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functional block diagram buf vdd clkin ad7740 vin fout 2.5v reference voltage-to- frequency modulator clkout gnd clock generation refin/out x1 rev. a 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 which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of analog devices. a ad7740 * features synchronous operation full-scale frequency set by external system clock 8-lead sot-23 and 8-lead microsoic packages 3 v or 5 v operation low power: 3 mw (typ) nominal input range: 0 to v ref true C150 mv capability without charge pump v ref range: 2.5 v to vdd internal 2.5 v reference 1 mhz max input frequency selectable high impedance buffered input minimal external components required applications isolation of high common-mode voltages low-cost analog-to-digital conversion battery monitoring automotive sensing general description the ad7740 is a low-cost, ultrasmall synchronous voltage-to- frequency converter (vfc). it works from a single 3.0 v to 3.6 v or 4.75 v to 5.25 v supply consuming 0.9 ma. the ad7740 is available in an 8-lead sot-23 and also in an 8-lead microsoic package. small package, low cost and ease of use were m ajor design goals for this product. the part contains an on-chip 2.5 v bandgap reference but the user may overdrive this using an external reference. this external reference range includes vdd. the full-scale output frequency is synchronous with the clock signal on the clkin pin. this clock can be generated with the addition of an external crystal (or resonator) or supplied from a cmos-compatible clock source. the part has a maximum input frequency of 1 mhz. for an analog input signal that goes from 0 v to v ref , the out- put frequency goes from 10% to 90% of f clkin. in buffered mode, the part provides a very high input impedance and accepts a range of 0.1 v to vdd C 0.2 v on the vin pin. there is also an unbuffered mode of operation that allows vin to go from C0.15 v to vdd + 0.15 v. the modes are interchangeable using the buf pin. the ad7740 (y grade) is guaranteed over the automotive temperature range of C40 c to +105 c. the ad7740 (k grade) is guaranteed from 0 c to 85 c. one technology way, p.o. box 9106, norwood, ma 02062-9106, u.s.a. tel: 781/329-4700 world wide web site: http://www.analog.com fax: 781/326-8703 ? analog devices, inc., 2001 product highlights 1. the ad7740 is a single channel, single-ended vfc. it is available in 8-lead sot-23 and 8-lead microsoic packages, and is intended for low-cost applications. the ad7740 offers considerable space saving over alternative solutions. 2. the ad7740 operates from a single 3.0 v to 3.6 v or 4.75 v to 5.25 v supply and consumes typically 0.9 ma when the input is u nbuffered. it also contains an automatic power-down function. 3. the ad7740 does not require external resistors and capaci- tors to set the output frequency. the maximum output frequency is set by a crystal or a clock. no trimming or cali- bration is required. 4. the analog input can be taken to 150 mv below gnd for true bipolar operation. 5. the speci?ed voltage reference range on refin is from 2.5 v to the supply voltage, vdd. 3 v/5 v low power, synchronous voltage-to-frequency converter * protected under u.s. patent # 6,147,528.
ad7740 specifications k, y versions 1 parameter 2 min typ max unit test conditions/comments dc performance integral nonlinearity clkin = 32 khz 3 0.012 % of span 4 unbuffered mode, external clock at clkin clkin = 1 mhz 0.012 % of span unbuffered mode, crystal at clkin clkin = 32 khz 3 0.018 % of span buffered mode, external clock at clkin clkin = 1 mhz 0.018 % of span buffered mode, crystal at clkin offset error 7 35 mv unbuffered mode, vin = 0 v 7 35 mv buffered mode, vin = 0.1 v gain error 0.1 0.7 % of span offset error drift 3 20 v/ c gain error drift 3 4 ppm of span/ c power supply rejection ratio 3 C55 db ? vdd = 5% (5 v) C65 db ? vdd = 10% (3.3 v) analog input, vin nominal input span 0 C v ref v 150 mv overrange available 0.1 vdd C 0.2 v buffered mode input current 8 10 a unbuffered mode, vin = 5.4 v, refin = 5.25 v 5 100 na buffered mode, vin = 0.1 v, refin = 2.5 v reference voltage refin 5 nominal input voltage 2.5 vdd v refout output voltage 2.3 2.5 2.7 v output impedance 3 1k ? see pin function description reference drift 3 50 ppm/ c line rejection 3 C75 db ? vdd = 5% (5 v) line rejection 3 C60 db ? vdd = 10% (3.3 v) reference noise (0.1 hz to 10 hz) 3 100 v pCp fout output nominal frequency span 0.1 f clkin to 0.9 f clkin hz vin = 0 v to v ref . see figure 2 logic inputs (clkin, buf) 3 clkin input frequency 32 1000 khz for speci?ed performance input high voltage, v ih 3.5 v vdd = 5 v 5% input high voltage, v ih 2.5 v vdd = 3.3 v 10% input low voltage, v il 0.8 v vdd = 5 v 5% input low voltage, v il 0.4 v vdd = 3.3 v 10% input current 2 a vin = 0 v to v dd pin capacitance 3 10 pf buf input high voltage, v ih 2.4 v vdd = 5 v 5% input high voltage, v ih 2.1 v vdd = 3.3 v 10% input low voltage, v il 0.8 v vdd = 5 v 5% input low voltage, v il 0.4 v vdd = 3.3 v 10% input current 100 na pin capacitance 3 10 pf logic outputs (fout, clkout) 3 output high voltage, v oh 4.0 v output sourcing 200 a 6 . vdd = 5 v 5% output high voltage, v oh 2.1 v output sourcing 200 a 6 . vdd = 3.3 v 10% output low voltage, v ol 0.1 0.4 v output sinking 1.6 ma 6 power requirements v dd 7 3.0 5.25 v i dd (normal mode) 8 0.9 1.25 ma v ih = vdd, v il = gnd. unbuffered mode i dd (normal mode) 8 1.1 1.5 ma v ih = vdd, v il = gnd. buffered mode i dd (power-down) 30 100 a power-up time 3 30 s exiting power-down (ext. clock at clkin) notes 1 temperature range: k version, 0 c to +85 c; y version, C40 c to +105 c; typical speci?cations are at 25 c. 2 see terminology. 3 guaranteed by design and characterization, not production tested. 4 span = max output frequencyCmin output frequency. 5 because this pin is bidirectional, any external reference must be capable of sinking/sourcing 400 a in order to overdrive the internal reference. 6 these logic levels apply to clkout only when it is loaded with one cmos load. 7 operation at vdd = 2.7 v is also possible with degraded specifications. 8 outputs unloaded. i dd increases by c l v out f fout when fout is loaded. if using a crystal/resonator as the clock source, i dd will vary depending on the crystal/resonator type (see clock generation section). speci?cations subject to change without notice. rev. a C2C (vdd = 3.0 v to 3.6 v, 4.75 v to 5.25 v, gnd = 0 v, refin = 2.5 v; clkin = 1 mhz; all specifications t min to t max unless otherwise noted.)
ad7740 rev. a C3C (vdd = 3.0 v to 3.6 v, 4.75 v to 5.25 v, gnd = o v, refin = 2.5 v) limit at t min , t max limit at t min , t max parameter vdd = 3.0 v to 3.6 v vdd = 4.75 v to 5.25 v unit conditions/comments f clkin 32 32 khz min clock frequency 1 1 mhz max t high :t low 40:60 40:60 min clock mark/space ratio 60:40 60:40 max t 1 50 35 ns typ clkin edge to fout edge delay t 2 2.3 1.8 ns typ fout rise time t 3 1.6 1.4 ns typ fout fall time t 4 t high 20 t high 8 ns typ fout pulsewidth notes 1 guaranteed by design and characterization, not production tested. 2 all input signals are speci?ed with tr = tf = 5 ns (10% to 90% of vdd) and timed from a voltage level of (v il + v ih )/2. 3 see figure 1. speci?cations subject to change without notice. t 3 t 2 t 1 t 4 t high t low clkin fout figure 1. timing diagram timing characteristics 1, 2, 3 absolute maximum ratings * (t a = 25 c unless otherwise noted) vdd to gnd . . . . . . . . . . . . . . . . . . . . . . . . . C0.3 v to +7 v analog input voltage to gnd . . . . . . . . C0.3 v to v dd + 0.3 v reference input voltage to gnd . . . . . C0.3 v to v dd + 0.3 v logic input voltage to gnd . . . . . . . . C0.3 v to vdd + 0.3 v fout voltage to gnd . . . . . . . . . . . C0.3 v to vdd + 0.3 v operating temperature range commercial (k version) . . . . . . . . . . . . . . . . 0 c to +85 c automotive (y version) . . . . . . . . . . . . . . C40 c to +105 c storage temperature range . . . . . . . . . . . . C65 c to +150 c junction temperature (t j max) . . . . . . . . . . . . . . . . . . 150 c sot-23 package power dissipation . . . . . . . . . . . . . . . . . . (t j max C t a )/ ja ja thermal impedance . . . . . . . . . . . . . . . . . . . . . 240 c/w lead temperature (10 secs) . . . . . . . . . . . . . . . . . . 300 c reflow soldering peak temperature . . . . . . . . . . . . . . . . . . . . 220 + 5/0 c time at peak temperature . . . . . . . . . . . . 10 sec to 40 sec microsoic package power dissipation . . . . . . . . . . . . . . . . . (t j max C t a )/ ja ja thermal impedance . . . . . . . . . . . . . . . . . . . . . 206 c/w jc thermal impedance . . . . . . . . . . . . . . . . . . . . . . 44 c/w lead temperature (10 secs) . . . . . . . . . . . . . . . . . . . 300 c reflow soldering peak temperature . . . . . . . . . . . . . . . . . . . . . . 220 +5/0 c time at peak temperature . . . . . . . . . . . . . 10 sec to 40 sec * 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 other conditions above those listed in the operational sections of this speci?cation is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. 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 ad7740 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
C4C pin function descriptions 8-lead microsoic pin numbers * pin no. mnemonic function 1 clkout the crysta l/resonator is tied between this pin and clkin. in the c ase of an external clock driving clkin, an inverted clock signal appears on this pin and can be used to drive other circuitry provided it is buffered ?rst. 2 clkin the master clock for the device may be in the form of a crystal/resona tor tied between this pin and clkout. an external cmos-compatible clock may also be applied to this input as the clock for the device. if clkin is inactive low for 1 ms (typ), the ad7740 automatically enters power-down. 3 gnd ground reference for all the circuitry on-chip. 4 refin/out voltage reference input. this is the reference input to the core of the vfc and de?nes the span of the vfc. if this pin is left unconnected, the internal 2.5 v reference is the default reference. alternatively, a precision external reference may be used to overdrive the internal reference. the i nternal reference has high output impedance in order to allow it to be overdriven. 5 vin the analog input to the vfc. it has a nominal input range from 0 v to v ref which corresponds to an output frequency of 10% f clkin to 90% f clkin . it has a 150 mv overrange. if buffered, it draws virtually no current from whatever source is driving it. 6 vdd power supply input. these parts can be operated at 3.3 v 10% or 5 v 5%. the supply should be adequately decoupled with a 10 f and a 0.1 f capacitor to gnd. 7 fout frequency output. fout goes from 10% to 90% of f clkin , depending on vin. 8 buf buffered mode select pin. when buf is tied low, the vin input is unbuffered and the range on the vin pin is C0.15 v to vdd + 0.15 v. when it is tied high, vin is buffered and the range on the vin pin is restricted to 0.1 v to vdd C 0.2 v. * note that the sot-23 and microsoic packages have different pinouts. ordering guide package branding model temperature range package description option information ad7740krm 0 c to 85 c microsoic package rm-8 vok ad7740yrt C40 c to +105 c sot-23 package rt-8 voy ad7740yrm C40 c to +105 c microsoic package rm-8 voy pin configurations 8-lead microsoic microsoic top view (not to scale) 8 7 6 5 1 2 3 4 clkout clkin gnd refin/out buf fout vdd vin ad7740 8-lead sot-23 sot-23 top view (not to scale) 8 7 6 5 1 2 3 4 buf fout vdd vin clkout clkin gnd refin/out ad7740 rev. a ad7740
ad7740 rev. a C5C terminology integral nonlinearity for the vfc, integral nonlinearity (inl) is a measure of the maxi- mum deviation from a straight line passing through the actual endpoints of the vfc transfer function. the error is expressed in % of the actual frequency span: frequency span = fout(max) ?fout(min ) offset error ideally, the output frequency for 0 v input voltage is 10% of f clkin in unbuffered mode. the deviation from this value referred to the input is the offset error at buf = 0. in buffered mode the minimum output frequency (corresponding to 0.10 v minimum input voltage) is 13.2% of f clkin at v ref = 2.5 v. the deviation from this value referred to the input is the offset error at buf = 1. offset error is expressed in mv. gain error this is a measure of the span error of the vfc. the gain is the scale factor that relates the input vin to the output fout. the gain error is the deviation in slope of the ac tual vfc transfer characteristic from the ideal expressed as a percentage of the full- scale span. see figure 2. offset error drift this is a measure of the change in offset error with changes in temperature. it is expressed in v/ c. gain error drift this is a measure of the change in gain error with changes in temperature. it is expressed in (ppm of span)/ c. power supply rejection ratio (psrr) this indicates how the apparent input voltage of the vfc is affected by changes in the supply voltage. the input voltage is kept constant at 2 v, v ref is 2.5 v and the vdd supply is varied 10% at 3.3 v and 5% at 5 v. the ratio of the apparent change in input voltage to the change in vdd is measured in dbs. 0 refin with offset error only ideal gain error output frequency fout 0.9 f clkin offset error with offset error and gain error input voltage vin 0.1 f clkin figure 2. offset and gain
C6C ?.015 ?.005 0 0.005 0.01 ?.01 ?.5 0 0.5 1.5 1 2 2.5 vin ?v vdd = 5v refin = 2.5v clkin = 1mhz t a = 25 c buffer off buffer on inl error ?% of span 0.015 tpc 1. inl vs. vin (buffered and unbuffered) vdd v offset error mv 12 8 8 03 6 45 4 4 0 refin = 2.5v clkin = 1mhz t a = 25 c buf = 0 gain error gain error offset error 0.08 0.04 0.12 0 0.08 0.04 gain error % span tpc 4. offset and gain error vs. vdd vdd v refout v 2.520 2.500 2.5 3.0 5.5 3.5 2.515 2.510 t a = 25 c 2.505 4.0 4.5 5.0 tpc 7. refout vs. vdd clkin frequency khz offset error mv 4 5 8 0 200 1000 400 6 7 600 800 v dd = 5v refin = 2.5v t a = 25 c buffer on buffer off tpc 2. offset error vs. clkin (buffered and unbuffered) vin v 80 1 0 5 1 23 4 psrr db 60 50 70 vdd = 5v refin = 4.75v clkin = 1mhz t a = 25 c buffer on buffer off tpc 5. psrr vs. vin (buffered and unbuffered) 2.00v ch2 2.00v m 2.00 c clkin 1 mhz 1 ch1 tpc 8. typical fout pulse train (vin = v ref /4) clkin frequency khz 0.1 0 0.05 0.05 0.1 0 200 800 600 400 1000 gain error % span buffer off buffer on v dd = 5v refin = 2.5v t a = 25 c tpc 3. gain error vs. clkin (buffered and unbuffered) clkin frequency khz i dd ma 0 0 200 0.25 400 600 800 1000 0.5 0.75 1 1.25 buffer off buffer on vdd = 5v refin = 5v t a = 25 c c fout = 43pf c clkout = 22pf tpc 6. i dd vs. clkin (buffered and unbuffered) ad7740?ypical performance characteristics rev. a
ad7740 rev. a C7C general description the ad7740 is a cmos synchronous voltage-to-frequency converter (vfc) which uses a charge-balance conversion technique. the input voltage signal is applied to a proprietary front-end based around an analog modulator which converts the input voltage into an output pulse train. the part also contains an on-chip 2.5 v bandgap reference and operates from a single 3.3 v or 5 v supply. a block diagram of the ad7740 is shown in figure 3. vin integrator fout comparator buf gnd switched caps switched caps ad7740 figure 3. block diagram input ampli?r buffering and voltage range the analog input vin can be buffered by setting buf = 1. this presents a high impedance, typically 100 m ? , which allows signi?cant external source impedances to be tolerated. the vin voltage range is now 0.1 v to vdd C 0.2 v. by setting buf = 0 the ad7740 input circuit accepts an analog input below gnd and the analog input vin has a voltage range from C0.15 v to vdd + 0.15 v. in this case the input impedance is typically 650 k ?. the transfer function for the ad7740 is represented by: fout = 0.1 f clkin + 0.8 ( vin/v ref ) f clkin it is shown in figure 4 for unbuffered mode. v ref output frequency fout v ref + 0.15v 0.10 f clkin fout min input voltage vin fout max 0.90 f clkin 0 0.15v ad7740 figure 4. transfer function sample calculation: v ref = 2.5 v, buf = 0 fout (min) = 0.1 f clkin + 0.8(C0.15/2.5) f clkin = 0.052 f clkin fout (max) = 0.1 f clkin + 0.8(2.65/2.5) f clkin = 0.948 f clkin vfc modulator the analog input signal to the ad7740 is continuously sampled by a switched capacitor modulator whose sampling rate is set by a master clock. the input signal may be buffered on-chip (buf = 1) before being applied to the sampling capacitor of the modulator. this isolates the sampling capacitor charging currents from the analog input pin. this system is a negative feedback loop that acts to keep the net charge on the integrator capacitor at zero, by balancing charge injected by the input voltage with charge injected by v ref . the output of the comparator provides the digital input for the 1-bit dac, so that the system functions as a negative feedback loop that acts to minimize the difference signal. see figure 5. integrator comparator 1-bit bitstream +v ref input clk v ref ad7740 figure 5. modulator loop the digital data that represents the analog input voltage is con- tained in the duty cycle of the pulse train appearing at the output of the comparator. the output is a pulse train whose frequency depends on the analog input signal. a full-scale input gives an output frequency of 0.9 f clkin and zero-scale input gives an output frequency of 0.1 f clkin . the output allows simple inter- facing to either standard logic families or opto-couplers. the pulsewidth of fout is ?xed and is determined by the high period of clkin. the pulse is synchronized to the rising edge of the clock signal. the delay time between the edge of clkin and the edge of fout is typically 35 ns. figure 6 shows the waveform of this frequency output. (see tpc 8.) 3 t clkin fout = f clkin /2 vin = v ref /2 f clkin fout = f clkin /5 vin = v ref /8 fout = f clkin 3/10 vin = v ref /4 4t clkin average fout is f clkin 3/10 but the actual pulse stream varies between f clkin /3 and f clkin /4 figure 6. frequency output waveforms if there is a step change in input vol tage, there is a settling time that must elapse before valid data is obtained. this is typically two clkin cycles.
C8C clock generation as distinct from the asynchronous vfcs that rely on the stability of an external capacitor to set their full-scale frequency, the ad7740 uses an external clock to de?ne the full-scale output frequency. the result is a more stable transfer function, which allows the designer to determine the system stability and drift based upon the selected external clock. the ad7740 requires a master clock input, which may be an external cmos-compatible clock signal applied to the clkin pin (clkout not used). for a frequency of 1 mhz, a crystal or resonator can be connected between clkin and clkout so that the clock circuit functions as a crystal controlled oscilla- tor. figure 7 shows a simple model of this. clkin clkout 5m c1 c2 on-chip circuitry off-chip circuitry figure 7. on-chip oscillator using the part with a crystal or ceramic resonator between the clkin and clkout pins generally causes more current to be drawn from vdd than when the part is clocked from a driven clock signal at the clkin pin. this is because the on-chip oscillator is active in the case of the crystal or resonator. the amount of additional current depends on a number of factors. first, the larger the value of the capacitor on clkin and clkout pins, the larger the current consumption. typical values recommended by the crystal and resonator manufacturers are in the range of 30 pf to 50 pf. another factor that influ- ences i dd is effective series resistance of the crystal (esr). the lower the esr value, the lower the current taken by the oscillator circuit. the on-chip oscillator also has a start-up time associated with it before it oscillates at its correct frequency and voltage levels. the typical start-up time is 10 ms with a v dd of 5 v and 15 ms with a v dd of 3.3 v (both with a 1 mhz crystal). the ad7740 master clock appears inverted on the clkout pin of the device. the maximum recommended load on this pin is one cmos load. when using a crystal to generate the ad7740s clock it may be desirable to then use this clock as the clock source for the entire system. in this case, it is recommended that the clkout signal be buffered with a cmos buffer before being applied to the rest of the circuit (as shown in figure 7). reference input the ad7740 performs conversions relative to the applied refer- ence voltage. this reference may be taken from the internal 2.5 v bandgap reference by leaving refin/out unconnected. alterna- tively an external precision reference may be used. this is connected to the refin/out pin, overdriving the internal reference. drive capability, initial error, noise, and drift charac- teristics should be considered when selecting an external refer- ence. the ad780 and ref192 are suitable choices for external references. the internal reference is most suited to applications where ratiometric operation of the signal source is possible. using the internal reference in systems where the signal source varies with time, temperature, loading, etc., tends to cancel out errors. power-down mode when clkin is inactive low for 1 ms (typ), the ad7740 auto- matically enters a power-down mode. in this mode most of the digital and analog circuitry is shut down and refout floats. fout goes high. this reduces the power consumption to 525 w max (5 v) and 360 w (3.3 v). applications the basic connection diagram for the part is shown in figure 8. in the connection diagram shown, the ad7740 is con?gured in unbuffered mode. the 5 v power supply is used as a reference to the ad7740. a quartz crystal provides the master clock source for the part. it may be necessary to connect capacitors (c1 and c2 in the diagram) to the crystal to ensure that it does not oscil- late at overtones of its fundamental operating frequency. the values of capacitors will vary depending on the manufacturers speci?cations. refin ad7740 clkin clkout c1 c2 fout gnd buf 0.1 f 10 f vin vdd 5v figure 8. basic connection diagram ad7740 rev. a
ad7740 rev. a C9C a/d conversion techniques using the ad7740 one m ethod of using a vfc in an a/d sy stem is to count the output pulses of f out for a ?xed gate interval (see figure 9). this ?xed gate interval should be generated by dividing down the clock input frequency. this ensures that any errors due to clock jitter or clock frequency drift are eliminated. the ratio of the fout frequency to the clock frequency is what is important here, not the absolute value of fout. the frequency divi- sion can be done by a binary counter where clkin is the counter input. frequency divider gate signal fout vin to p ad7740 counter clock generator clkin figure 9. a/d conversion using the ad7740 vfc figure 10 shows the waveforms of clkin, fout, and the gate signal. a counter counts the rising edges of fout while the gate signal is high. since the gate interval is not synchronized with fout, there is a possibility of a counting inaccuracy. d epending on fout, an error of one count may occur. gate t gate fout clkin figure 10. waveforms in an a/d converter using a vfc the clock frequency and the gate time determine the resolution of such an adc. if 12-bit resolution is required and clkin is 1 mhz (therefore, fout max is 0.9 mhz), the minimum gate time required is calculated as follows: n counts at full scale (0.9 mhz) will take ( n /0.9 10 6 ) seconds = minimum gate time n is the total number of codes for a given resolution; 4096 for 12 bits. minimum gate time = (4096/0.9 10 6 ) seconds = 4.551 ms since t gate fout max = number of counts at full scale, the fastest conversion for a given resolution can be performed with the highest clkin frequency. if the output frequency is measured by counting pulses gated to a signal derived from the clock, the clock stability is unimportant and the device simply performs as a voltage-controlled frequency divider, producing a high-resolution adc. the inherent mono- tonicity of the transfer function and wide range of input clock frequencies allows the conversion time and resolution to be optimized for speci?c applications. another parameter is taken into account when choosing the length of the gate interval. because the integration period of the vfc is equal to the gate interval, any interfering signal can be rejected by counting for an integer number of periods of the inte rfering signal. for example, a gate interval of 100 ms will give normal-mode rejection of 50 hz and 60 hz signals. isolation applications the ad7740 can also be used in isolated analog signal trans- mission applications. due to noise, safety requirements or distance, it may be necessary to isolate the ad7740 from any controlling circuitry. this can easily be achieved by using opto-isolators. this is extremely useful in overcoming ground loops between equipment. the analog voltage to be transmitted is converted to a pulse train using the vfc. an opto-isolator circuit is used to couple this pulse train across an isolation barrier using light as the connecting medium. the input led of the isol ator is driven from the output of the ad7740. at the receiver side, the output transistor is operated in the photo-transistor mode. the pulse train can be reconverted to an analog voltage using a frequency- to-voltage converter; alternatively, the pulse train can be fed into a counter to generate a digital signal. the analog and digital sections of the ad7740 have been designed to allow operation from a single-ended power source, simplify- ing its use with isolated power supplies. figure 11 shows a general purpose vfc circuit using a low cost opto-isolator. a 5 v power supply is assumed for both the iso- lated (v dd ) and local (v cc ) supplies. vin fout gnd1 v cc gnd2 isolation barrier r 0.1 f 10 f ad7740 v dd optocoupler figure 11. opto-isolated application
C10C temperature sensor application the ad7740 can be used with an ad22100s temperature sensor to give a digital measure of ambient temperature. the output voltage of the ad22100s is proportional to the tempera- ture times the supply voltage. it uses a single 5 v supply, and its output swings from 0.25 v at C50 c to 4.75 v at +150 c. by feed ing its output through the ad7740, the value of ambient temperature is converted into a digital pulse train. see figure 12. refin ad7740 ad22100s v+ clkin clkout c1 c2 fout gnd buf 0.1 f 10 f 0.1 f vin vdd 5v 10 f figure 12. using the ad7740 with a temperature sensor due to its ratiometric nature this application provides an extremely cost-effective solution. the need for an external pre- cision reference is eliminated since the 5 v power-supply is used as a reference to both the vfc and the ad22100s. 32 khz operation the ad7740 oscillator circuit will not operate at 32 khz. if the user wishes to use a 32 khz watch crystal, some additional external circuitry is required. the circuit in figure 13 is for a crystal with a required drive of 1 w. resistors r1 and r2 reduce the power to this level. r3 1m clkin r2 100k r1 220k 32khz 40106 40106 figure 13. 32 khz watch crystal circuit power supply bypassing and grounding in any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. the printed circuit board housing the ad7740 should be designed such that the analog and digital sections are separated and con?ned to certain areas of the board. to minimize capacitive coupling between them, digital and analog ground planes should only be joined in one place, close to the ad7740, and should not overlap. avoid running digital lines under the device, as these will couple noise onto the die. the analog ground plane should be allowed to run under the ad7740 to avoid noise coupling. the power supply lines to the ad7740 should use as large a trace as pos- sible to provide low impedance paths and reduce the effects of glitches on the power supply line. fast switching signals like clocks should be shielded with digital ground to avoid radiating noise to other parts of the board, and clock signals should never be run near analog inputs. avoid crossover of digital and analog signals. traces on opposite sides of the board should run at right angles to each other. this reduces the effect of feedthrough through the board. a microstrip technique is by far the best but is not always possible with a double-sided board. in this technique, the component side of the board is dedicated to the ground plane while the signal traces are placed on the solder side. good decoupling is also important. all analog supplies should be decoupled to gnd with surface mount capacitors, 10 f in parallel with 0.1 f located as close to the package as possible, ideally right up against the device. the lead lengths on the by- pass capacitor should be as short as possible. it is essential that these capacitors be placed physically close to the ad7740 to minimize the inductance of the pcb trace between the capacitor and the supply pin. the 10 f are the tantalum bead type and are located in the vicinity of the vfc to reduce low-f requency ripple. the 0.1 f capacitors should have low effective s eries resistance (esr) and effective series inductance (esi), such as the common ceramic types, which provide a low im ped- ance path to ground at high frequencies to handle transient currents due to internal logic switching. additionally, it is ben- e?cial to have large capacitors (> 47 f) located at the point where the power connects to the pcb. ad7740 rev. a
ad7740 rev. a C11C outline dimensions dimensions shown in inches and (mm). 8-lead microsoic (rm-8) 0.011 (0.28) 0.003 (0.08) 0.028 (0.71) 0.016 (0.41) 33 27 0.120 (3.05) 0.112 (2.84) 85 4 1 0.122 (3.10) 0.114 (2.90) 0.199 (5.05) 0.187 (4.75) pin 1 0.0256 (0.65) bsc 0.122 (3.10) 0.114 (2.90) seating plane 0.006 (0.15) 0.002 (0.05) 0.018 (0.46) 0.008 (0.20) 0.043 (1.09) 0.037 (0.94) 0.120 (3.05) 0.112 (2.84) 8-lead sot-23 (rt-8) 1234 8765 0.122 (3.10) 0.110 (2.80) pin 1 0.077 (1.95) bsc 0.026 (0.65) bsc 0.071 (1.80) 0.059 (1.50) 0.009 (0.23) 0.003 (0.08) 0.022 (0.55) 0.014 (0.35) 10 0 0.015 (0.38) 0.009 (0.22) 0.006 (0.15) 0.000 (0.00) 0.051 (1.30) 0.035 (0.90) seating plane 0.057 (1.45) 0.035 (0.90) c01030aC2.5C2/01 (rev. a) printed in u.s.a.

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