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3-287 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 TC9400 tc9401 tc9402 voltage-to-frequency/frequency-to-voltage converters features voltage-to-frequency n choice of guaranteed linearity: tc9401 ......................................................... 0.01% TC9400 ......................................................... 0.05% tc9402 ......................................................... 0.25% n dc to 100 khz (f/v) or 1hz to 100khz (v/f) n low power dissipation .......................... 27mw typ n single/dual supply operation ................................. + 8v to + 15v or 4v to 7.5v n gain temperature stability .......... 25 ppm/ c typ n programmable scale factor frequency-to-voltage n operation ........................................... dc to 100 khz n choice of guaranteed linearity: tc9401 ......................................................... 0.02% TC9400 ......................................................... 0.05% tc9402 ......................................................... 0.25% n programmable scale factor applications n m p data acquisition n 13-bit analog-to-digital converters n analog data transmission and recording n phase-locked loops n frequency meters/tachometer n motor control n fm demodulation general description the TC9400/tc9401/tc9402 are low-cost voltage-to- frequency (v/f) converters utilizing low power cmos technology. the converters accept a variable analog input signal and generate an output pulse train whose frequency is linearly proportional to the input voltage. the devices can also be used as highly-accurate fre- quency-to-voltage (f/v) converters, accepting virtually any input frequency waveform and providing a linearly-propor- tional voltage output. a complete v/f or f/v system only requires the addition of two capacitors, three resistors, and reference voltage. i in i ref TC9400 r in integrator opamp integrator capacitor threshold detector one shot pulse output pulse/2 output 2 input voltage reference capacitor reference voltage ordering information linearity temperature part no. (v/f) package range TC9400cod 0.05% 14-pin 0 c to +70 c soic (narrow) TC9400cpd 0.05% 14-pin 0 c to +70 c plastic dip TC9400ejd 0.05% 14-pin C 40 c to +85 c cerdip tc9401cpd 0.01% 14-pin 0 c to +70 c plastic dip tc9401ejd 0.01% 14-pin C 40 c to +85 c cerdip tc9402cpd 0.25% 14-pin 0 c to +70 c plastic dip tc9402ejd 0.25% 14-pin C 40 c to +85 c cerdip functional block diagram TC9400/1/2-5 11/6/96
3-288 telcom semiconductor, inc. electrical characteristics: v dd = +5v, v ss = C 5v, v gnd = 0v, v ref = C 5v, r bias = 100k w , full scale = 10khz, unless otherwise specified. t a = +25 c, unless temperature range is specified (C 40 c to +85 c for e device, 0 c to +70 c for c device). voltage-to-frequency tc9401 TC9400 tc9402 parameter definition min typ max min typ max min typ max unit accuracy linearity 10 khz output deviation from straight 0.004 0.01 0.01 0.05 0.05 0.25 % full line between normalized zero scale and full-scale input linearity 100 khz output deviation from straight 0.04 0.08 0.1 0.25 0.25 0.5 % full line between normalized zero scale reading and full-scale input gain temperature variation in gain a due to 25 40 25 40 50 100 ppm/ c drift (note 1) temperature change full scale gain variance variation from ideal accuracy 10 C 10 10 C % of nominal zero offset (note 2) correction at zero adjust for zero 10 50 10 50 20 100 mv output when input is zero zero temperature variation in zero offset due to 25 50 25 50 50 100 m v/ c drift (note 1) temperature change analog input i in full scale full-scale analog input current to 10 10 10 m a achieve specified accuracy i in overrange overrange current 50 50 50 m a response time settling time to 0.1% full scale 2 2 2 cycle digital section v sat @ i ol = 10ma logic "0" output voltage (note 3) 0.2 0.4 0.2 0.4 0.2 0.4 v v out max C v out voltage range between output 18 18 18 v common (note 4) and common pulse frequency 3 3 3 m sec output width absolute maximum ratings* v dd C v ss ................................................................. +18v i in ...........................................................................10ma v out max Cv out common .......................................... 23v v ref C v ss ..............................................................C 1.5v storage temperature range ................ C 65 c to +150 c operating temperature range c device ................................................ 0 c to +70 c e device ........................................... C 40 c to +85 c package dissipation (t a 70 c) 8-pin cerdip .................................................. 800mw 8-pin plastic dip ............................................. 730mw 8-pin soic .....................................................470mw lead temperature (soldering, 10 sec) ................. +300 c *static-sensitive device. unused devices must be stored in conductive material. protect devices from static discharge and static fields. stresses above those listed under absolute maximum ratings may cause perma- nent damage to the device. these are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-289 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 notes: 1. full temperature range. guaranteed, not tested. 2. i in = 0. 3. full temperature range, i out = 10ma. 4. i out = 10 m a. 5. threshold detect = 5v, amp out = 0v, full temperature range electrical characteristics: (cont.) v dd = +5v, v ss = C 5v, v gnd = 0, v ref = C 5v, r bias = 100k w , full scale = 10khz, unless otherwise specified. t a = +25 c, unless temperature range is specified C 40 c to +85 c for e device, 0 c to +70 c for c device. frequency-to-voltage tc9401 TC9400 tc9402 parameter definition min typ max min typ max min typ max unit supply current i dd quiescent current required from positive (note 5) supply during operation 1.5 6 1.5 6 3 10 ma i ss quiescent current required from negative (note 5) supply during operation C 1.5 C 6 C 1.5 C 6 C 3 C 10 ma v dd supply operating range of positive supply 4 7.5 4 7.5 4 7.5 v v ss supply operating range of negative supply C 4 C 7.5 C 4 C 7.5 C 4 C 7.5 v reference voltage v ref Cv ss range of voltage reference input C 2.5 C 2.5 C 2.5 v accuracy nonlinearity (note 10) deviation from ideal transfer 0.01 0.02 0.02 0.05 0.05 0.25 % full function as a percentage scale full-scale voltage input frequency frequency range for specified 10 100k 10 100k 10 100k hz range (note 7 and 8) nonlinearity frequency input positive excursion voltage required to turn 0.4 v dd 0.4 v dd 0.4 v dd v threshold detector on negative excursion voltage required to turn C 0.4 C 2 C 0.4 C 2 C 0.4 C 2 v threshold detector off minimum positive time between threshold 5 5 5 m sec pulse width (note 8) crossings minimum negative time between threshold 0.5 0.5 0.5 m sec pulse width (note 8) crossings input impedance 10 10 10 m w analog outputs output voltage voltage range of op amp output v dd C 1 v dd C 1 v dd C 1 v (note 9) for specified nonlinearity output loading resistive loading at output of 2 2 2 k w op amp supply current i dd quiescent current required from positive (note 10) supply during operation 1.5 6 1.5 6 3 10 ma i ss quiescent current required from negative (note 10) supply during operation C 1.5 C 6 C 1.5 C 6 C 3 C 10 ma v dd supply operating range of positive supply 4 7.5 4 7.5 4 7.5 v v ss supply operating range of negative supply C 4 C 7.5 C 4 C 7.5 C 4 C 7.5 v reference voltage v ref Cv ss range of voltage reference input C 2.5 C 2.5 C 2.5 v 6. 10hz to 100khz.; guaranteed, not tested 7. 5 m sec minimum positive pulse width and 0.5 m sec minimum negative pulse width. 8. t r = t f = 20 nsec. 9. r l 3 2k w .; tested @ 10k w 10. full temperature range, v in = C 0.1v. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-290 telcom semiconductor, inc. pin no. symbol description 1i bias this pin sets bias current in the TC9400. connect to v ss through a 100 k w resistor. see text. 2 zero adj low frequency adjustment input. see text. 3i in input current connection for the v/f converter. 4v ss negative power supply voltage connection, typically C 5v. 5v ref out reference capacitor connection. 6 gnd analog ground. 7v ref voltage reference input, typically C 5v. 8 pulse freq out frequency output. this open drain output will pulse low each time the freq threshold detector limit is reached. the pulse rate is proportional to input voltage. 9 output common source connection for the open drain output fets. see text. 10 freq/2 out this open drain output is a square wave at one half the frequency of the pulse output (pin 8). output transitions of this pin occur on the rising edge of pin 8. 11 threshold detect input to the threshold detector. this pin is the frequency input during f/v operation. 12 amplifier out output of the integrator amplifier. 13 nc no internal connection 14 v dd positive power supply connection, typically +5v. pin descriptions pin configurations 1 2 3 4 5 6 7 14 13 12 11 10 9 8 v dd nc amplifier out threshold detector freq/2 out output common pulse freq out i bias zero adj i in v ss v ref out gnd v ref 1 2 3 4 5 6 7 14 13 12 11 10 9 8 v dd nc amplifier out threshold detector freq/2 out output common pulse freq out i bias zero adj i in v ss v ref out gnd v ref TC9400 tc9401 tc9402 TC9400 tc9401 tc9402 nc = no internal connection 14-pin plastic dip/cerdip 14-pin soic (narrow) voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-291 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 figure 1. 10 hz to 10 khz v/f converter + +5v + 5v 14 v dd + 5v r l 10k w r l 10k w 8 10 9 f out f out /2 11 3 sec delay self- start 12 5 20k w 60pf opamp c int 820pf c ref 180pf 12pf r in 1m w v in input +5v ?v 50k w 510k w 10k w 3 1 offset adjust i in zero adjust 0v ?0v i bias v ss 4 ?v 2 output common v ref out r bias 100k w amp out TC9400 tc9401 tc9402 gnd 6 threshold detector threshold detect reference voltage (typically ?v) 2 v ref 7 ?v at the end of the charging period, c ref is shorted out. this dissipates the charge stored on the reference capaci- tor, so that when the output again crosses zero the system is ready to recycle. in this manner, the continued discharg- ing of the integrating capacitor by the input is balanced out by fixed charges from the reference voltage. as the input voltage is increased, the number of reference pulses re- quired to maintain balance increases, which causes the output frequency to also increase. since each charge in- crement is fixed, the increase in frequency with voltage is linear. in addition, the accuracy of the output pulse width does not directly affect the linearity of the v/f. the pulse must simply be long enough for full charge transfer to take place. voltage-to-frequency (v/f) circuit description the TC9400 v/f converter operates on the principal of charge balancing. the operation of the TC9400 is easily understood by referring to figure 1. the input voltage (v in ) is converted to a current (i in ) by the input resistor. this current is then converted to a charge on the integrating capacitor and shows up as a linearly decreasing voltage at the output of the op amp. the lower limit of the output swing is set by the threshold detector, which causes the reference voltage to be applied to the reference capacitor for a time period long enough to charge the capacitor to the reference voltage. this action reduces the charge on the integrating capacitor by a fixed amount (q = c ref v ref ), causing the op amp output to step up a finite amount. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-292 telcom semiconductor, inc. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 pin functions threshold detector input in the v/f mode, this input is connected to the amplifier output (pin 12) and triggers a 3 m sec pulse when the input voltage passes through its threshold. in the f/v mode, the input frequency is applied to this input. the nominal threshold of the detector is halfway be- tween the power supplies, or (v dd + v ss )/2 400mv. the TC9400's charge balancing v/f technique is not dependent on a precision comparator threshold, because the threshold only sets the lower limit of the op-amp output. the op-amp's peak-to-peak output swing, which determines the frequency, is only influenced by external capacitors and by v ref . pulse freq out this output is an open-drain n-channel fet which provides a pulse waveform whose frequency is proportional to the input voltage. this output requires a pull-up resistor and interfaces directly with mos, cmos, and ttl logic. freq/2 out this output is an open-drain n-channel fet which provides a square wave one-half the frequency of the pulse frequency output. the freq/2 output will change state on the rising edge of pulse freq out. this output requires a pull- up resistor and interfaces directly with mos, cmos, and ttl logic. the TC9400 contains a "self-start" circuit to ensure the v/f converter always operates properly when power is first applied. in the event that, during power-on, the op amp output is below the threshold and c ref is already charged, a positive voltage step will not occur. the op-amp output will continue to decrease until it crosses the C3.0v threshold of the "self-start" comparator. when this happens, an internal resistor is connected to the op-amp input, which forces the output to go positive until the TC9400 is in its normal operating mode. the TC9400 utilizes low power cmos processing for low input bias and offset currents with very low power dissipation. the open-drain n-channel output fets provide high voltage and high current sink capability. voltage-to-time measurements the TC9400 output can be measured in the time do- main as well as the frequency domain. some microcom- puters, for example, have extensive timing capability but limited counter capability. also, the response time of a time domain measurement is only the period between two out- put pulses, while the frequency measurement must accu- mulate pulses during the entire counter timebase period. time measurements can be made from either the TC9400's pulse freq out output or from the freq/2 output. the freq/2 output changes state on the rising edge of pulse freq out, so freq/2 is a symmetrical square wave at one half the pulse output frequency. timing measurements can therefore be made between successive pulse freq out pulses, or while freq/2 is high (or low). figure 2 . output waveforms 3 ?ec typ 1/f f out f out /2 amp out v ref 0v c ref c int notes: 1. to adjust f min , set v in = 10mv and adjust the 50k w offset for 10hz output. 2. to adjust f max , set v in = 10v and adjust r in or v ref for 10 khz output. 3. to increase f out max to 100khz, change c ref to 2pf and c int to 75pf. 4. for high-performance applications, use high-stability components for r in , c ref , v ref (metal film resistors and glass capacitors). also, separate output ground (pin 9) from input ground (pin 6).
3-293 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 output common the sources of both the freq/2 out and the pulse freq out are connected to this pin. an output level swing from the drain voltage to ground or to the v ss supply may be obtained by connecting this pin to the appropriate point. r bias an external resistor, connected to v ss , sets the bias point for the TC9400. specifications for the TC9400 are based on r bias = 100k w 10%, unless otherwise noted. increasing the maximum frequency of the TC9400 beyond 100khz is limited by the pulse width of the pulse output (typically 3 m sec). reducing r bias will decrease the pulse width and increase the maximum operating frequency, but linearity errors will also increase. r bias can be reduced to 20k w , which will typically produce a maximum full scale frequency of 500khz. amplifier out the output stage of the operational amplifier. during v/f operation, a negative-going ramp signal is available at this pin. in the f/v mode, a voltage proportional to the frequency input is generated. zero adjust this pin is the noninverting input of the operational amplifier. the low-frequency set point is determined by adjusting the voltage at this pin. i in the inverting input of the operational amplifier and the summing junction when connected in the v/f mode. an input current of 10 m a is specified, but an overrange current up to 50 m a can be used without detrimental effect to the circuit operation. i in connects the summing junction of an operational amplifier. voltage sources cannot be attached directly, but must be buffered by external resistors. v ref a reference voltage from either a precision source or the v ss supply is applied to this pin. accuracy of the TC9400 is dependent on the voltage regulation and temperature char- acteristics of the reference circuitry. since the TC9400 is a charge balancing v/f converter, the reference current will be equal to the input current. for this reason, the dc impedance of the reference voltage source must be kept low enough to prevent linearity errors. for linearity of 0.01%, a reference impedance of 200 w or less is recommended. a 0.1 m f bypass capacitor should be connected from v ref to ground. v ref out the charging current for c ref is supplied through this pin. when the op amp output reaches the threshold level, this pin is internally connected to the reference voltage and a charge, equal to v ref x c ref , is removed from the integrator capacitor. after about 3 m sec, this pin is internally connected to the summing junction of the op amp to dis- charge c ref . break-before-make switching ensures that the reference voltage is not directly applied to the summing junction. v/f converter design information input/output relationships the output frequency (f out ) is related to the analog input voltage (v in ) by the transfer equation: frequency out = external component selection r in the value of this component is chosen to give a full- scale input current of approximately 10 m a: r in @ . example: note that the value is an approximation and the exact relationship is defined by the transfer equation. in practice, the value of r in typically would be trimmed to obtain full- scale frequency at v in full scale (see "adjustment proce- dure"). metal film resistors with 1% tolerance or better are recommended for high-accuracy applications because of their thermal stability and low-noise generation. c int the exact value is not critical but is related to c ref by the relationship: 3c ref c int 10 c ref . improved stability and linearity are obtained when c int 4c ref . low-leakage types are recommended, although mica and ceramic devices can be used in applica- tions where their temperature limits are not exceeded. locate as close as possible to pins 12 and 13. v in r in 1 (v ref ) (c ref ) v in full scale 10 m a r in @ = 1m w . 10v 10 m a
3-294 telcom semiconductor, inc. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 c ref the exact value is not critical and may be used to trim the full-scale frequency (see "input/output relationships"). glass film or air trimmer capacitors are recommended because of their stability and low leakage. locate as close as possible to pins 5 and 3. v dd , v ss power supplies of 5v are recommended. for high- accuracy requirements, 0.05% line and load regulation and 0.1 m f disc decoupling capacitors located near the pins are recommended. adjustment procedure figure 1 shows a circuit for trimming the zero location. full scale may be trimmed by adjusting r in , v ref , or c ref . recommended procedure for a 10khz full-scale frequency is as follows: (1) set v in to 10 mv and trim the zero adjust circuit to obtain a 10hz output frequency. (2) set v in to 10v and trim either r in , v ref , or c ref to obtain a 10khz output frequency. if adjustments are performed in this order, there should be no interaction and they should not have to be repeated. figure 3. recommended c ref vs v ref 500 400 300 200 100 0 ? ? ? ? ? ? ? v ref (v) c ref (pf) +12pf 1 khz 100khz v dd = +5v v ss = ?5v r in = 1m w v in = +10v t a = +25? improved single supply v/f converter operation a TC9400 which operates from a single 12 to 15v variable power source is shown in figure 5. this circuit uses two zener diodes to set stable biasing levels for the TC9400. the zener diodes also provide the reference voltage, so the output impedance and temperature coefficient of the zeners will directly affect power supply rejection and temperature performance. full scale adjustment is accomplished by trimming the input current. trimming the reference voltage is not recom- mended for high accuracy applications unless an op amp is used as a buffer, because the TC9400 requires a low impedance reference (see the v ref pin description section for more information). the circuit of figure 5 will directly interface with cmos logic operating at 12v to 15v. ttl or 5v cmos logic can be accommodated by connecting the output pullup resistors to the +5v supply. an optoisolator can also be used if an isolated output is required.
3-295 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 v + = 8v to 15v (fixed) 14 8 10k w 10k w f out f out /2 10 149 100 k w 0v?0v i in 180 pf 820 pf 3 5 12 11 7 0.01 f 2 k w 8.2 k w 6 2 v 2 r 2 0.9 r 1 0.2 r 1 r in 1m w i in v ref TC9400 offset adjust gain adjust v + 10v 12v 15v 1 m w 1.4 m w 2 m w r 1 r 2 10k w 14k w 20k w f out = i in 1 (v 2 v 7 ) (c ref ) (v in ? 2 ) (v + ? 2 ) + i in = r in (0.9 r 1 +0.2 r 1 ) 5v 0.01 f v in figure 4 . fixed voltage single supply operation figure 5. voltage to frequency r1 910k r4 100k 1 f d2 5.1vz r2 910k r5 91k rp offset 20k 100k d1 5.1vz 0.1 100k c ref c int 1.2k* +12 to +15v 10k 10k output frequency digital ground analog ground input voltage (0 to 10v) r3 gain TC9400 11 12 5 3 2 6 7 1 4 14 9 10 8 threshold detect amp out c ref i in zero adjust gnd v ref i bias output common f out /2 f out v dd v ss component selection f/s freq . 1 khz 10 khz 100 khz c ref 2200pf 180pf 27pf c int 4700pf 470pf 75pf
3-296 telcom semiconductor, inc. frequency-to-voltage (f/v) circuit description when used as an f/v converter, the TC9400 generates an output voltage linearly proportional to the input frequency waveform. each zero crossing at the threshold detector's input causes a precise amount of charge (q = c ref v ref ) to be dispensed into the op amp's summing junction. this charge in turn flows through the feedback resistor, generating voltage pulses at the output of the op amp. a capacitor (c int ) across r int averages these pulses into a dc voltage which is linearly proportional to the input frequency. f/v converter design information input/output relationships the output voltage is related to the input frequency (f in ) by the transfer equation: v out = [v ref c ref r int ] f in . the response time to a change in f in is equal to (r int c int ). the amount of ripple on v out is inversely proportional to c int and the input frequency. c int can be increased to lower the ripple. values of 1 m f to 100 m f are perfectly acceptable for low frequencies. when the TC9400 is used in the single-supply mode, v ref is defined as the voltage difference between pin 7 and pin 2. +5v 14 64 +5v ?v v dd 1.0m 11 33k in914 v ss det TC9400 0.01 f frequency input 0v gnd +8v to +5v 14 10k 4 +5v v dd 1.0m 11 33k in914 v ss det TC9400 0.01 f frequency input 0v 0.1 f 10k (b) single supply (a) 5v supply figure 6. frequency input level shifter input voltage levels the input frequency is applied to the threshold detector input (pin 11). as discussed in the v/f circuit section of this data sheet, the threshold of pin 11 is approximately (v dd + v ss ) /2 400mv. pin 11's input voltage range extends from v dd to about 2.5 v below the threshold. if the voltage on pin 11 goes more than 2.5 volts below the threshold, the v/f mode startup comparator will turn on and corrupt the output voltage. the threshold detector input has about 200 mv of hysteresis. in 5 v applications, the input voltage levels for the TC9400 are 400mv, minimum. if the frequency source being measured is unipolar, such as ttl or cmos operat- ing from a +5v source, then an ac coupled level shifter should be used. one such circuit is shown in figure 6a. the level shifter circuit in figure 6b can be used in single supply f/v applications. the resistor divider ensures that the input threshold will track the supply voltages. the diode clamp prevents the input from going far enough in the negative direction to turn on the startup comparator. the diode's forward voltage decreases by 2.1 mv/ c, so for high ambient temperature operation two diodes in series are recommended. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-297 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 0.5?ec min 5.0?ec min delay = 3?ec input f out f out /2 figure 7. dc 10 khz f/v converter TC9400a tc9401a tc9402a +5v 14 v dd v + v + f out/2 f out output common 10 9 8 5 3 12 12pf c ref 56 pf see equation, page 12 c int 1000pf r int 1 m w 60pf amp out v o v ss i bias 14 10 k w 2.2k w 100k w 2 k w ?v +5v zero adjust 2 7 ( typically ?v ) v ref f in 11 threshold detector 3 sec delay * * * * optional if buffer is needed offset adjust v ref out i in 4 2 + op amp + v ref see figure 6 6 gnd threshold detect figure 8 . f/v digital outputs input buffer f out and f out /2 are not used in the f/v mode. however, these outputs may be useful for some applications, such as a buffer to feed additional circuitry. then, f out will follow the input frequency waveform, except that f out will go high 3 m sec after f in goes high; f out /2 will be squarewave with a frequency of one-half f out . if these outputs are not used, pins 8, 9 and 10 should be connected to ground. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402
3-298 telcom semiconductor, inc. voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 figure 10. ripple filter offset adjust 10k .01 f 6.2v in914 33k 100k 500k 0.1 f 100k v + = 10v to 15v 1m 47pf v out frequency input TC9400 6 10k 2 11 1.0m 4 14 12 3 5 gnd v ref out i in zero adjust v ref i bias amp out v dd v ss gnd 6 7 1.0k v + 1.0k 0.01 f .001 f det note: the output is referenced to pin 6, which is at 6.2v (vz). for frequency meter applications, a 1 ma meter with a series-scaling resistor can be placed across pins 6 and 12. 1m 47pf v out TC9400 12 3 5 v ref out i in gnd amp out 6 .001 f + 1m 3 2 .01 f 1m 0.1 f +5 7 6 4 ? tl071 200 output filtering the output of the TC9400 has a sawtooth ripple super- imposed on a dc level. the ripple will be rejected if the TC9400 output is converted to a digital value by an integrat- ing analog to digital converter, such as the tc7107 or tc7109. the ripple can also be reduced by increasing the value of the integrating capacitor, although this will reduce the response time of the f/v converter. the sawtooth ripple on the output of an f/v can be eliminated without affecting the f/v's response time by using the circuit in figure 10. the circuit is a capacitance multiplier, where the output coupling capacitor is multiplied by the ac gain of the op amp. a moderately fast op amp, such as the tl071, should be used. figure 9. f/v single supply f/v converter
3-299 telcom semiconductor, inc. 7 6 5 4 3 1 2 8 voltage-to-frequency/ frequency-to-voltage converters TC9400 tc9401 tc9402 in some cases, however, the TC9400 output must be zero at power-on without a frequency input. in such cases, a capacitor connected from pin 11 to v dd will usually be sufficient to pulse the TC9400 and provide a power-on reset (see figure 11a). where predictable power-on operation is critical, a more complicated circuit, such as figure 11b, may be required. v dd 14 11 1000pf threshold detector 1k w f in v dd 100k w 1 f 3 4 8 6 f in 1 2 5 16 v cc b r c q v ss a clra cd4538 (a) (b) TC9400 to tc 9400 figure 11. power-on operation/reset f/v power-on reset in f/v mode, the TC9400 output voltage will occasion- ally be at its maximum value when power is first applied. this condition remains until the first pulse is applied to f in . in most frequency-measurement applications this is not a problem, because proper operation begins as soon as the frequency input is applied.

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