SNAS348G May 2006 – April 2016 DAC124S085
While the simplicity of the DAC124S085 implies ease of use, it is important to recognize that the path from the reference input (VREFIN) to the VOUTs has essentially zero Power Supply Rejection Ratio (PSRR). Therefore, it is necessary to provide a noise-free supply voltage to VREFIN. To use the full dynamic range of the DAC124S085, the supply pin (VA) and VREFIN can be connected together and share the same supply voltage. Because the DAC124S085 consumes very little power, a reference source may be used as the reference input or the supply voltage. The advantages of using a reference source over a voltage regulator are accuracy and stability. Some low noise regulators can also be used. Listed below are a few reference and power supply options for the DAC124S085.
The LM4132, with its 0.05% accuracy over temperature, is a good choice as a reference source for the DAC124S085. The 4.096-V version is useful if a 0-V to 4.095-V output range is desirable or acceptable. Bypassing the LM4132 VIN pin with a 0.1-µF capacitor and the VOUT pin with a 2.2-µF capacitor improves stability and reduce output noise. The LM4132 comes in a space-saving 5-pin SOT23.
Available with accuracy of 0.44%, the LM4050 shunt reference is also a good choice as a reference for the DAC124S085. It is available in 4.096-V and 5-V versions and comes in a space-saving 3-pin SOT23.
The minimum resistor value in the circuit of Figure 38 must be chosen such that the maximum current through the LM4050 does not exceed its 15-mA rating. The conditions for maximum current include the input voltage at its maximum, the LM4050 voltage at its minimum, and the DAC124S085 drawing zero current. The maximum resistor value must allow the LM4050 to draw more than its minimum current for regulation plus the maximum DAC124S085 current in full operation. The conditions for minimum current include the input voltage at its minimum, the LM4050 voltage at its maximum, the resistor value at its maximum due to tolerance, and the DAC124S085 draws its maximum current. These conditions can be summarized with Equation 4 and Equation 5.
The LP3985 is a low-noise, ultra-low dropout voltage regulator with a 3% accuracy over temperature. It is a good choice for applications that do not require a precision reference for the DAC124S085. It comes in 3.0-V, 3.3-V, and 5-V versions, among others, and sports a low 30-µV noise specification at low frequencies. Because low frequency noise is relatively difficult to filter, this specification could be important for some applications. The LP3985 comes in a space-saving 5-pin SOT-23 and 5-bump DSBGA packages.
An input capacitance of 1 µF without any ESR requirement is required at the LP3985 input, while a 1-µF ceramic capacitor with an ESR requirement of 5 mΩ to 500 mΩ is required at the output. Careful interpretation and understanding of the capacitor specification is required to ensure correct device operation.
The LP2980 is an ultra-low dropout regulator with a 0.5% or 1.0% accuracy over temperature, depending upon grade. It is available in 3.0-V, 3.3-V, and 5-V versions, among others.
Like any low dropout regulator, the LP2980 requires an output capacitor for loop stability. This output capacitor must be at least 1 µF over temperature, but values of 2.2 µF or more provides even better performance. The ESR of this capacitor must be within the range specified in the LP2980 data sheet. Surface-mount solid tantalum capacitors offer a good combination of small size and ESR. Ceramic capacitors are attractive due to their small size but generally have ESR values that are too low for use with the LP2980. Aluminum electrolytic capacitors are typically not a good choice due to their large size and have ESR values that may be too high at low temperatures.