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Current-sink D/A converter measures 3 x 3-mm

Sunnyvale, Calif. — Maxim Integrated Products Inc. says this 10-bit, dual, digital-to-analog (D/A) converter for fiber-optic applications can sink 3.6 milliamps of current per output.

Each output has two current sink ranges; 0 to 1.2 mA and 0 to 3.6 mA. The range and value of sink current for each output is independently set using a 10-MHz, SPI-compatible serial interface. For applications requiring higher sink currents, the two outputs of the MAX5547 can be connected in parallel to sink a maximum of 7.2 mA.

This D/A converter integrates a precision 4 ppm/°C bandgap reference, operates from a single +2.7 volt to +5.25-V supply, and fits in a 3 x 3-mm, 8-pin TDFN package.

 
 
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The D/A converters are stabilized over temperature by the MAX5547's precision +2.5-V internal 4 ppm/°C reference. This reference is available externally and may be used as the system reference. Additionally, the internal reference may be turned off and an external 2.5-V reference can be used.

The MAX5547 D/A converter is also accurate (±4 least significant bit (LSB) at 1.2 mA, and guaranteed monotonic over the extended operating temperature range of -40°C to +85°C. The device settles to within 1 LSB of full scale in 10 microseconds (typical).

The current-sink operation of this D/A converter makes it suited for laser-driver control applications, including pin-diode bias current, modulation currents, and average power and extinction ratios, according to Maxim.

Pricing for the MAX5547 starts at $3.19 (1000-up, FOB USA). Click here for the preliminary MAX5547 data sheet.

Maxim Integrated Products Inc. , 1-800-998-8800, www.Maxim-ic.com.

While current output D/A converters, which are essentially unbuffered, appear to be plentiful, buffered current sink D/A converters like Maxim's MAX5547 for laser drivers aren't.

Laser drivers have pins that control laser bias current, modulation current, average power, etc. These pins operate at a fixed voltage (typically 1.2 V,) and the parameter is controlled by the amount of current flowing from the pin to ground. “Therefore, something is needed to sink current at less than 1.2 V and the MAX5547 does this very easily,” said Joe Neubauer, a business manager in Maxim's Signal Processing and Conversion Group. To do the same thing with a voltage output D/A converter requires extra resistors. Sometimes digital potentiometers (pot) can be used instead, but they aren't great because you wind up with a non linear response, Neubauer said.

On a simple low cost laser driver system used for short distances, the control may be as simple as a fixed resistor from the pin to ground. Only slightly more sophisticated is a pot to ground that is set once and never touched again. As laser transmission distances increase, more precise control of the laser is needed.

A nonvolatile temperature-controlled digital pot, such as Dallas Semi's DS1848, for instance, can change value with temperature. “This provides better control than the set-and-forget pot,” Neubauer said. For even more precise laser control, a microcontroller is typically needed. The MCU can actively monitor the system and make adjustments in bias current, modulation current, average power, etc. The MCU does this through an external digital pot, or by using a D/A converter with two resistors to set the current range. Neubauer expects the MAX5547 to replace the old resistor-type method (DS1848) for controlling laser drivers.

A D/A converter with one resistor could be used, but then only the voltages below 1.2 V would be useable, Neubauer explained. Although voltage output D/A converters with additional resistors work well, they require additional board space and there is a placement cost for the extra resistors, he said.

Maxim provides a good voltage reference, with two D/A converters, which do not require external resistors for use with the A/D converter in the microcontroller, Neubauer said.

The MAX5547 D/A converter doesn't provide a performance improvement over a voltage D/A converter with resistors. The improvement is in the size.

Look for additional versions of this device, which is a first for Maxim, for different applications like cell phone cameras.

Click here to see Maxim’s Analog Design Guide, which features the MAX5547.

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