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ML2035IP Datasheet(PDF) 5 Page - Micro Linear Corporation |
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ML2035IP Datasheet(HTML) 5 Page - Micro Linear Corporation |
5 / 9 page ML2035 5 FUNCTIONAL DESCRIPTION The ML2035 is composed of a programmable frequency generator, a sine wave generator, a crystal oscillator, and a serial digital interface. The ML2035 frequency and sine wave generator functional block diagram is shown in Figure 3. PROGRAMMABLE FREQUENCY GENERATOR The programmable frequency generator produces a digital output whose frequency is determined by a 16-bit digital word. The frequency generator is composed of a phase accumulator which is clocked at fCLK IN/4. The value stored in the data latch is added to the phase accumulator every 4 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 equation: f f D15 - D0) 2 OUT CLKIN DEC 23 = × ( (1) The frequency resolution and the minimum frequency are the same and is given by the following equation: ∆f f 2 MIN CLKIN 23 = (2) When fCLK IN = 12.352MHz, DfMIN = 1.5Hz (±0.75Hz). Lower frequencies are obtained by using a lower input clock frequency. Due to the phase quantization nature of the frequency generator, spurious tones can be present in the output range of –55dB relative to fundamental. The energy from these tones is included in the signal to noise + distortion specification. The frequency of these tones can be very close to the fundamental. Therefore, it is not practical to filter them out. SINEWAVE GENERATOR The sinewave generator is composed of a sine look-up table, a DAC, and an output smoothing filter. The sine look-up table is addressed by the phase accumulator. The DAC is driven by the output of the look-up table and generates a staircase representation of a sine wave. The output filter smoothes 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 45dB below the fundamental. The ML2035 provides a peak sinewave voltage of ±VCC/2, referenced to GND. The analog section is designed to operate over a range from DC to 25kHz. Due to slew rate limitations, the peak- to-peak output voltage must be limited to VOUT(P-P) £ (125kV x Hz)/fOUT. Since the ML2035 peak-to-peak output voltage is equal to VCC, the maximum output frequency must be limited to 25kHz for VCC = 5V. VOUT can drive a 1k W, 100pF loads, provided the slew rate limitations mentioned above are not exceeded. The output offset voltage, VOS, is a function of the peak-to- peak output voltage and is specified as: (3) For example, if VOUT(P-P) = 2.5V: 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 GND of the ML2035. An on-chip crystal oscillator will then generate the internal clock. No other external capacitors or components are required. The crystal should be a parallel-resonant type with a frequency between 3MHz to 12.4MHz. It should be placed physically as close as possible to the CLK IN and GND. An external clock can drive CLK IN directly if desired. The frequency of this clock can be anywhere between 0 and 12MHz. The crystal must have the following characteristics: 1. Parallel resonant type 2. Frequency: 3MHz to 12.4MHz 3. Maximum equivalent series resistance of 15 W at a drive levels of 1µW to 200µW, and 30 W at drive levels of 10nW to 1µW 4. Typical load capacitance: 18pF 5. Maximum case capacitance: 7pF The frequency of oscillation will be a function of the crystal parameters and PC board capacitance. Crystals that meet these requirements at 12.352000MHz are M-tron 3709-010 12.352 for 0ºC to 70ºC and 3709-020 12.352 for -40ºC to 85ºC operation. |
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