Automated Calibration Technique Reduces DAC Offset to Less than 1 mV
The transfer function for an N-bit bipolar digital-to-analog converter (DAC) is
where A is the analog output, D is the digital input, G is the gain, VFS is the nominal full-scale voltage, and VOS is the offset voltage. For an ideal DAC, G = 1 and VOS = 0.
The offset error specification, combined with the system requirements, will determine if calibration is needed. The AD5360 16-bit, 16-channel DAC is factory trimmed, but an offset of several millivolts can still exist. The following example shows how a simple software algorithm can reduce an unknown offset to less than 1 mV (typical). This technique can be used for factory calibration, or for offset correction at any point in the DAC's life cycle.
The AD5360's Offset DAC is used to set the output range, which can be unipolar positive, unipolar negative, bipolar centered, or bipolar skewed. The default value of the Offset DAC sets a ±10-V output range when a 5-V reference is used. The Offset DAC also has an offset error. The sixteen DAC outputs are factory trimmed with the Offset DAC at its default value, so this error is trimmed out. As the Offset DAC is changed, its offset error affects the offset errors of the main DAC outputs.
Two features of the AD5360 simplify offset calibration: a GPIO pin, which can have its status determined by reading a register; and an integrated monitor multiplexer, which allows any of the 16 DAC outputs or two external voltages to be switched to a single pin under software control.
Theory of Operation
The AD5360's multiplexer connects the selected DAC output to MON_OUT. Its switches have a low but finite RDSON, so any current drawn from MON_OUT creates a voltage drop across RDS, and hence an output error. To prevent this, MON_OUT is buffered by an AD8597 low-noise amplifier. The low pass filter following the amplifier reduces the amount of noise seen by the AD790 fast, precision comparator and prevents false triggering. The AD790 can be operated from ±15V supplies, making it compatible with the AD5360. In addition, the AD790 has a 15-V maximum differential input voltage, so it can tolerate the output voltages from the AD5360 without attenuation. In Figure 1, the comparator output will be low if the channel offset is positive, indicating that the output voltage needs to be reduced to remove the offset. The comparator output will be high if the channel offset is negative, indicating that the output voltage needs to be increased to remove the offset.
Figure 1. Circuit Diagram
Monitor Multiplexer and GPIO of the AD5360.
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