Data acquisition
radiated emissions as an outcome of its inherently symmetrical topology.
As shown in Figure 2, the power circuitry on the reference platform was designed such that the 12V from the FMC connector can provide the power needed by the data acquisition board while enabling isolation. To achieve those, the LT3999 on the circuit drives a Pulse Electronics PH9085.083NL 2.5kVrms isolation power transformer. The LT3999 power converter produces an output voltage that is unregulated. The output voltage decreases with increasing load as shown in Figure 3. A low dropout, linear post-regulator (ADP7105) is included on the reference platform as an option to use a regulated 3.3V output if desired. Hence, galvanically isolating the entire measurement or data acquisition circuitry through the interposer board minimises the impact of common-mode voltage variations and external noise sources. This proves the method is an accurate, economical, and efficient way to employ an isolated measurement circuitry.
PRESERVING ACCURACY Figure 3. LT3999 push-pull converter output voltage regulation.
power consumption. These isolation devices, besides being easy to implement, offer exceptional performance characteristics vs. typical alternatives such as optocouplers. Specifically, the maximum propagation delay is 13ns with a pulse width distortion of less than 5ns. Channel-to-channel matching of propagation delay is tight at 4.0ns and 3.0ns maximum, respectively.
Subsequently, an isolated precision signal chain would likewise require an isolated power circuitry that satisfies the operating requirements of the signal chain. The isolated power circuitry should not affect the performance of the precision
signal chain. Low emissions must be ensured from the power circuitry while also being efficient and conforming to the required safety requirements. A good choice for power isolation is the LT3999 low noise, push-pull DC-to-DC driver, which features 1A internal dual switches with programmable current limit, adjustable switching frequency from 50kHz to 1MHz (which can also be synchronised to an external clock), a wide operating input range from 2.7V to 36V, and a shutdown current of less than 1µA. The push- pull topology is simple to design and implement, uses few components, and operates with low
Aside from implementing isolation techniques, the building blocks inside the signal chain must be matched well. Each component contributes to the entire signal chain performance and is critical in preserving the accuracy of the whole system. Precision amplifiers - characterised by their high accuracy, low noise, and low offset voltage - provide accurate signal conditioning and amplification to ensure that the acquired signal is faithfully represented without introducing additional distortion or offsets. Furthermore, filtering elements such as low-pass filters are often employed to attenuate high frequency noise and unwanted signals, allowing only the desired signal to pass through the signal chain. This further enhances
Figure 4. Precision medium bandwidth signal chain. Continued on page 60... Instrumentation Monthly September 2025 59
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