility is assumed by analog devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a 2.5 v/3.0 v high precision reference AD780 functional block diagram r16 r10 +v in r4 r5 q6 q7 r11 r14 r13 r15 nc v out trim gnd o/p select 2.5v ?nc 3.0v ?gnd temp nc nc = no connect AD780 product description the AD780 is an ultrahigh precision band gap reference voltage that provides a 2.5 v or 3.0 v output from inputs between 4.0 v and 36 v. low initial error and temperature drift combined with low output noise and the ability to drive any value of capacitance make the AD780 the ideal choice for enhancing the per formance of high resolution adcs and dacs and for any general-purpose preci- sion reference application. a unique low headroom design facili- tates a 3.0 v output from a 5.0 v 10% input, providing a 20% boost to the dynamic range of an adc, over performance with existing 2.5 v references. the AD780 can be used to source or sink up to 10 ma and can be used in series or shunt mode, thus allowing positive or negative output voltages without external components. this makes it suitable for virtually any high-perform ance reference application. unlike some competing references, the AD780 has no ?egion of possible instability.?the part is stable under all load conditions when a 1 f bypass capacitor is used on the supply. a temperature output pin is provided on the AD780. this pro- vides an output voltage that varies linearly with temperature, allowing the AD780 to be configured as a temperature transd ucer while providing a stable 2.5 v or 3.0 v output. the AD780 is a pin-compatible performance upgrade for the lt1019(a)?.5 and the ad680. the latter is targeted toward low power applications. the AD780 is available in three grades in plastic dip and soic packages. the AD780an, AD780ar, AD780bn, AD780br, and AD780cr are specified for operation from ?0 c to +85 c. features pin-programmable 2.5 v or 3.0 v output ultralow drift: 3 ppm/ c max high accuracy: 2.5 v or 3.0 v 1 mv max low noise: 100 nv/ hz noise reduction capability low quiescent current: 1 ma max output trim capability plug-in upgrade for present references temperature output pin series or shunt mode operation ( 2.5 v, 3.0 v) product highlights 1. the AD780 provides a pin-programmable 2.5 v or 3.0 v out put from a 4 v to 36 v input. 2. laser trimming of both initial accuracy and temperature coefficients results in low errors over temperature without the use of external components. the AD780bn has a maximum variation of 0.9 mv from ?0 c to +85 c. 3. for applications requiring even higher accuracy, an optional fine-trim connection is provided. 4. the AD780 noise is extremely low, typically 4 mv p-p from 0.1 hz to 10 hz and a wideband spectral noise density of typically 100 nv/ hz . this can be further reduced if desired, by simply using two external capacitors. 5. the temperature output pin enables the AD780 to be con- figured as a temperature transducer while providing a stable output reference. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700www.analog.com fax: 781/326-8703 ? analog devices, inc., 2002
(t a = 25 c, v in = 5 v, unless otherwise noted.) AD780an/ar AD780cr AD780bn/br parameter min typ max min typ max min typ max unit output voltage 2.5 v out 2.495 2.505 2.4985 2.5015 2.499 2.501 volts 3.0 v out 2.995 3.005 2.9950 3.0050 2.999 3.001 volts output voltage drift 1 e40 c to +85 c7 73 ppm/ c e55 c to +125 c2 02 0 ppm/ c line regulation 2.5 v output, 4 v +v in 36 v t min to t max 10 10 10 v/v 3.0 v output, 4.5 v +v in 36 v t min to t max 10 10 10 v/v load regulation, series mode sourcing 0 < i out < 10 ma 50 50 50 v/ma t min to t max 75 75 75 v/ma sinking e10 < i out < 0 ma 75 75 75 v/ma e40 c to +85 c757575 v/ma e55 c to +125 c 150 150 150 v/ma load regulation, shunt mode i < i shunt < 10 ma 75 75 75 v/ma quiescent current, 2.5 v series mode 2 e40 c to +85 c 0.75 1.0 0.75 1.0 0.75 1.0 ma e55 c to +125 c 0.8 1.3 0.8 1.3 0.8 1.3 ma minimum shunt current 0.7 1.0 0.7 1.0 0.7 1.0 ma output noise 0.1 hz to 10 hz 4 4 4 v p-p spectral density, 100 hz 100 100 100 nv/ hz h h
AD780 rev. c ?3? ordering guide initial temperature temperature package model error range coefficient options AD780an 5.0 mv e40 c to +85 c7 ppm/ cp lastic dip AD780ar 5.0 mv e40 c to +85 c7 ppm/ c soic AD780ar-reel7 5.0 mv e40 c to +85 c7 ppm/ c soic AD780bn 1.0 mv e40 c to +85 c3 ppm/ cp lastic dip AD780br 1.0 mv e40 c to +85 c3 ppm/ c soic AD780br-reel 1.0 mv e40 c to +85 c3 ppm/ c soic AD780br-reel7 1.0 mv e40 c to +85 c3 ppm/ c soic AD780cr 1.5 mv e40 c to +85 c7 ppm/ c soic AD780cr-reel7 1.5 mv e40 c to +85 c7 ppm/ c soic absolute maximum ratings * +v in to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 v trim pin to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 v temp pin to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 v power dissipation (25 c) . . . . . . . . . . . . . . . . . . . . . 500 mw storage temperature . . . . . . . . . . . . . . . . . . e65 c to +150 c lead temperature (soldering 10 sec) . . . . . . . . . . . . . . 300 c output protection: output safe for indefinite short to ground and momentary short to v in . esd classification . . . . . . . . . . . . . . . . . . . . . class 1 (1000 v) * stresses above those listed under absolute maximum ratings may cause perma- nent damage to the device. this is a stress rating only; functional operation of the device at these or any conditions above those indicated in the operational specifi- cation is not implied. exposure to absolute maximum specifications for extended periods may affect device reliability. pin configuration 8-lead plastic dip and soic packages 1 2 3 4 8 7 6 5 top view (not to s cale) AD780 2.5v/3.0v o/p select (nc or gnd) nc v out trim nc +v in temp gnd nc = no connect die layout gnd temp +v in trim v out 2.5v/3.0v o/p select gnd notes both v out pads should be connected to the output. die thickness: the standard thickness of analog devices bipolar dice is 24 mil 2 mil. die dimensions: the dimensions given have a tolerance of 2 mil. backing: the standard backside surface is silicon (not plated). analog devices does not recommend gold-backed dice for most applications. edges: a diamond saw is used to separate wafers into dice thus providing per- pendicular edges halfway through the die. in contrast to scribed dice, this tech- nique provides a more uniform die shape and size. the perpendicular edges facilitate handling (such as tweezer pickup), while the uniform shape and size simplify substrate design and die attach. top surface: the standard top surface of the die is covered by a layer of glassivation. all areas are covered except bonding pads and scribe lines. surface metalization: the metalization to analog devices bipolar dice is aluminum. minimum thickness is 10,000 a. bonding pads: all bonding pads have a minimum size of 4.0 mil by 6.0 mil. the passivation windows have a 3.6 mil by 5.6 mil minimum size. caution esd (electrostatic discharge) sensitive device. electrostatic charges as high as 4000 v readily accumulate on the human body and test equipment and can discharge without detection. although the AD780 features proprietary esd protection circuitry, permanent damage may occur on devices subjected to high-energy electrostatic discharges. therefore, proper esd precautions are recommended to avoid performance degradation or loss of functionality. warning! esd sensitive device
rev. c ?4? theory of operation band gap references are the high performance solution for low supply voltage and low power voltage reference applications. in this technique, a voltage with a positive temperature coefficient is combined with the negative coefficient of a transistor?s vbe to produce a constant band gap voltage. in the AD780, the band gap cell contains two npn transistors (q6 and q7) that differ in emitter area by 12 . the differ ence in their vbes produces a ptat current in r5. this, in turn, pro duces a ptat voltage across r4 that, when combined with the vbe of q7, produces a voltage vbg that does not vary with tem- perature. precision laser trimming of the resistors and other pat- ented circuit techniques are used to further en hance the drift perfor mance. r16 r10 +v in r4 r5 q6 q7 r11 r14 r13 r15 nc v out trim gnd o/p select 2.5v e nc 3.0v e gnd temp nc nc = no connect AD780 figure 1. schematic diagram the output voltage of the AD780 is determined by the configu- ration of resistors r13, r14, and r15 in the amplifier?s feedback loop. this sets the output to either 2.5 v or 3.0 v depending on whether r15 (pin 8) is grounded or not connected. a unique feature of the AD780 is the low headroom design of the high gain amplifier which produces a precision 3 v output from an input voltage as low as 4.5 v (or 2.5 v from a 4.0 v in put). the amplifier design also allows the part to work with +v in = v out when current is forced into the output terminal. this allows the AD780 to work as a two terminal shunt regulator providing a e2.5 v or e3.0 v reference voltage output without external components. the ptat voltage is also used to provide the user with a ther- mometer output voltage (at pin 3) that increases at a rate of ap- proximately 2 mv/ c. the AD780?s nc (pin 7) is a 20 k resistor to v+ that is used solely for production test purposes. users who are currently using the lt1019 self-heater pin (pin 7) must take into account the different load on the heater supply. applying the AD780 the AD780 can be used without any external components to achieve specified performance. if power is supplied to pin 2 and pin 4 is grounded, pin 6 provides a 2.5 v or 3.0 v output depending on whether pin 8 is left unconnected or grounded. a bypass capacitor of 1 f (+v in to gnd) should be used if the load capacitance in the application is expected to be greater than 1 nf. the AD780 in 2.5 v mode typically draws 700 a of iq at 5v . this increases by ~2 a/v up to 36 v. nc temp +v in v out trim gnd o/p select 2.5v e nc 3.0v e gnd nc r null r pot 1 f AD780 nc = no connect figure 2. optional fine trim circuit initial error can be nulled using a single 25 k potentiometer connected between v out , trim, and gnd. this is a coarse trim with an adjustment range of 4% and is only included here for compatibility purposes with other references. a fine trim can be implemented by inserting a large value resistor (e.g., 1e5 m ) in series with the wiper of the potentiometer (see figure 2 above). t he trim range, expressed as a fraction of the output, is simply greater than or equal to 2.1 k /r null for either the 2.5 v or 3.0 v mode. the external null resistor affects the overall temperature coeffi- cient by a factor equal to the percentage of v out nulled. for example, a 1 mv (0.03%) shift in the output caused by the trim circuit, with a 100 ppm/ c null resistor, will add less than 0.06 ppm/ c to the output drift (0.03% 200 ppm/ c, since the resistors internal to the AD780 also have temperature coeffi cients of less than 100 ppm/ c). noise performance the impressive noise performance of the AD780 can be further improved if desired by the addition of two capacitors: a load capacitor, c1, between the output and ground, and a compensa- tion capacitor, c2, between the temp pin and ground. suitable values are shown in figure 3. AD780
AD780 rev. c ?5? 100 1 0.1 0.1 1 100 10 10 load capacitor, c1 e f compensation capacitor, c2 e nf figure 3. compensation and load capacitor combinations c1 and c2 also improve the settling performance of the AD780 when subjected to load transients. the improvement in noise performance is shown in figures 4, 5, and 6. 10 90 100 0% 100 v1s 0.1 to 10hz amplifier gain = 100 10 90 100 0% 20 v 10ms 10hz to 10khz no amplifier figure 4. standalone noise performance nc temp +v in v out trim gnd o/p select 2.5v e nc 3.0v e gnd nc nc = no connect c2 1 f c1 AD780 figure 5. noise reduction circuit noise comparison the wideband noise performance of the AD780 can also be expressed in ppm. the typical performance with c1 and c2 is 0.6 ppm and without external capacitors is 1.2 ppm. this performance is respectively 7 and 3 lower than the specified performance of the lt1019. 10 90 100 0% 20 v 10ms 10hz to 10khz no amplifier figure 6. reduced noise performance with c1 = 100 f, c2 = 100 nf temperature performance the AD780 provides superior performance over temperature by means of a combination of patented circuit design techniques, precision thin film resistors, and drift trimming. temperature performance is specified in terms of ppm/ c, but because of non- linearity in the temperature characteristic, the box test method is used to test and specify the part. the nonlinearity takes the form of the characteristic s-shaped curve shown in figure 7. the box test method forms a rectangular box around this curve, enclosing the maximum and minimum output voltages over the specified tem perature range. the specified drift is equal to the slope of th e di agonal of this box. 2.0 e0.8 140 0.4 e0.4 e40 0 e60 1.6 0.8 1.2 120 100 80 60 40 20 0 e20 temperature e c error e mv figure 7. typical AD780bn temperature drift temperature output pin the AD780 provides a temp output (pin 3) that varies lin early with temperature. this output can be used to monitor changes in system ambient temperature and to initiate calibra tion of the system if desired. the voltage v temp is 560 mv at 25 c, and the temperature coefficient is approximately 2 mv/ c.
rev. c ?6? figure 8 shows the typical v temp characteristic curve over temperature taken at the output of the op amp with a noninverting gain of five. 4.25 2.00 150 2.50 2.25 e50 e75 2.75 3.00 3.25 3.50 3.75 4.00 125 100 75 50 25 0 e25 temperature e c voltage e v out circuit calibrated at 25 c refer to figure 9 10mv per c figure 8. temperature pin transfer characteristic since the temp voltage is acquired from the band gap core circuit, current pulled from this pin will have a significant effect on v out . care must be taken to buffer the temp output with a suitable op amp, e.g., an op07, ad820, or ad711 (all of which would result in less than a 100 v change in v out ). the relation- ship between i temp and v out is as follows: v out = 5.8 mv/ a i temp (2.5 v range ) or v out = 6.9 mv/ a i temp (3.0 v range ) notice how sensitive the current dependent factor on v out is. a large amount of current, even in tens of microamp, drawn from the temp pin can cause v out and temp output to fail. the choice of c1 and c2 was dictated primarily by the need for a relatively flat response that rolled off early in the high-frequency noise at the output. but there is considerable margin in the choice of these capacitors. for example, the user can actually put a huge c2 on the temp pin with none on the output pin. however, one must either put very little or a lot of capacitance at the temp pin. intermediate values of capacitance can sometimes cause oscilla tion. in any case, the user should follow the recommendation in fig ure 3. temperature transducer circuit the circuit shown in figure 9 is a temperature transducer that amplifies the temp output voltage by a gain of a little over +5 to provide a wider full-scale output range. the trimpot can be used to adjust the output so it varies exactly by 10 mv/ c. to minimize resistance changes with temperature, resistors with low temperature coefficients, such as metal film resistors, should be used. 5v +v in temp gnd ad820 6.04k (1%) 1.27k (1%) 200 r f r b r bp 10mv/ c 1 f AD780 + e figure 9. differential temperature transducer supply current over temperature the AD780?s quiescent current will vary slightly over tempera ture and input supply range. the test limit is 1 ma over the in dus- trial and 1.3 ma over the military temperature range. typical performance with input voltage and temperature variation is shown in figure 10. 0.85 0.60 36 0.75 0.65 0.70 4 0.80 input voltage e v quiescent current e ma e55 c +125 c +25 c figure 10. typical supply current over temperature AD780
AD780 rev. c ?7? turn-on time the time required for the output voltage to reach its final value within a specified error band is defined as the turn-on settling time. the two major factors that affect this are the active circuit settling time and the time for the thermal gradients on the chip to stabilize. typical settling performance is shown in figure 11. the AD780 settles to within 0.1% of its final value within 10 s. 2.499v 2.498v 2.500v 0v 5v 10 s/div v in v out figure 11. turn-on settling time performance dynamic performance the output stage of the AD780 has been designed to provide superior static and dynamic load regulation. figures 12a and 12b show the performance of the AD780 while driving a 0 ma to 10 ma load. +v in v out AD780 v l 0v v out 249 1 f figure 12a. transient resistive load test circuit output change e 50mv/div 10ma 0ma 10 s/div i load v out (c l = 0pf) figure 12b. settling under transient resistive load the dynamic load may be resistive and capacitive. for example, the load may be connected via a long capacitive cable. figures 13a and 13b show the performance of the AD780 driving a 1000 pf, 0 ma to 10 ma load. +v in v out AD780 v l 0v v out 249 1 f c l 1000pf figure 13a. capacitive load transient response test circuit output change e 50mv/div 10 s/div 0ma 10ma i load v out (c l = 1000pf) figure 13b. settling under dynamic capacitive load
rev. c ?8? line regulation line regulation is a measure of the change in output voltage due to a specified change in input voltage. it is intended to simulate worst-case unregulated supply conditions and is measured in v/v. figure 14 shows typical performance with 4.0 v < v in < 15.0 v. 200 e200 415 100 e100 10 0 input voltage e v output change e v t = 25 c figure 14. output voltage change vs. input voltage precision reference for high resolution 5 v data converters the AD780 is ideally suited to be the reference for most 5 v high resolution adcs. the AD780 is stable under any capaci- tive load, it has superior dynamic load performance, and the 3.0 v output provides the converter with the maximum dynamic range without requiring an additional and expensive buffer a mplifier. one of the many adcs that the AD780 is suited for is t he ad7884, a 16-bit, high-speed sampling adc (see figure 15). this part previously needed a precision 5 v reference, resistor divider, and buffer amplifier to do this function. 5v +v in v out gnd AD780 1 f v ref +f v ref +s ad7884 2.5/3.0v select figure 15. precision 3 v reference for the ad7884 16-bit, high-speed adc the AD780 is also ideal for use with higher resolution convert- ers, such as the ad7710/ad7711/ad7712 (see figure 16). while these parts are specified with a 2.5 v internal reference, the AD780 in 3 v mode can be used to improve the absolute accuracy, temperature stability, and dynamic range. it is shown below with the two optional noise reduction capacitors. 5v +v in v out gnd AD780 1 f refin+ refine ad7710 2.5v/3.0v o/p select 100 f 100nf figure 16. precision 2.5 v or 3.0 v reference for the ad7710 high resolution, sigma-delta adc 4.5 v reference from 5 v supply some 5 v high resolution adcs can accommodate reference voltages up to 4.5 v. the AD780 can be used to provide a precision 4.5 v reference voltage from a 5 v supply using the circuit shown in figure 17. this circuit will provide a regulated 4.5 v output from a supply voltage as low as 4.7 v. the high quality tantalum 10 f capacitor in parallel with the ceramic AD780 0.1 f capacitor and the 3.9 resistor ensures a low output impedance around 50 mhz. AD780 2n2907 10 f 3.9 4k 0.01% 5k 0.01% 0.1 f 0.1 f 1k v supply v out op90 2.5k 0.1 f + e figure 17. 4.5 v reference from a single 5 v supply AD780
AD780 rev. c ?9? negative (e2.5 v) reference the AD780 can produce a negative output voltage in shunt mode, simply by connecting the input and output to ground connecting the AD780?s gnd pin to a negative supply via a bias resistor as shown in figure 18. nc temp +v in v out trim gnd o/p select 2.5v e nc 3.0v e gnd nc 1 f AD780 ve e2.5 v out note 1. i l = load current 2. i s min = minimum shunt current 3. nc = no connect v out e (ve) i l + i s min r = figure 18. negative (?2.5 v) shunt mode reference a precise e2.5 v reference capable of supplying up to 100 ma to a load can be implemented with the AD780 in series mode using the bootstrap circuit shown below. AD780 2n3906 +5v +5v op07 connect if e3v output desired e5v e2.5v (i l 100ma) e5v out 1k 1000pf +v in e + figure 19. ?2.5 v high load current reference
rev. c ?10? outline dimensions dimensions shown in millimeters and (inches) 8-lead standard small outline package [soic] narrow body (r-8) 0.25 (0.0098) 0.19 (0.0075) 1.27 (0.0500) 0.40 (0.0157) 0.50 (0.0196) 0.25 (0.0099) 45 8 0 1.75 (0.0688) 1.35 (0.0532) seating plane 0.25 (0.0098) 0.10 (0.0040) 85 4 1 5.00 (0.1968) 4.80 (0.1890) pin 1 4.00 (0.1574) 3.80 (0.1497) 1.27 (0.0500) bsc 6.20 (0.2440) 5.80 (0.2284) 0.51 (0.0201) 0.33 (0.0130) coplanarity controlling dimensions are in millimeters; inch dimensions (in parentheses) are rounded-off millimeter equivalents for reference only and are not appropriate for use in design compliant to jedec standards ms-012 aa dimensions shown in inches and (mm) 8-lead plastic dual-in-line package [pdip] (n-8) seating plane 0.060 (1.52) 0.015 (0.38) 0.210 (5.33) max 0.022 (0.56) 0.014 (0.36) 0.160 (4.06) 0.115 (2.93) 0.070 (1.77) 0.045 (1.15) 0.130 (3.30) min 8 1 4 5 pin 1 0.280 (7.11) 0.240 (6.10) 0.100 (2.54) bsc 0.430 (10.92) 0.348 (8.84) 0.195 (4.95) 0.115 (2.93) 0.015 (0.38) 0.008 (0.20) 0.325 (8.25) 0.300 (7.62) AD780
AD780 rev. c ?11? revision history location page 5/02?data sheet changed from rev. b to rev. c. updates to packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
printed in u.s.a. c00841?0?5/02(c) ?12?

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