VOLTAGE-TO-FREQUENCY CONVERTER

Transcription

1 VOTAGE-TO-FEQUENY ONVETE Internal circuit of M I VOTAGE EFEENE M OP AMP UENT MIO Q,9V Q P B Vin T ONE SHOT TIME VOT OMP VOT OMP S Q Q Q Q Q Q In direct transmission of an analog signal (below), V AN will be more easily corrupted by noise. TEM PEATUE SENSO INTEFAE SIGNA V AN SIGNA NOISE FITE V AN By converting V AN to a large signal whose frequency is proportional to V AN, the transmission of the signal becomes much less corruptible by external noise. TEM PEATUE SENSO F to V O NVETE V AN INTEFAE V AN V to F O NVETE SIGNA SIGNA NOISE FITE- EV E DETETO. Sauriol ev. // Page -

2 Timing Diagram m = I s -(V ave / ) m - = V ave V ave V = V V IPPE VOTAGE m = I s I initial transient charging from to V VAIABE SW FIXED PW T out -, T, T V cc / V cc V T = V V out V cc SET ESET The D current, or average current, through capacitor is A once the V waveform has settled. TIME (ave ) PW T = I ( ave) I (ave) =, V if V in V V in A ave = T out PW = V ref n() T =,9 T V = PW m = PW I V S in = SW m = ( T PW). Sauriol ev. // Page -

3 NOTE: To obtain good linearity at low values of, the ripple voltage V must be very small which means that the time constant must be very large and this makes the converter very slow to react to changes of. When ripple is very small, the frequency of the V to F depends on a very small signal easily corruptible with noise or spurious signals which may result in an erratic O/P frequency. For small values of, the equation derived for is not accurate because ripple must be factored in and becomes a non-linear function of. Basic V-F onverter in BY, µf, µf V Vin HYS BA in out thr Ire M gnd Isw P T mv to V K Vin µf K K K Fmax adj ust in out thr Ire M Isw gnd K,K Fout Hz to khz, µf From =,9 T we find that s = will produce a khz full scale output given the standard values on the diagram. The k pot will have to be adjusted to some other value because of component tolerance. PW =, T =,**,µ =, µs fixed I s =,9V/Ω = µa For F = Hz to khz, duty ycle =,µ/(, to µ) * =,% to,% NOTE: PW and duty cycle refer to the switched current source I sw (pin#). The duty cycle of the output waveform is inverted, that is D/ out = 99,9% to,%. V o V and V - o V ipple Voltage ( ) k µ V = PW m = PW I =,µ µ m = 9,9m,mV pp NOTE: Those small ripple values make the V to F vulnerable to noise and unreliable operation may result. At Hz the discharge of is not linear (it is exponential) because =,s = T SW. The speed of the V-F converter is determine by the time constant. For the above circuit, the rise time of V thr is : t r =,/(F c ) =,* π* =,s which is rather slow. For a faster response of V thr (and ) should be made smaller but this will entail more ripple voltage across and therefore less accuracy at low and values. Hysteresis The amount of hysteresis of the input comparator, for clean and faster switching (no crossover oscillations), is V HYS = I s HYS = µ * =, mv - this will impair low operation.. Sauriol ev. // Page -

4 hoice of in in The input low-pass filter produces a cutoff frequency of F c = (π*k*,µ) - =,9 Hz. It is used to filter down any HF noise that contaminates the D input voltage. If the input D voltage is changed abruptly to a new value, the M input ( at pin ) will settle to its new value in about in in =*K*,µ = ms. Therefore the response time of the output frequency will the compounded value of t r of t r of V thr. hoice of T Maximum duty cycle (PW/T)* =, T * F max < 9% and should leave enough time for the internal circuit to reset when SW = T-PW becomes very small. PW should not be too small either. hoice of Select for desired maximum ripple V = PW = ( T PW ) and for desired rise time of V thr : t r =,*π* Waveforms showing effect of hysteresis provided by HYS on V waveform. When is detected by the comparator, V pulls up abruptly to prevent oscillations (chattering) at the internal comparator output. V V HYS V HYST IMPOVED V to F ONVETE -ve INTEGATO I E Aave F Vo V to F ONVETE V I sw A P s I s in out thr Ire M Isw gnd T p s. Sauriol ev. // Page -

5 The above converter has a very linear V to F characteristic because its output frequency is totally independant of the ripple voltage present at (pin ) of the M. For fast and reliable operation of the V to F converter, F can be made very small in order to get subtantial ripple - V pp recommended onverter equations = PW T = I E I F ave = I E = PW T = ( ) V ref P S n() T = ( P S ) = T = P S = V ref n() T. 9. T ( ) = I E ( T PW)= I I sw E F F PW The basic waveforms are shown below. m = I E / F m - = -(I sw -I E )/ F V thr V thr -, I sw A PW = T n() V V out V The output frequency can be made to respond very rapidly to an abrupt change in by selecting a small value for F which entails a large ripple voltage for of M.. Sauriol ev. // Page -

6 BASI F TO V ONVETE V Hz to khz Vmax adj ust Vin pf K K k k k k k in thr out M gnd k Ire Isw, µf,k k, µf, µf I SUPPY BY- PASS AP, µf D OUTPUT Vout mv to V V V V TH,V (I) 9V,V,V SET PW SET V V T V ESET A V EF PW T V V OUT (D) V. Sauriol ev. // Page -

7 The D output voltage is : V out (ave) = (ave ) = PW () T in = V ref, T F in =,9 T F in The output ripple voltage is given by - see appendix for derivation: V out (pp ) = PW The output filter has the following TF: F PW for F > (π ) - F(S) = ( ) S S = S S = S, ( ) S, The above TF allows us to determine the rise and fall times of V out in response to an input frequency jump. The approximate value can be determined from the first cutoff frequency: t r = t f. F =.. π =. The internal latch SET pulsewidth (PW set ) must be less than the current pulsewidth PW T =, T, otherwise the circuit will not work properly. PW set is determined by the input coupling capacitor and the input resistance K K. PW SET = in in n V V F.. = K p n V V F 9. =. µs PW T =, T =,.K.µ =. µs NOTE: PW set =. µs may not be long enough to set the internal S latch - minimum time not specified. NOTE: If the input square wave has a large amplitude, the protection diodes will turn on and the exponential pulses at the junction of the two K resistors will have two time constants, that is: ( K K K) p when one of the diodes is ON (K K ) x p when the diodes are OFF.. Sauriol ev. // Page -

8 APPENDIX Derivation Of Output ipple Voltage NOTON THEVENIN V V V V V TH V TH PW T V OUT (D ) V V V OUT (D ) V t PW ( () T ) V (PP) = V H ( A) dt = PW ( I t S V ave )= PW I S V (PP) = PW PW T ( ()) = PW ( ) PW T V (PP) = t V (A ) dt = ( V ( PP) T )= V (PP) T t V (PP) = T PW I S ( ) PW T = I PW ( ) T PW NOTE: =. Sauriol ev. // Page -