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ALD1801APC Datasheet(PDF) 5 Page - Advanced Linear Devices |
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ALD1801APC Datasheet(HTML) 5 Page - Advanced Linear Devices |
5 / 6 page ALD1801A/ALD1801B Advanced Linear Devices 5 ALD1801 APPLICATIONS NOTES: The ALD1801 is an 8 bit multiplying D/A converter. It has been designed to operate with standard single or split power supplies of 5V or ±5V. Functionality extends down to 3V or ±1.5V power supply, making it ideal for lithium battery or rechargeable battery operated systems where power efficiency and performance are important design parameters. The ALD1801 features ultra low quiescent bias current which depends on the operating current in the internal current steering circuit. A high gain internal operational amplifier is fully compensated for stable operation without any need for external capacitor compensation. The operational amplifier is placed in a negative feedback loop to generate the reference current for ratio matched transistors. It is trimmed to provide nonlinearity as specified. In some applications this feature can be used along with the multiplying nature of the ALD1801 to obtain 9 or 10 bit digital to analog conversion with external switches. The ALD1801 is a multiplying D/A converter in which the output is a current that is a function of an 8 bit digital input word multiplied by the reference current. The reference current may be a fixed reference current or a varying input current. If a fixed reference voltage or a varying low impedance voltage output source is used, a fixed value resistor (referred to as RREF) can be used to convert the voltage into a reference current for most applications. RREF is a precision fixed resistor without trimming. Trimming can be accomplished by adjusting RREF through either connecting a potentiometer in series with a fixed resistor or by substituting the fixed resistor with a multi- turn potentiometer. The recommended nominal value of RREF is 25KΩ. The ALD1801 can operate from single power supply or dual supplies up to ±6.6V. Symmetrical supplies are not required in the case of dual supply operation. However, VREF- input voltage must be set at 1V or higher above the V- voltage. Due to the internal operational amplifier feedback, VREF+ input terminal would be set at the same potential as VREF- input terminal, with the input reference current determined by: Input reference current IREF = { (VREF+)- (VREF-) } / RREF The ALD1801 is fabricated in CMOS technology and provides direct logic interface to all CMOS logic families including logic levels of the CD4000 and 74C logic families operating at logic voltage levels other than 5V. It is primarily designed to operate in a power supply environment where interface takes place between digital logic circuitry and analog circuitry. Both types of circuitry operate with the same power supplies. The nonlinearity and monotonicity of the ALD1801 applies over the full rated operating temperature. The device has been developed with minimal temperature drift, typically at ±10ppm/°C. A single array of matched and tracked resistor networks of nominal 50K Ω can be used for all the resistors required for the D/A converter. As an example, RREF can be set to 25KΩ by connecting two 50KΩ resistors in parallel where output resistors are set at 50K Ω. Due to the very low quiescent and reference currents used, layout of the circuit board to minimize any parasitic capacitance is important in getting high frequency operation, as the settling time is due in large part to the output currents charging and discharging the node capacitances at the output. As the internal bias current levels is adaptive to the reference current, the switching speed of the device depends on the reference current selected. Characterization of the settling time has been performed at full scale current set at 100 µA and device operation at single 5V supply. Faster operation can be obtained by using short leads, minimizing output capacitance, load resistor values, and by adequate bypass capacitors placed on the board at the supply and reference nodes. Both Io and Io outputs can be used simultaneously. If one of the outputs is not used, then it must be connected to ground or a potential where the sum of Io and Io can maintain a constant value. Due to internal leakage currents that vary with temperature, it is recommended that IFS of 50 µA to 100µA be used so that 1/2LSB bit current is set at as high a level as possible. The temperature coefficient of the reference resistor RREF and output resistors ROUT and ROUT should match to minimize temperature drift. |
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