Datasheet MAX038 (Analog Devices) - 9

MAX038
High-Frequency Waveform Generator
A stable 2.5V reference voltage, REF, allows simple VFADJ = 0V, the fundamental output frequency (Fo) is determination of IIN, FADJ, or DADJ with fixed resistors, given by the formula: and permits adjustable operation when potentiometers Fo (MHz) = IIN (µA) ÷ CF (pF) [1] are connected from each of these inputs to REF. FADJ and/or DADJ can be grounded, producing the nominal The period (to) is: frequency with a 50% duty cycle. to (µs) = CF (pF) ÷ IIN (µA) [2] The output frequency is inversely proportional to where: capacitor CF. CF values can be selected to produce IIN = current injected into IIN (between 2µA and frequencies above 20MHz. 750µA) A sine-shaping circuit converts the oscillator triangle CF = capacitance connected to COSC and GND wave into a low-distortion sine wave with constant (20pF to >100µF). amplitude. The triangle, square, and sine waves are input to a multiplexer. Two address lines, A0 and A1, For example: control which of the three waveforms is selected. The 0.5MHz = 100µA ÷ 200pF output amplifier produces a constant 2VP-P amplitude and (±1V), regardless of wave shape or frequency. 2µs = 200pF ÷ 100µA The triangle wave is also sent to a comparator that pro- duces a high-speed square-wave SYNC waveform that Optimum performance is achieved with IIN between can be used to synchronize other oscillators. The SYNC 10µA and 400µA, although linearity is good with IIN circuit has separate power-supply leads and can be between 2µA and 750µA. Current levels outside of this disabled. range are not recommended. For fixed-frequency oper- ation, set IIN to approximately 100µA and select a suit- Two other phase-quadrature square waves are gener- able capacitor value. This current produces the lowest ated in the basic oscillator and sent to one side of an temperature coefficient, and produces the lowest fre- "exclusive-OR" phase detector. The other side of the quency shift when varying the duty cycle. phase-detector input (PDI) can be connected to an external oscillator. The phase-detector output (PDO) is The capacitance can range from 20pF to more than a current source that can be connected directly to 100µF, but stray circuit capacitance must be minimized FADJ to synchronize the MAX038 with the external by using short traces. Surround the COSC pin and the oscillator. trace leading to it with a ground plane to minimize cou- pling of extraneous signals to this node. Oscillation
Waveform Selection
above 20MHz is possible, but waveform distortion The MAX038 can produce either sine, square, or trian- increases under these conditions. The low frequency gle waveforms. The TTL/CMOS-logic address pins (A0 limit is set by the leakage of the COSC capacitor and and A1) set the waveform, as shown below: by the required accuracy of the output frequency. Lowest frequency operation with good accuracy is usu- ally achieved with 10µF or greater non-polarized
A0 A1 WAVEFORM
capacitors. X 1 Sine wave An internal closed-loop amplifier forces IIN to virtual 0 0 Square wave ground, with an input offset voltage less than ±2mV. IIN 1 0 Triangle wave may be driven with either a current source (IIN), or a voltage (VIN) in series with a resistor (RIN). (A resistor X = Don’t care. between REF and IIN provides a convenient method of Waveform switching can be done at any time, without generating IIN: IIN = VREF/RIN.) When using a voltage in regard to the phase of the output. Switching occurs series with a resistor, the formula for the oscillator fre- within 0.3µs, but there may be a small transient in the quency is: output waveform that lasts 0.5µs. Fo (MHz) = VIN ÷ [RIN x CF (pF)] [3]
Waveform Timing
and: to (µs) = CF(pF) x RIN ÷ VIN [4]
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 Maxim Integrated 9 Document Outline Blank Page