...the analog plus company TM XR-45 Voltage-to-Frequency Converter FEATURES APPLICATIONS June 99- Single Supply Operation (+V to +V) Voltage-to-Frequency Conversion Pulse Output Compatible with All Logic Forms A/D and D/A Conversion Programmable Scale Factor (K) Data Transmission Linearity 0.05% Typical-precision Mode Frequency-to-Voltage Conversion Temperature Stability 00% ppm/ C Typical Transducer Interface High Noise Rejection System Isolation Inherent Monotonicity Easily Transmittable Output Simple Full Scale Trim Single-Ended Input, Referenced to Ground Also Provides Frequency-to-Voltage Conversion Direct Replacement for RC/RV/RM-45 GENERAL DESCRIPTION The XR-45 is a device designed to provide a simple, low-cost method for converting a DC voltage into a proportional pulse repetition frequency. It is also capable of converting an input frequency into a proportional output voltage. The XR-45 is useful in a wide range of applications including A/D and D/A conversion and data transmission. ORDERING INFORMATION Operating Part No. Package Temperature Range XR-45P Lead 00 Mil PDIP -40 C to +5 C XR-45CP Lead 00 Mil PDIP 0 C to +0 C XR-45MD Lead 4.4mm EIAJ SOP 0 C to +0 C BLOCK DIAGRAM GND 4 SCFA INPV TRSH Comp One Shot Switch OUTL CSO RC 5 Figure. Block Diagram 99 EXAR Corporation, 40 Kato Road, Fremont, CA 945 (50) -000 FAX (50) -0
PIN CONFIGURATION CSO SCFA OUTL GND 4 5 INPV TRSH RC CSO SCFA OUTL GND 4 5 INPV TRSH RC Lead PDIP (0.00 ) Lead SOP (EIAJ, 4.4mm) PIN DESCRIPTION Pin # Symbol Type Description CSO O Current Source Output. SCFA I Scale Factor Input. OUTL O Logic Output. 4 GND Supply Ground. 5 RC I One Shot Timing Input. TRSH I Comparator Input. INPV I Input Voltage. O Positive Supply.
ELECTRICAL CHARACTERISTICS Test Conditions: = 5V, T A = +5 C, Unless Otherwise Specified Supply Current XR-45MD, CP XR-45P Parameter Min. Max. Typ. Unit Conditions Conversion Accuracy Scale Factor XR-45MD, CP XR-45P.0.0.0 0.90 0.9.0.5.4.0.0.5 4.5 4.5.00.00 ma ma ma khz/v khz/v V < < 5V 5V < < V 5V < < V Circuit of Figure, V I =0V RS=4.0K Drift With Temperature 00 ppm/ C Circuit of Figure, V I =0V Drift With XR-45MD, CP XR-45P -0.9 0.9 Input Comparator 0. 0. %/V %/V Offset Voltage 0 5 mv Offset Current 00 50 na Input Bias Current -00-00 na Common Mode Range 0-0 to - One-Shot V Circuit of Figure, V I =.0V V < < V Threshold Voltage, Pin 5 0. 0.0 0. x Input Bias Current, Pin 5-500 -00 na Reset V SAT 0.5 0.5 V Pin 5=.mA Current Source Output Current. µa Pin, V=0, RS=4.0kΩ Change With Voltage.5.0 µa Pin, V=0V to V=0V Off Leakage 50 0.5 na Pin, V=0V Reference Voltage.0.0.9 V Pin Logic Output V SAT 0.50 0.5 V Pin, =.0mA V SAT 0.0 0.0 V Pin, =.0mA Off Leakage.0 0. µa Notes Input Common Mode Range includes ground. Bold face parameters are covered by production test and guaranteed over operating temperature range. Specifications are subject to change without notice
ABSOLUTE MAXIMUM RATINGS Power Supply............................... V Output Sink Current....................... 0mA Internal Power Dissipation................ 500mW Input Voltage...................... -0.V to + Output Short Circuit to Ground......... Continuous SYSTEM DESCRIPTION The XR-45 is a precision voltage-to-frequency converter featuring 0.05% conversion linearity (precision mode), high noise rejection, monotonicity, and single supply operation from V to V. An RC network on Pin 5 gets the maximum full wave frequency. Input voltage on Pin is compared with the voltage on Pin (which is generally controlled by the current source output, Pin ). Frequency output is proportioned to the voltage on Pin. The current source is controlled by the resistance on Pin (nominally 4k with I =.9 V/R. The output is an open collector at Pin. PRINCIPLES OF OPERATION Single Supply Mode Voltage-to-Frequency Converter In this application, the XR-45 functions as a stand alone voltage-to-frequency converter operating on a single positive power supply. Refer to the functional block diagram and Figure, the circuit connection for single supply voltage-to-frequency conversion. The XR-45 contains a voltage comparator, a one-shot, and a precision switched current source. The voltage comparator compares a positive input voltage applied at pin to the voltage at pin. If the input voltage is higher, the comparator will fire the one-shot. The output of the one-shot is connected to both the logic output and the precision switched current source. During the one-shot period, T, the logic output will go low and the current source will turn on with current. At the end of the one shot period the logic output will go high and the current source will shut off. At this time the current source has injected an amount of charge Q = I O T into the network R B -C B. If this charge has not increased the voltage V B such that V B > V I, the comparator again fires the one-shot and the current source injects another, Q, into the R B -C B network. This process continues until V B > V I. When this condition is achieved, the current source remains off and the voltage V B decays until V B is again equal to V I. This completes one cycle. The VFC will now run in a steady state mode. The current source charges the capacitor C B at a rate such that V B >V I. Since the discharge rate of capacitor C B is proportional to V B /R B, the frequency at which the system runs will be proportional to the input voltage. 4
0.µF R S Voltage V I Input K 0.0µF 5K COMP One Shot C S 5 4 SW V L R L 5.K XR-45 Frequency fo Output R O.K C O 0.0µF C B µf R B fo T f0=kv I, Where K=0.4 T=.R O C O R S R B R O C O khz V Figure. Voltage-to-Frequency Converter TYPICAL APPLICATIONS Single Supply Voltage-to-Frequency Converter Figure shows the simplest type of VFC that can be made with the XR-45. The input voltage range is from 0 to +0V, and the output frequency is from 0 to 0kHz. The full scale frequency can be tuned by adjusting R S, the output current set resistor. This circuit has the advantage of being simple and low in cost, but it suffers from inaccuracy due to a number of error sources. Linearity error is typically %. A frequency offset will also be introduced by the input comparator offset voltage. Also, response time for this circuit is limited by the passive integration network R B C B. For the component values shown in Figure, response time for a step change input from 0 to +0V will be 5msec. For applications which require fast response time and high accuracy, use the circuit of Figure. 5
Precision Voltage-to-Frequency Converter In this application (Figure ) the XR-45 is used with an operational amplifier integrator to provide typical linearity of 0.05% over the range of 0 to -0V. Offset is adjustable to zero. Unlike many VFC designs which lose linearity below 0mV, this circuit retains linearity over the full range of input voltage, all the way to 0V. Trim the full scale adjust pot at V I = -0V for an output frequency of 0kHz. The offset adjust pot should be set for 0Hz with an input voltage of -0mV. The operational amplifier integrator improves linearity of this circuit over that of Figure by holding the output of the source, Pin, at a constant 0V. Therefore, the linearity error due to the current source output conductance is eliminated. The diode connected around the operational amplifier prevents the voltage at pin of the XR-45 from going below 0. Use a low-leakage diode here, since any leakage will degrade the accuracy. This circuit can be operated from a single positive supply if an XR-40 ground-sensing operational amplifier is used for the integrator. In this case, the diode can be left out. Note that even though the circuit itself will operate from a single supply, the input voltage is necessarily negative. For operations above 0kHz, bypass pin of the XR-45 with.0µf. RS Full Scale Trim 0.µF K C S VL 5.K comp One Shot SW RL 5.K Frequency fo VI 0K RB RO.K 0.0µF 5 4 CO XR-45 Output LM4 CI nf N94 00 V EE Offset Adjust V EE 5K Figure. Precision Voltage to Frequency Converter
Frequency-to-Voltage Conversion The XR-45 can be used as a frequency-to-voltage converter. Figure 4 shows the single-supply FVC configuration. With no signal applied, the resistor bias networks tied to pins and hold the input comparator in the off state. A negative going pulse applied to pin (or positive pulse to pin ) will cause the comparator to fire the one-shot. For proper operation, the pulse width must be less than the period of the one-shot, T =. R 0 C 0. For a 5Vpp square-wave input the differentiator network formed by the input coupling capacitor and the resistor bias network will provide pulses which correctly trigger the one-shot. An external voltage comparator can be used to square-up sinusoidal input signals before they are applied to the XR-45. Also, the component values for the input signal differentiator and bias network can be altered to accommodate square waves with different amplitudes and frequencies. The passive integrator network R B C B filters the current pulses from the pin output. For less output ripple, increase the value of C B. For increased accuracy and linearity, use an operational amplifier integrator as shown in Figure 5, the precision FVC configuration. Trim the offset to give -0mV out with 0Hz in and trim the full scale adjust for -0V out with 0kHz in. Input signal conditioning for this circuit is necessary just as for the single supply mode and the scale factor can be programmed by the choice of component values. A tradeoff exists between the amount of output ripple and the response time, through the choice or integration capacitor C. If C = 0.µF the ripple will be about 00mV. Response time constant τ R = R B C I. For R B = 00kΩ and C I = 0.µF, τ R = 0msec. 0K R C 0.µF 0K R R S 4K C S V L Frequency Input f nf I C 5V P-P Square R Wave 0K R4 5.K COMP One Shot 5 4 SW R L 5.K XR-45 Pulse fo Output Voltage Output R O.K 0.0µF C O C B µf RB V O Up to 0V Design Equations V O = fi/k, Where K=0.4 T =.R O /C O R S R B R O C O Hz V Figure 4. Frequency to Voltage Converter
Precautions. The voltage applied to comparator input pins and should not be allowed to go below ground by more than 0.V.. Pins and 5 are open-collector outputs. Shorts between these pins and can cause overheating and eventual destruction.. Reference voltage terminal pin is connected to the emitter of an NPN transistor and is held at approximately.9v. This terminal should be protected from accidental shorts to ground or supply voltages. Permanent damage may occur if the current in pin exceeds 5mA. 4. Avoid stray coupling between pins 5 and ; it could cause false triggering. For the circuit of Figure, bypass pin to ground with at least 0.0µF. This is necessary for operation above 0kHz. 0.µF R S Full Scale Trim K C S Frequency f I Input 0<f<0kHz 5K 0K 0K 5.K nf 0K COMP R O.K 0.0µF One Shot 5 4 C O SW XR-45 C I R B V EE 5pF - + LM4 V EE Offset Adjust 5K Voltage Output V O -0<V O <0 Figure 5. Precision Frequency-to-Voltage Converter
Programming the XR-45 The XR-45 can be programmed to operate with a full scale frequency anywhere from.0hz to 00kHz. In the case of the VFC configuration, nearly any full scale input voltage from.0v and up can be tolerated if proper scaling is employed. Here is how to determine component values for any desired full scale frequency.. Set R S = 4kΩ or use a K resistor and 5K pot as shown in the figures. (The only exception to this is Figure ).. Set T =.R 0 C 0 = 0.5[/fo] where fo is the desired full scale frequency. For optimum performance make.kω > R 0 > 0kΩ and 0.00µF < C 0 <.0µF.. a) For the circuit of Figure make C B = 0 - [/fo] Farads. Smaller values of C B will give a faster response time, but will also increase the frequency offset and nonlinearity. b) For the active integrator circuit make C I = 5 x 0-5 [/fo] Farads. The operational amplifier integrator must have a slew rate of at least 5 x 0 - [/C ] volts per second where the value of C is in Farads. 4. a) For the circuit of Figure keep the values of R B as shown and use an input attenuator to give the desired full scale input voltage. b) For the precision mode circuit of Figure, set R B = V IO /00µA where V IO is the full scale input voltage. Alternately, the operational amplifier inverting input (summing node) can be used as a current input with the full scale input current I IO = -00µA. 5. For the FVC s, pick the value of C B or C I to give the optimum tradeoff between the response time and output ripple for the particular application. Design Example. T = 0.5 [/0 5 ] =.5µsec. Let R 0 =.kω and C 0 = 0.00µF.. C I = 5 x 0-5 [/0 5 ] = 500pF. Op amp slew rate must be at least SR = 5 x 0 - [/500pF] = 0.V/µsec. 4. R B = 0V/00µA = 00kΩ. II. Design a precision VFC with fo = Hz and V IO = 0V.. Let R S = 4.0kΩ.. T = 0.5 [/] = 0.5 sec. Let R 0 = 0kΩ and C 0 =.0µF.. C I = 5 x 0-5 [/]F = 50µF. 4. R B = 00kΩ. III. Design a single supply FVC to operate with a supply voltage of 9V and full scale input frequency fo =.Hz. The output voltage must reach at least 0. of its final value in 00msec. Determine the output ripple.. Set R S = 4.0kΩ.. T = 0.5 [.] = 9msec. Let R 0 = kω and C O = 0.µF.. Since this FVC must operate from.0v, we shall make the full scale output voltage at pin equal to 5.0V. 4. R B = 5V/00µA = 50kΩ. 5. Output response time constant is τ R 00msec. Therefore, C B τ R /R B = (00 x 0 - )/(50 x 0 ) = 4µF. Worst case ripple voltage is V R = (9ms x 5µA)/4µF = 04mV. I. Design a precision VFC (from Figure 4) with fo = 00kHz and V IO = -0V.. Set R S = 4.0kΩ. 9
5 4 Figure. Equivalent Schematic Diagram 0
LEAD PLASTIC DUAL-IN-LINE (00 MIL PDIP) Rev..00 5 4 E D E Seating Plane A L A A α C B e B e A e B INCHES MILLIMETERS SYMBOL MIN MAX MIN MAX A 0.45 0.0. 5. A 0.05 0.00 0.. A 0.05 0.95.9 4.95 B 0.04 0.04 0. 0.5 B 0.00 0.00 0.. C 0.00 0.04 0.0 0. D 0.4 0.40.4 0.9 E 0.00 0.5.. E 0.40 0.0.0. e 0.00 BSC.54 BSC e A 0.00 BSC. BSC e B 0.0 0.40. 0.9 L 0.5 0.0.9 4.0 α 0 5 0 5 Note: The control dimension is the inch column
LEAD EIAJ SMALL OUTLINE (4.4 mm EIAJ SOP) Rev..00 D 5 4 E H Seating Plane A C A α e B A L INCHES MILLIMETERS SYMBOL MIN MAX MIN MAX A 0.05 0.0.45.0 A 0.00 0.00 0.05 0.0 A 0.055 0.0.40.0 B 0.0 0.00 0.0 0.50 C 0.004 0.00 0.0 0.0 D 0.9 0.0 4.90 5.0 E 0.9 0. 4.0 4.50 e 0.050 BSC. BSC H 0. 0.5.00.40 L 0.0 0.00 0.0 0. α 0 0 0 0 Note: The control dimension is the millimeter column
Notes
Notes 4
Notes 5
NOTICE EXAR Corporation reserves the right to make changes to the products contained in this publication in order to improve design, performance or reliability. EXAR Corporation assumes no responsibility for the use of any circuits described herein, conveys no license under any patent or other right, and makes no representation that the circuits are free of patent infringement. Charts and schedules contained herein are only for illustration purposes and may vary depending upon a user s specific application. While the information in this publication has been carefully checked; no responsibility, however, is assumed for inaccuracies. EXAR Corporation does not recommend the use of any of its products in life support applications where the failure or malfunction of the product can reasonably be expected to cause failure of the life support system or to significantly affect its safety or effectiveness. Products are not authorized for use in such applications unless EXAR Corporation receives, in writing, assurances to its satisfaction that: (a) the risk of injury or damage has been minimized; (b) the user assumes all such risks; (c) potential liability of EXAR Corporation is adequately protected under the circumstances. Copyright 99 EXAR Corporation Datasheet June 99 Reproduction, in part or whole, without the prior written consent of EXAR Corporation is prohibited.