CN0028| Application Note
Circuit Note
CN0028
A
Verified Circuit Designs Using Analog Devices Signal Processing Devices
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Precision, Bipolar, Configuration for the AD5547/57
Contributed November 7, 2008 Devices connected in this Circuit Note: AD5547/57 Dual Current-Output Parallel-Input, 16/14-Bit DAC AD8512 Low Noise, Low Input Bias, JFET Operational Amplifier ADR01 Ultracompact, Precision 10.0 V Voltage Reference
Circuit Function and Benefits
The circuit described in this document and shown in Figure 1 provides precision, bipolar data conversion using the AD5547/57, current-output DAC along with the ADR03 precision reference and the AD8512 operational amplifier (op-amp). This circuit provides accurate, low-noise, high speed output voltage capability and is well-suited for process control, automatic test equipment and digital calibration applications.
Circuit Description
The AD5547/AD5557 are dual-channel, precision 16-/14-bit, multiplying, low power, current output, parallel input D/A converters. They operate from a single 2.7 V to 5.5 V supply with 15 V multiplying Figure 1: 4-Quadrant Multiplying Mode, VOUT = VREF to +VREF references for 4-quadrant outputs. Built-in 4-quadrant resistors facilitate the resistance matching and temperature tracking that minimizes the number of components needed for multi-quadrant applications. This circuit uses the ADR01 which is a highly accuracy, high stability, 10V precision voltage reference. As voltage reference temperature coefficient and long-term drift are primary considerations for applications requiring highprecision conversion, this device is an ideal candidate. An op-amp is used in the current-to-voltage (I-V) stage of this circuit. An op-amp's bias current and offset voltage are both important selection criteria for use with precision current output DACs. Therefore, this circuit employs the AD8512 op-amp which has ultralow off
set voltage and bias current. C9 is a compensation capacitor. The value of C9 for this application is 2.2pF, which is optimized to compensate for the external output capacitance of the DAC. The input offset voltage of the op-amp is multiplied by the variable gain (due to the code-dependent output resistance of the DAC) of the circuit. A change in this noise gain between two adjacent digital fractions produces a step change in the output voltage due to the amplifier's input offset voltage. This output voltage change is superimposed on the desired change in output between the two codes and gives rise to a differential linearity error, which, if large enough, could cause the DAC to be non-monotonic. In general, the input offset voltage should be a
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