MAX038EPP

MAX038CPP MAXIM | Alldatasheet

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Technical content

The MAX038 is a high-frequency, precision function generator producing accurate, high-frequency triangle, sawtooth, sine, square, and pulse waveforms with a minimum of external components. The output frequency can be controlled over a frequency range of 0.1Hz to 20MHz by an internal 2.5V bandgap voltage reference and an external resistor and capacitor. The duty cycle can be varied over a wide range by applying a ±2.3V control signal, facilitating pulse-width modula- tion and the generation of sawtooth waveforms. Frequency modulation and frequency sweeping are achieved in the same way. The duty cycle and frequency controls are independent. Sine, square, or triangle waveforms can be selected at the output by setting the appropriate code at two TTL-compatible select pins. The output signal for all waveforms is a 2V P-P signal that is symmetrical around ground. The low-impedance output can drive up to ±20mA. The TTL-compatible SYNC output from the internal oscillator maintains a 50% duty cycle—regardless of the duty cycle of the other waveforms—to synchronize other devices in the system. The internal oscillator can be synchronized to an external TTL clock connected to PDI. Precision Function Generators Voltage-Controlled Oscillators Frequency Modulators Pulse-Width Modulators Phase-Locked Loops Frequency Synthesizer FSK Generator—Sine and Square Waves ' 0.1Hz to 20MHz Operating Frequency Range ' Triangle, Sawtooth, Sine, Square, and Pulse Waveforms ' Independent Frequency and Duty-Cycle Adjustments ' 350 to 1 Frequency Sweep Range ' 15% to 85% Variable Duty Cycle ' Low-Impedance Output Buffer: 0.1Ω ' Low-Distortion Sine Wave: 0.75% ' Low 200ppm/°C Temperature Drift *Contact factory for dice specifications. MAX038 High-Frequency Waveform Generator OUT GND V+A1 GND REF TOP VIEW MAX038 DV+ DGND SYNC PDIFADJ DADJ GND COSC PDO GNDIIN GND DIP/SO 19-0266; Rev 2a; 9/96 PART TEMP. RANGE PIN-PACKAGE MAX038CPP 0°C to +70°C 20 Plastic DIP MAX038CWP 0°C to +70°C 20 SO MAX038C/D 0°C to +70°C Dice* MAX038EPP -40°C to +85°C 20 Plastic DIP MAX038EWP -40°C to +85°C 20 SO EVALUATION KIT AVAILABLE For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468.

High-Frequency Waveform Generator ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1, GND = DGND = 0V, V+ = DV+ = 5V, V- = -5V, V DADJ = V FADJ = V PDI = V PDO = 0V, C F = 100pF, RIN = 25kΩ , RL = 1kΩ , CL = 20pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25° C.) Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. PARAMETER SYMBOL MIN TYP MAX UNITS Frequency Temperature Coefficient ∆Fo/° C 200 ppm/° C600 IIN Offset Voltage VIN ±1.0 ±2.0 mV Frequency Programming Current IIN 1.25 375 µA (∆Fo/Fo) ∆V+ ±0.4 ±2.00Frequency Power-Supply Rejection (∆Fo/Fo) ∆V- ±0.2 ±1.00 %/V Output Peak-to-Peak Symmetry VOUT ±4 mV Maximum Operating Frequency Fo 20.0 40.0 MHz 2.50 750 Output Resistance ROUT 0.1 0.2 Ω Output Short-Circuit Current IOUT 40 mA Amplitude VOUT 1.9 2.0 2.1 VP-P Rise Time tR 12 ns Fall Time tF 12 ns Duty Cycle dc 47 50 53 % Amplitude VOUT 1.9 2.0 2.1 VP-P Nonlinearity 0.5 % Duty Cycle dc 47 50 53 % CONDITIONS VFADJ = -3V VFADJ = 0V VFADJ = -3V Short circuit to GND 10% to 90% 90% to 10% VDADJ = 0V, dc = tON/t x 100% 15pCF ≤ 15pF, IIN = 500µA VFADJ = 0V Fo = 100kHz, 5% to 95% VDADJ = 0V (Note 1) Pin Voltages Continuous Power Dissipation (TA = +70° C) Operating Temperature Ranges Amplitude VOUT 1.9 2.0 2.1 VP-P Duty cycle adjusted to 50% 0.75Total Harmonic Distortion THD Duty cycle unadjusted 1.50 % Fo/° C FREQUENCY CHARACTERISTICS OUTPUT AMPLIFIER (applies to all waveforms) SQUARE-WAVE OUTPUT (R L = 100Ω ) TRIANGLE-WAVE OUTPUT (R L = 100Ω ) SINE-WAVE OUTPUT (R L = 100Ω )

High-Frequency Waveform Generator ELECTRICAL CHARACTERISTICS (continued) (Circuit of Figure 1, GND = DGND = 0V, V+ = DV+ = 5V, V- = -5V, V DADJ = V FADJ = V PDI = V PDO = 0V, C F = 100pF, RIN = 25kΩ , RL = 1kΩ , CL = 20pF, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25° C.) Note 1:Guaranteed by duty-cycle test on square wave. Note 2:VREF is independent of V-. PARAMETER DADJ Nonlinearity SYMBOL MIN TYP MAX dc/VFADJ 2 4 UNITS Duty Cycle dcSYNC 50 % Fall Time tF 10 ns Rise Time tR 10 ns Change in Output Frequency with DADJ DADJ Input Current IDADJ 190 250 320 µA DADJ Voltage Range VDADJ ±2.3 V Fo/VDADJ ±2.5 ±8 % Duty-Cycle Adjustment Range dc 15 85 % Maximum DADJ Modulating Frequency FDC 2 MHz Output Low Voltage FADJ Input Current IFADJ 190 250 320 µA FADJ Voltage Range VFADJ ±2.4 V Frequency Sweep Range VOL 0.3 0.4 V Fo ±70 % FM Nonlinearity with FADJ Output High Voltage Fo/VFADJ ±0.2 % VOH 2.8 3.5 V Change in Duty Cycle with FADJ dc/VFADJ ±2 % Output Voltage VREF 2.48 2.50 2.52 V CONDITIONS -2V ≤ VDADJ ≤ 2V 90% to 10%, RL = 3kΩ , CL = 15pF 10% to 90%, RL = 3kΩ , CL = 15pF -2V ≤ VDADJ ≤ 2V -2.3V ≤ VDADJ ≤ 2.3V ISINK = 3.2mA -2.4V ≤ VFADJ ≤ 2.4V -2V ≤ VFADJ ≤ 2V ISOURCE = 400µA -2V ≤ VFADJ ≤ 2V IREF = 0 Temperature Coefficient VREF/° C 20 ppm/° C 0mA ≤ IREF ≤ 4mA (source) 1 2Load Regulation VREF/IREF -100µA ≤ IREF ≤ 0µA (sink) 1 4 mV/mA Line Regulation VREF/V+ 4.75V ≤ V+ ≤ 5.25V (Note 2) 1 2 mV/V Input Low Voltage VIL 0.8 V Input High Voltage VIH 2.4 V Input Current (A0, A1) IIL, IIH VA0, VA1 = VIL, VIH ±5 µA Input Current (PDI) IIL, IIH VPDI = VIL, VIH ±25 µA Positive Supply Voltage V+ 4.75 5.25 V SYNC Supply Voltage DV+ 4.75 5.25 V Negative Supply Voltage V- -4.75 -5.25 V Positive Supply Current I+ 35 45 mA SYNC Supply Current IDV+ 1 2 mA Negative Supply Current I- 45 55 mA Maximum FADJ Modulating Frequency FF 2 MHz SYNC OUTPUT DUTY-CYCLE ADJUSTMENT (DADJ) FREQUENCY ADJUSTMENT (FADJ) VOLTAGE REFERENCE LOGIC INPUTS (A0, A1, PDI) POWER SUPPLY

High-Frequency Waveform Generator (Circuit of Figure 1, V+ = DV+ = 5V, V- = -5V, VDADJ = VFADJ = VPDI = VPDO = 0V, RL = 1kΩ , CL = 20pF, TA = +25° C, unless otherwise noted.) 0.1 1 100 1000 OUTPUT FREQUENCY vs. IIN CURRENT 100 MAX038-08 IIN CURRENT (µA) OUTPUT FREQUENCY (Hz) 10k 100k 10M 100M 100µF 47µF 10µF 3.3µF 1µF 100nF 33nF 3.3nF 330pF 100pF 33pF 1.0 -3 2 NORMALIZED OUTPUT FREQUENCY vs. FADJ VOLTAGE 0.2 0.8 MAX038-09 VFADJ (V) FOUT NORMALIZED 0.4 -2 -1 1 0.6 1.2 1.4 1.6 1.8 2.0 IIN = 100µA, COSC = 1000pF 0.85 NORMALIZED OUTPUT FREQUENCY vs. DADJ VOLTAGE 0.90 1.10 MAX038-17 DADJ (V) NORMALIZED OUTPUT FREQUENCY 1.00 0.95 1.05 IIN = 10µA IIN = 25µA IIN = 50µA IIN = 100µA IIN = 250µA IIN = 500µA 2.0 -2.5 -2.0 -1.0 1.0 2.5 DUTY-CYCLE LINEARITY vs. DADJ VOLTAGE -2.0 1.0 MAX038-18 DADJ (V) DUTY-CYCLE LINEARITY ERROR (%) 0 1.5 -1.0 -1.5 -0.5 0.5 1.5 IIN = 10µA IIN = 25µA IIN = 50µA IIN = 100µA IIN = 250µA IIN = 500µA -3 2 DUTY CYCLE vs. DADJ VOLTAGE MAX038-16B DADJ (V) DUTY CYCLE (%) -2 -1 1 100 IIN = 200µA

High-Frequency Waveform Generator SINE-WAVE OUTPUT (50Hz) TOP: OUTPUT 50Hz = Fo BOTTOM: SYNC IIN = 50µA CF = 1µF TRIANGLE-WAVE OUTPUT (50Hz) TOP: OUTPUT 50Hz = Fo BOTTOM: SYNC IIN = 50µA CF = 1µF SQUARE-WAVE OUTPUT (50Hz) TOP: OUTPUT 50Hz = Fo BOTTOM: SYNC IIN = 50µA CF = 1µF SINE-WAVE OUTPUT (20MHz) IIN = 400µA CF = 20pF (Circuit of Figure 1, V+ = DV+ = 5V, V- = -5V, VDADJ = VFADJ = VPDI = VPDO = 0V, RL = 1kΩ , CL = 20pF, TA = +25° C, unless otherwise noted.) TRIANGLE-WAVE OUTPUT (20MHz) IIN = 400µA CF = 20pF

High-Frequency Waveform Generator (Circuit of Figure 1, V+ = DV+ = 5V, V- = -5V, VDADJ = VFADJ = VPDI = VPDO = 0V, RL = 1kΩ , CL = 20pF, TA = +25° C, unless otherwise noted.) FREQUENCY MODULATION USING FADJ TOP: OUTPUT BOTTOM: FADJ 0.5V -0.5V FREQUENCY MODULATION USING IIN TOP: OUTPUT BOTTOM: IIN FREQUENCY MODULATION USING IIN TOP: OUTPUT BOTTOM: IIN PULSE-WIDTH MODULATION USING DADJ TOP: SQUARE-WAVE OUT, 2VP-P BOTTOM: VDADJ, -2V to +2.3V +1V -1V +2V -2V SQUARE-WAVE OUTPUT (20MHz) IIN = 400µA CF = 20pF

High-Frequency Waveform Generator *The five GND pins are not internally connected. Connect all five GND pins to a quiet ground close to the device. A ground plane is recommended (see Layout Considerations). -100 0 20 60 100 OUTPUT SPECTRUM, SINE WAVE (Fo = 11.5MHz) -80 -20 MAX038-12A FREQUENCY (MHz) ATTENUATION (dB) 40 80 -40 -60 -10 -30 -50 -70 -90 10 30 50 70 90 RIN = 15kΩ (VIN = 2.5V), CF = 20pF, VDADJ = 40mV, VFADJ = -3V -100 0 10 30 50 OUTPUT SPECTRUM, SINE WAVE (Fo = 5.9kHz) -80 -20 MAX038 12B FREQUENCY (kHz) ATTENUATION (dB) 20 40 -40 -60 -10 -30 -50 -70 -90 5 15 25 35 45 RIN = 51kΩ (VIN = 2.5V), CF = 0.01µF, VDADJ = 50mV, VFADJ = 0V (Circuit of Figure 1, V+ = DV+ = 5V, V- = -5V, VDADJ = VFADJ = VPDI = VPDO = 0V, RL = 1kΩ , CL = 20pF, TA = +25° C, unless otherwise noted.) -5V supply inputV-20 Sine, square, or triangle outputOUT19 +5V supply inputV+17 Digital +5V supply input. Can be left open if SYNC is not used.DV+16 Digital groundDGND15 TTL/CMOS-compatible output, referenced between DGND and DV+. Permits the internal oscillator to be synchronized with an external signal. Leave open if unused.SYNC14 Current input for frequency controlIIN10 Phase detector output. Connect to GND if phase detector is not used.PDO12 Phase detector reference clock input. Connect to GND if phase detector is not used.PDI13 External capacitor connectionCOSC5 Duty-cycle adjust inputDADJ7 Frequency adjust inputFADJ8 Waveform selection input; TTL/CMOS compatibleA14 Waveform selection input; TTL/CMOS compatibleA03 PIN Ground*GND2, 6, 9, 11, 18 2.50V bandgap voltage reference outputREF1 FUNCTIONNAME

ing tracking to an external signal source. The MAX038 operates with ±5V ±5% power supplies. = BYPASS CAPACITORS ARE 1µF CERAMIC OR 1µF ELECTROLYTIC IN PARALLEL WITH 1nF CERAMIC. Figure 1. Block Diagram and Basic Operating Circuit

A stable 2.5V reference voltage, REF, allows simple determination of IIN, FADJ, or DADJ with fixed resistors, and permits adjustable operation when potentiometers are connected from each of these inputs to REF. FADJ and/or DADJ can be grounded, producing the nominal frequency with a 50% duty cycle. The output frequency is inversely proportional to capacitor C F. C F values can be selected to produce frequencies above 20MHz. A sine-shaping circuit converts the oscillator triangle wave into a low-distortion sine wave with constant amplitude. The triangle, square, and sine waves are input to a multiplexer. Two address lines, A0 and A1, control which of the three waveforms is selected. The output amplifier produces a constant 2V P-P amplitude (±1V), regardless of wave shape or frequency. The triangle wave is also sent to a comparator that pro - duces a high-speed square-wave SYNC waveform that can be used to synchronize other oscillators. The SYNC circuit has separate power-supply leads and can be disabled. Two other phase-quadrature square waves are gener - ated in the basic oscillator and sent to one side of an “exclusive-OR” phase detector. The other side of the phase-detector input (PDI) can be connected to an external oscillator. The phase-detector output (PDO) is a current source that can be connected directly to FADJ to synchronize the MAX038 with the external oscillator. Waveform Selection The MAX038 can produce either sine, square, or trian - gle waveforms. The TTL/CMOS-logic address pins (A0 and A1) set the waveform, as shown below: X = Don’t care Waveform switching can be done at any time, without regard to the phase of the output. Switching occurs within 0.3µs, but there may be a small transient in the output waveform that lasts 0.5µs. Waveform Timing Output Frequency The output frequency is determined by the current injected into the IIN pin, the COSC capacitance (to ground), and the voltage on the FADJ pin. When V FADJ = 0V, the fundamental output frequency (F o) is given by the formula: Fo (MHz) = IIN (µA) ÷ CF (pF) [1] The period (to) is: to (µs) = CF (pF) ÷ IIN (µA) [2] where: IIN = current injected into IIN (between 2µA and 750µA) CF = capacitance connected to COSC and GND (20pF to >100µF). For example: 0.5MHz = 100µA ÷ 200pF and 2µs = 200pF ÷ 100µA Optimum performance is achieved with I IN between 10µA and 400 µA, although linearity is good with I IN between 2µA and 750 µA. Current levels outside of this range are not recommended. For fixed-frequency oper - ation, set I IN to approximately 100 µA and select a suit - able capacitor value. This current produces the lowest temperature coefficient, and produces the lowest fre - quency shift when varying the duty cycle. The capacitance can range from 20pF to more than 100µF, but stray circuit capacitance must be minimized by using short traces. Surround the COSC pin and the trace leading to it with a ground plane to minimize cou - pling of extraneous signals to this node. Oscillation above 20MHz is possible, but waveform distortion increases under these conditions. The low frequency limit is set by the leakage of the COSC capacitor and by the required accuracy of the output frequency. Lowest frequency operation with good accuracy is usu - ally achieved with 10 µF or greater non-polarized capacitors. An internal closed-loop amplifier forces IIN to virtual ground, with an input offset voltage less than ±2mV. IIN may be driven with either a current source (I IN), or a voltage (VIN) in series with a resistor (R IN). (A resistor between REF and IIN provides a convenient method of generating I IN: I IN = VREF/RIN.) When using a voltage in series with a resistor, the formula for the oscillator fre- quency is: Fo (MHz) = VIN ÷ [RIN x CF (pF)] [3] and: to (µs) = CF (pF) x RIN ÷ VIN [4] MAX038 High-Frequency Waveform Generator A0 A1 WAVEFORM X 1 Sine wave 0 0 Square wave 1 0 Triangle wave

When the MAX038’s frequency is controlled by a volt - age source (VIN) in series with a fixed resistor (RIN), the output frequency is a direct function of V IN as shown in the above equations. Varying VIN modulates the oscilla- tor frequency. For example, using a 10k Ω resistor for RIN and sweeping V IN from 20mV to 7.5V produces large frequency deviations (up to 375:1). Select R IN so that IIN stays within the 2µA to 750µA range. The band- width of the IIN control amplifier, which limits the modu - lating signal’s highest frequency, is typically 2MHz. IIN can be used as a summing point to add or subtract currents from several sources. This allows the output frequency to be a function of the sum of several vari - ables. As V IN approaches 0V, the I IN error increases due to the offset voltage of IIN. Output frequency will be offset 1% from its final value for 10 seconds after power-up. FADJ Input The output frequency can be modulated by FADJ, which is intended principally for fine frequency control, usually inside phase-locked loops. Once the funda - mental, or center frequency (F o) is set by I IN, it may be changed further by setting FADJ to a voltage other than 0V. This voltage can vary from -2.4V to +2.4V, causing the output frequency to vary from 1.7 to 0.30 times the value when FADJ is 0V (F o ±70%). Voltages beyond ±2.4V can cause instability or cause the frequency change to reverse slope. The voltage on FADJ required to cause the output to deviate from Fo by Dx (expressed in %) is given by the formula: VFADJ = -0.0343 x Dx [5] where V FADJ , the voltage on FADJ, is between -2.4V and +2.4V. Note:While IIN is directly proportional to the fundamen - tal, or center frequency (Fo), VFADJ is linearly related to % deviation from F o. V FADJ goes to either side of 0V, corresponding to plus and minus deviation. The voltage on FADJ for any frequency is given by the formula: VFADJ = (Fo - Fx) ÷ (0.2915 x Fo) [6] where: Fx = output frequency Fo = frequency when VFADJ = 0V. Likewise, for period calculations: VFADJ = 3.43 x (tx - to) ÷ tx [7] where: tx = output period to = period when VFADJ = 0V. Conversely, if V FADJ is known, the frequency is given by: Fx = Fo x (1 - [0.2915 x VFADJ]) [8] and the period (tx) is: tx = to ÷ (1 - [0.2915 x VFADJ]) [9] Programming FADJ FADJ has a 250µA constant current sink to V- that must be furnished by the voltage source. The source is usu - ally an op-amp output, and the temperature coefficient of the current sink becomes unimportant. For manual adjustment of the deviation, a variable resistor can be used to set V FADJ, but then the 250 µA current sink’s temperature coefficient becomes significant. Since external resistors cannot match the internal tempera - ture-coefficient curve, using external resistors to pro - gram V FADJ is intended only for manual operation, when the operator can correct for any errors. This restriction does not apply when V FADJ is a true voltage source. A variable resistor, RF, connected between REF (+2.5V) and FADJ provides a convenient means of manually setting the frequency deviation. The resistance value F) is: RF = (VREF - VFADJ) ÷ 250µA [10] VREF and V FADJ are signed numbers, so use correct algebraic convention. For example, if V FADJ is -2.0V (+58.3% deviation), the formula becomes: = (4.5V) ÷ 250µA = 18kΩ Disabling FADJ The FADJ circuit adds a small temperature coefficient to the output frequency. For critical open-loop applica - tions, it can be turned off by connecting FADJ to GND (not REF) through a 12k Ω resistor (R1 in Figure 2). The -250µA current sink at FADJ causes -3V to be devel - oped across this resistor, producing two results. First, the FADJ circuit remains in its linear region, but discon - nects itself from the main oscillator, improving tempera - ture stability. Second, the oscillator frequency doubles. If FADJ is turned off in this manner, be sure to correct equations 1-4 and 6-9 above, and 12 and 14 below by doubling F o or halving to. Although this method doubles the normal output frequency, it does not double the upper frequency limit. Do not operate FADJ open cir - cuit or with voltages more negative than -3.5V. Doing so may cause transistor saturation inside the IC, lead - ing to unwanted changes in frequency and duty cycle. High-Frequency Waveform Generator

offsetting the sweep voltage. DADJ can be used to reduce the sine-wave distortion. tion can be minimized (see Figure 2).

19 SINE-WAVE

Figure 2. Operating Circuit with Sine-Wave Output and 50% Duty Cycle; SYNC and FADJ Disabled

DADJ is similar to FADJ; it has a 250 µA constant cur - rent sink to V- that must be furnished by the voltage source. The source is usually an op-amp output, and the temperature coefficient of the current sink becomes unimportant. For manual adjustment of the duty cycle, a variable resistor can be used to set V DADJ, but then the 250µA current sink’s temperature coefficient becomes significant. Since external resistors cannot match the internal temperature-coefficient curve, using external resistors to program V DADJ is intended only for manual operation, when the operator can correct for any errors. This restriction does not apply when V DADJ is a true voltage source. A variable resistor, R D, connected between REF (+2.5V) and DADJ provides a convenient means of manually setting the duty cycle. The resistance value D) is: RD = (VREF - VDADJ) ÷ 250µA [15] Note that both V REF and VDADJ are signed values, so observe correct algebraic convention. For example, if V DADJ is -1.5V (23% duty cycle), the formula becomes: Varying the duty cycle in the range 15% to 85% has minimal effect on the output frequency—typically less than 2% when 25µA < I IN < 250µA. The DADJ circuit is wideband, and can be modulated at up to 2MHz (see photos, Typical Operating Characteristics). Output The output amplitude is fixed at 2V P-P, symmetrical around ground, for all output waveforms. OUT has an output resistance of under 0.1 Ω , and can drive ±20mA with up to a 50pF load. Isolate higher output capaci - tance from OUT with a resistor (typically 50 Ω ) or buffer amplifier. Reference Voltage REF is a stable 2.50V bandgap voltage reference capa- ble of sourcing 4mA or sinking 100 µA. It is principally used to furnish a stable current to IIN or to bias DADJ and FADJ. It can also be used for other applications external to the MAX038. Bypass REF with 100nF to min- imize noise. Selecting Resistors and Capacitors The MAX038 produces a stable output frequency over time and temperature, but the capacitor and resistors that determine frequency can degrade performance if they are not carefully chosen. Resistors should be metal film, 1% or better. Capacitors should be chosen for low temperature coefficient over the whole tempera - ture range. NPO ceramics are usually satisfactory. The voltage on COSC is a triangle wave that varies between 0V and -1V. Polarized capacitors are generally not recommended (because of their outrageous tem - perature dependence and leakage currents), but if they are used, the negative terminal should be connected to COSC and the positive terminal to GND. Large-value capacitors, necessary for very low frequencies, should be chosen with care, since potentially large leakage currents and high dielectric absorption can interfere with the orderly charge and discharge of C F. If possi - ble, for a given frequency, use lower IIN currents to reduce the size of the capacitor. SYNC Output SYNC is a TTL/CMOS-compatible output that can be used to synchronize external circuits. The SYNC output is a square wave whose rising edge coincides with the output rising sine or triangle wave as it crosses through 0V. When the square wave is selected, the rising edge of SYNC occurs in the middle of the positive half of the output square wave, effectively 90° ahead of the output. The SYNC duty cycle is fixed at 50% and is indepen - dent of the DADJ control. Because SYNC is a very-high-speed TTL output, the high-speed transient currents in DGND and DV+ can radiate energy into the output circuit, causing a narrow spike in the output waveform. (This spike is difficult to see with oscilloscopes having less than 100MHz band - width). The inductance and capacitance of IC sockets tend to amplify this effect, so sockets are not recom - mended when SYNC is on. SYNC is powered from sep - arate ground and supply pins (DGND and DV+), and it can be turned off by making DV+ open circuit. If syn - chronization of external circuits is not used, turning off SYNC by DV+ opening eliminates the spike. Phase Detectors Internal Phase Detector The MAX038 contains a TTL/CMOS phase detector that can be used in a phase-locked loop (PLL) to synchro - nize its output to an external signal (Figure 3). The external source is connected to the phase-detector input (PDI) and the phase-detector output is taken from PDO. PDO is the output of an exclusive-OR gate, and produces a rectangular current waveform at the MAX038 output frequency, even with PDI grounded. PDO is normally connected to FADJ and a resistor, R PD, and a capacitor C PD, to GND. R PD sets the gain of the phase detector, while the capacitor attenuates high-frequency components and forms a pole in the phase-locked loop filter. High-Frequency Waveform Generator

Figure 6. Crystal-Controlled, Digitally Programmed Frequency Synthesizer—8kHz to 16MHz with 1kHz Resolution

High-Frequency Waveform Generator Layout Considerations Realizing the full performance of the MAX038 requires careful attention to power-supply bypassing and board layout. Use a low-impedance ground plane, and con - nect all five GND pins directly to it. Bypass V+ and V- directly to the ground plane with 1 µF ceramic capaci - tors or 1 µF tantalum capacitors in parallel with 1nF ceramics. Keep capacitor leads short (especially with the 1nF ceramics) to minimize series inductance. If SYNC is used, DV+ must be connected to V+, DGND must be connected to the ground plane, and a second 1nF ceramic should be connected as close as possible between DV+ and DGND (pins 16 and 15). It is not necessary to use a separate supply or run separate traces to DV+. If SYNC is disabled, leave DV+ open. Do not open DGND. Minimize the trace area around COSC (and the ground plane area under COSC) to reduce parasitic capaci - tance, and surround this trace with ground to prevent coupling with other signals. Take similar precautions with DADJ, FADJ, and IIN. Place C F so its connection to the ground plane is close to pin 6 (GND). Frequency Synthesizer Figure 6 shows a frequency synthesizer that produces accurate and stable sine, square, or triangle waves with a frequency range of 8kHz to 16.383MHz in 1kHz incre- ments. A Motorola MC145151 provides the crystal-con - trolled oscillator, the ÷ N circuit, and a high-speed phase detector. The manual switches set the output frequency; opening any switch increases the output frequency. Each switch controls both the ÷ N output and an MX7541 12-bit DAC, whose output is converted to a cur- rent by using both halves of the MAX412 op amp. This current goes to the MAX038 IIN pin, setting its coarse frequency over a very wide range. Fine frequency control (and phase lock) is achieved from the MC145151 phase detector through the differ - ential amplifier and lowpass filter, U5. The phase detec- tor compares the ÷ N output with the MAX038 SYNC output and sends differential phase information to U5. U5’s single-ended output is summed with an offset into the FADJ input. (Using the DAC and the IIN pin for coarse frequency control allows the FADJ pin to have very fine control with reasonably fast response to switch changes.) A 50MHz, 50 Ω lowpass filter in the output allows pas - sage of 16MHz square waves and triangle waves with reasonable fidelity, while stopping high-frequency noise generated by the ÷ N circuit. PDI SYNC AO DADJ PDOFADJ 0.118" (2.997mm) 0.106" (2.692mm) COSC GND IINGND GND DGND DV+ GND GND REF V- OUT TRANSISTOR COUNT: 855 SUBSTRATE CONNECTED TO GND