Signal conditioning
boards used for testing to the power sup- ply and other nearby instruments. To mitigate this noise, all the hardware was enclosed in an electromagnetic field (EMF) blocking enclosure, effectively shielding the hardware under test from external radiated noise. The enclosure used is shown in Figure 2.
Variations in ambient temperature can also affect measurements. The stability of the voltage reference can be affected if it does not operate at a constant temperature. The LTZ1000 addresses this issue. This voltage reference incorporates an internal resistor, and, with the use of external components and a feedback loop, it regulates the temperature of the die. The internal temperature of the component remains constant, ensuring that variations in the external ambient temperature have no effect on voltage output stability.
Active components, including power supply devices, can cause changes in the output voltages of the supply rails and affect the output voltage of the DAC. The impact of voltage supply variations on the output voltage is reflected in the DCPSRR specification of the DAC. The operational amplifiers used for the references and output buffer also exhibit temperature dependency.
For high accuracy applications, special attention should be given to the selection of resistors. It is important to choose resistors with a low temperature coefficient, ideally around 0.01%, and, if possible, operate the system at a constant temperature to minimise variations in resistance.
Changes in the external temperature of the voltage reference IC result in proportional fluctuations in the output voltage due to the temperature coefficient drift. The impact of these fluctuations on the integral nonlinearity (INL) is illustrated in Figure 3. The INL graph was obtained using the ADR445 reference at room temperature without the use of an EMF blocking enclosure. The board used for testing contained resistors with a typical temperature coefficient of 3 ppm/K. The observed jumps in the INL are attributed to temperature changes in the room, such as variations in the number of people present and the cycling of the air conditioning system. The measurement was conducted over a duration of approximately 24 hours.
To minimise temperature variations during the tests, options are available, such as using a temperature forcing machine that can provide a highly stable temperature throughout the duration of the tests. To maintain simplicity, the same EMF blocking enclosure used to isolate the boards from external radiated electromagnetic noise was utilised to maintain a relatively stable temperature
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Figure 3. An INL plot using the ADR445 voltage reference.
during the testing. The power dissipation of the boards was calculated to be below 0.5 W, resulting in an interior temperature range of 25°C to 30°C within the EMF blocking enclosure throughout the duration of the tests.
Having identified all the elements that can impact the output voltage of a signal chain DAC, the next step is to program the two DACs to effectively obtain a 21-bit DAC. From a digital perspective, when
dealing with a given 21-bit code, the DAC code needs to be split into two halves. If the original code is even, the remainder of the division is zero. If the original 21-bit code is an odd number, the remain- der of the division is one. In this case, one DAC should be programmed with the result of the division, while the other DAC should be programmed with the split code plus one. An example is shown in Table 1. This concept can be extended further by dividing
Figure 4. A 21-bit INL plot. March 2025 Instrumentation Monthly
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