20-Bit, Linear, Low Noise, Precision, Unipolar +10 V DC Voltage Source (cn0257)
The circuit, shown in Figure 1, is a 20-bit, linear, low noise, precision, unipolar (+10 V) voltage source with a minimum amount of external components. The AD5790 DAC is a 20-bit, unbuffered voltage output DAC that operates from a bipolar supply of up to 33 V. The AD5790 accepts a positive reference input range of 5 V to VDD − 2.5 V, and a negative reference input range of VSS + 2.5 V to 0 V. Both reference inputs are buffered on the chip, and external buffers are not required. The AD5790 offers a relative accuracy specification of ±2 LSB maximum, and operation is guaranteed monotonic with a −1 to +2 LSB maximum DNL specification.
The AD8675 precision op amp has low offset voltage (75 μV maximum) and low noise (2.8 nV/√Hz typical) and is an optimum output buffer for the AD5790. The AD5790 has two internal matched 6.8 kΩ feedforward and feedback resistors, which can either be connected to the AD8675 op amp to provide a 10 V offset voltage for a ±10 V output swing, or connected in parallel to provide bias current cancellation. In this example, a unipolar +10 V output is demonstrated, and the resistors are used for bias current cancellation. The internal resistor connection is controlled by setting a bit in the AD5790 control register (see AD5790 data sheet).
The digital input to the circuit is serial and is compatible with standard SPI, QSPI, MICROWIRE®, and DSP interface standards. For high accuracy applications, the compact circuit offers high precision, as well as low noise—this is ensured by the combination of the AD5790 and AD8675 precision components.
Figure 1. 20-Bit Accurate, 0 V to +10 V Voltage Source (Simplified Schematic: All Connections and Decoupling Not Shown)
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