Signal conditioning
TEST AND MEASUREMENT Automatic test equipment (ATE) Mass spectrometry
Source measure units (SMUs) Data acquisition/analyzers
INDUSTRIAL AUTOMATION Semiconductor manufacturing Process automation Power supply control Advanced robotics
In test and measurement systems, the 0.5 ppm resolution and accuracy of the AD5791 improve overall equipment accuracy and granularity, enabling finer control and excitation of external sources and nano- actuators. In industrial automation, the 0.5 ppm resolution and accuracy provide the precision that is required to move, alter, or position an actuator on the nanoscale. The AD5791 is a single 20-bit, bipolar output, unbuffered voltage output DAC. It achieves a relative accuracy specification (INL) of ±1 LSB, and it guarantees a monotonic operation with a ±1 LSB differential nonlinearity (DNL). Other important parameters include a temperature drift of 0.05 ppm/°C, peak-to-peak noise of 0.1 ppm, and long-term stability of less than 1 ppm. The internal architecture of this IC is an R-2R digital-to-analogue converter made using thin film resistor matching techniques. It operates from a bipolar supply of up to 33 V, and it can be driven by a positive reference in the range of +5 V to VDD –2.5 V and a negative reference in the range of VSS 2.5 V to 0 V. It uses a 3-wire serial interface that operates at clock rates up to 35 MHz and that is compatible with standard serial peripheral interface (SPI), QSPI, MICROWIRE, and DSP interface standards.
The LTZ1000 is an ultrastable temperature- controllable reference. It provides a 7.2 V output with only 1.2 µV p-p of noise, long-term stability of 2 µV/ kHr, and temperature drifts of 0.05 ppm/°C. The part contains a buried Zener reference, a heater resistor for temperature stabilisation, and a temperature sensing transistor. External components are used to set operating currents and temperature to stabilise the reference, providing maximum flexibility and ensuring the best long- term stability and noise performance. This voltage reference, with temperature stabilisation, is almost insensitive to external temperature variations. For the operational amplifiers, a low offset, low noise, and low drift operational amplifier was needed. The AD8675/AD8676 operational
Instrumentation Monthly March 2025
Figure 2. An EMF blocking enclosure used for testing.
amplifiers were chosen for their precision rail-to- rail capabilities, featuring an ultralow offset of 12 µV, a drift of 0.6 µV/°C, a voltage noise of 2.8 nV/ Hz at 1 kHz, and input bias currents of 2 nA over the full operating temperature range. The principle of operation for achieving a 21-bit DAC from 20-bit DACs is based on a resistor divider. The output impedance of the AD5791 is 3.4 kΩ. When connecting two outputs of two of these ICs together, the equivalent circuit becomes a resistor divider. When the code difference between the two DACs is one LSB, the output voltage of the DAC resistor divider will be half of that voltage difference, equivalent to half an LSB. In other words, this configuration allows for obtaining an equivalent 21-bit DAC by connecting the outputs of two 20-bit DACs in parallel. The interconnection diagram is shown in Figure 1. Voltage references VREP and VREFN are set to +10 V and –10 V respectively,
then the output voltage range at VOUT can be programmed to any voltage within that voltage range. For the measurements shown in this article, the hardware used connects two off- the-shelf AD5791 evaluation boards (ordering information EVAL- AD5791). The boards share the same voltage reference, the LTZ1000 module, which is mounted on only one evaluation module. The connection of the reference between the two boards is established using three twisted wires. Additionally, an extra wire is used to connect the outputs of the two DACs. The performance demonstrated in this article could be further improved by mounting the two AD5791 DACs on the same board and ensuring short connections between components using optimised PCB traces. During the collection of linearity data, the results were affected by the presence of external radiated noise at low frequencies (below 1 MHz). This noise primarily originated from the proximity of the
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