Headstage PCBs and Their Specifications

Formation Headstage PCBs and Their Specifications, tutoriel & guide de travaux pratiques en pdf.

Final Headstage PCB

The final headstage PCB, being the removable part of the headstage system, has been design on a rigid-flex PCB with 6 layers. However this PCB has not been fabricated yet. This PCB will carry the same parts that are mounted on the prototype PCB (expect for the LEDs and the electrodes) but has the required form factor of the final headstage. As mentioned in the introduction of this chapter, the final headstage PCB is fabricated using the rigid-flex technology resulting in less occupied volume and better EMC/EMI robustness.
The final headstage PCB measures 65×20.9 mm2 when unrolled, has minimum trace width/spacing of 0.2 mm and the minimum hole diameter is 0.2 mm. Table 11 shows the layer stacking of this PCB.
Table 11. Final headstage PCB layer stack.
The rationale behind this stacking is that most PCB manufacturers prefer to have signal layers as the inner layers when fabricating rigid-flex PCBs.
This rigid-flex PCB has 3 rigid sections that are connected to each other using the flexible sections. This design strategy eliminates the need for bulky board-to-board connectors. Each rigid section measures 15×21 mm2 and each flexible section measures 10×21 mm2.
When the rigid parts are folded together, as is the goal of designing this rigid-flex PCB, the headstages measures less than 20×20 mm2 depending on the folding angle, as stated by the design requirements. Figure 24 shows the unrolled PCB in 2D view.
Figure 24. Final headstage PCB unrolled 2D view.
It can be seen that the three rigid parts are connected to each other using light blue and yellow traces. The rigid sections are those populated with components while the flex sections contain only PCB traces. The board-to-board connector is placed on the back of the rigid section at the right side of Figure 24. In Figure 25 and Figure 26, we can see the top view, bottom view and the unrolled final headstage PCB. More specifically, on the right-hand side of Figure 25, the white board-to-board connector can be seen.
Figure 25. Left: final PCB headstage rolled (top view). Right: final PCB headstage rolled (bottom view).
Figure 26. Final PCB headstage unrolled.
While Figure 25 and Figure 26 show the removable part of the headstage system, Figure 27† and Figure 28 show the complete headstage system that will be mounted on the animal head with and without proper packaging. The packaging might be required to protect the headstage from damages caused by animal movements. It can be seen that optical fibers, along with electrodes, will be implemented inside the animal brain.
Figure 27. Left: complete headstage system connected to the non-removable part. Right: cross-section view of the complete headstage system.
Figure 28. Left: complete headstage system in package. Right: cross-section view of the complete headstage system in package.

EMC/EMI Considerations

The EMC/EMI considerations of the final headstage PCB are similar to those of the prototype headstage.
However, the final headstage PCB benefits from the inherent separation of different rigid sections.
Furthermore, since the rigid section dedicated to the AFE is separate from the other subsystems, more EMC/EMI robustness is resulted.
† Figure 27 and Figure 28 were created by Doric Lenses Inc., Québec, Canada.

Measured Performance of the AFE

In this section, we present the measured performance of the analog front end (AFE). Three types of test have been carried out to test the AFE. Using these three tests, almost all characteristics of the AFE can be measured.
In the first type of test, the AFE differential inputs have been shorted out together and the noise characteristics of the AFE have been measured. Attention has been paid to the quality of the short circuit as using long wires will result in higher picked up noise. The input-referred noise can be calculated by dividing the noise at the AFE output by the overall AFE gain.
In the second type of test, the AFE characteristics including the gain and the cut-off frequencies have been measured using an Agilent 35670A dynamic signal analyzer.
Finally, in the third type of test, synthesized but realistic action potential signals with realistic amplitudes were fed to the AFE and the output of the signal chain (AFE and headstage outputs) were measured.
It should be noted that all the measurements were carried out while the system was working i.e., the LEDs were blinking and the radio was transmitting. In this scenario, all noise sources from other headstage subsystems are present. It can be verified that the high-current LED signals, fast digital signals and the radio RF signals do not affect the performance of the AFE.
Table 12 and Figure 29† provide the measurement results provided by the Agilent dynamic signal analyzer. It can be seen that the input-referred noise of the AFE is low enough for action potentials of 10 microvolts to be detected. Moreover, the AFE characteristics conform to those of the design requirements. The measurement results of the synthetic action potentials will be presented in a later subsection in this chapter.
Table 12. Measured AFE characteristics.

Measured Performance of the Optical Stimulation Circuitry

The optical stimulation circuitry has been tested by measuring the voltage on the LED terminals and the current that passes through the LED. The blue LED [87] has been used to carry out the tests. Table 13 summarizes the optical stimulation circuitry performance and Figure 30† shows the voltage on the LED terminals. It can be verified that the rise-time/fall-time of the LED stimulation pattern is negligible compared to the pulse length (i.e., when LED is on) and that the pattern is sharp. This fulfills the needs of the optogenetic experiments having millisecond-scale stimulation patterns.
Table 13. Measured characteristics of the optical stimulation circuitry.

Power Consumption Measurements

In this section, we present the measured power consumption of all headstage subsystems. The headstage has been powered by a 110 mAh Li-ion battery (GSP061225D2C, Great Power Battery Co., LTD [100]). This battery occupies a volume of 5.7×12×28 mm3 and has a maximum continuous discharge current of 200 mA. We have chosen this battery as it provides a compromise between size and capacity. Table 14 and Figure 31† provide the power consumption of different headstage subsystems (System LED is a small indication LED used to show the system operation). In our measurements, we have set the stimulation duty cycle to its maximum (10%).
Table 14. Power consumption of headstage subsystems.