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Data acquisition


meets the other necessary output requirements. Using the ADA4870 evaluation board


provides a relatively easy way to interface with an evaluation board and a signal generator since it has SMA inputs and SMA outputs.


Design ConsiDerations for evaluation BoarD Development


Designing an evaluation board that can also be used for ac PSRR testing is not grounds for drastically changing the design. Some key points to keep in mind are: For each supply that is going to be tested


Figure 7. Poor power plane connection design example.


for PSRR, provide an option to drive it via an SMA to maintain signal integrity from the signal source. Pay careful attention to reduce any


parasitic inductances and capacitances in the path from the SMA input to the associated power plane on the DUT. Any parasitic capacitances or inductances can cause unwanted resonances over the frequencies of interest. For each supply, make sure that its


Figure 8. High frequency schematic equivalent of Figure 7.


associated power plane is solid—that is, not split into multiple sections through passive components and multiple layers. For instance, a current sense resistor should not straddle two power planes (as shown in Figure 7). Also, minimise the number of times the power supply crosses layers to avoid parasitic inductances due to vias, as represented in the high frequency model shown in Figure 8. The resistors shown in Figure 7 could be used for current sensing but, in this case, they are 0 Ω. Figure 9 shows better PCB power plane routing, while Figure 10 shows the high frequency equivalent model. It is impor tant that the evaluation board be


tested without a DUT to make sure there are not any unwanted resonances in the frequency range of interest. If there are any resonances, then it should be accounted for during data processing. For each frequency, scope verify that the supply signal matches expectations—don’t trust the dials on the signal generator.


test setup Figure 9. Optimising PCB power plane routing: best performance.


As previously noted, the supply for the signal chain µModule solution under test must be able to provide the nominal dc offset to power the DUT plus an ac stimulus at sufficient current at the maximum input frequency. To accomplish this in the test setup shown here, the ADA4870 evaluation board is used in a noninver ting gain of two in combination with an AD3256 function generator. Figure 11 shows a customised ADA4870


Figure 10. High frequency equivalent of Figure 9. 40 November 2020 Instrumentation Monthly


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