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ML2037 Datasheet(PDF) 5 Page  Micro Linear Corporation 

ML2037 Datasheet(HTML) 5 Page  Micro Linear Corporation 
5 / 10 page ML2037 5 FUNCTIONAL DESCRIPTION The ML2037 is composed of a programmable frequency generator, a sine wave generator, a crystal oscillator, and a digital interface. The functional block diagram is shown in Figure 1. PROGRAMMABLE FREQUENCY GENERATOR The programmable frequency generator produces a digital output whose frequency is determined by a 16bit digital word. The frequency generator is composed of a phase accumulator which is clocked at ½fCLK IN. The value stored in the data latch is added to the phase accumulator every two cycles of CLK IN. The frequency of the analog output is equal to the rate at which the accumulator overflows and is given by the following equation: f fD D OUT CLKIN DEC = ´® 15 0 2 22 0 5 (1) Where (D15–D0) is the decimal value of the programming word. The frequency resolution and the minimum frequency are the same and can be calculated using: Df f MIN CLKIN = 2 22 (2) When fCLK IN = 25MHz, DfMIN = 5.96Hz (±2.98Hz). Lower output frequencies are obtained by using a lower clock frequency. The maximum frequency output can be easily calculated with the following equation: f f OUT MAX CLKIN () = 2 6 (3) When fCLK IN = 25MHz, fOUT(MAX) = 391kHz. Higher frequencies, up to 500kHz, are obtained by using an external clock, where 25MHz < fCLK IN < 32MHz. Due to the phase quantization nature of the frequency generator, spurious tones can be present in the output in the range of –50dB relative to fundamental. The energy from these tones is included in the signal to noise + distortion specification (SND) given in the electrical table. The frequency of these tones can be very close to the fundamental, and it is not practical to filter them out. SINEWAVE GENERATOR The sinewave generator is composed of a sine lookup table, an 8bit DAC, an output smoothing filter, and an amplifier. The sine lookup table is addressed by the phase accumulator. The DAC is driven by the output of the table and generates a staircase representation of a sine wave. The output filter smooths the analog output by removing the high frequency sampling components. The resultant voltage on VOUT is a sinusoid with the second and third harmonic distortion components at least 40dB below the fundamental. The ML2037 has a 2bit (G1, G0) digital gain control. With the gain input equal to logic 00, the sine wave amplitude is equal to 0.5VPP. Incrementing the gain control input increases the output amplitude in 0.5V steps to a maximum of 2.0VPP. The output amplitude is accurate to within ±0.5dB over the frequency range. G1 G0 P–P OUTPUT AMPLITUDE 0 0 .5V 0 1 1.0V 1 0 1.5V 1 1 2.0V The analog section is designed to operate over a frequency range of DC to 500kHz and is capable of driving 1k W, 50pF loads at the maximum amplitude of 2.0VPP. The sine wave output is typically centered about a 2.5V DC level, so for a 2VPP sine wave, the output will swing from 1.5V to 3.5V. CRYSTAL OSCILLATOR The crystal oscillator generates an accurate reference clock for the programmable frequency generator. The internal clock can be generated with a crystal or external clock. If a crystal is used, it must be placed between CLK IN and DGND. An onchip oscillator will then generate the internal clock. No other external components are required. The crystal should be a parallel resonant type with a frequency between 5MHz to 25.6MHz. It should be placed physically as close as possible to CLK IN and DGND, to minimize trace lengths. The crystal must have the following characteristics: • Parallel resonant type • Frequency: 5MHz to 25.6MHz • Maximum ESR: 120 W @ 5 to 10MHz, 80W @10 to 15MHz, and 50 W @ 15 to 25.6MHz • Drive level: 500µW • Typical load capacitance: 18  20pF • Maximum case capacitance: 7pF The frequency of oscillation will be a function of the crystal parameters and board capacitance. In general, 
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