A distributed transducer system for functional electrical stimulation
Summary (2 min read)
- Sensor chip coil close to the skin surface, and route the antenna signal to a single integrated transceiver/transducer.
- This is however not practical since biocompatible interconnect solutions like Cooper cable' have a high impedance at DC and RF.
- The authors adopted a solution with simple digital communication protocols between the central transceiver and control chip, and the outlying transducer chips.
- The sensor chip includes an amplifier for cuff electrode signals, which is itself the subject of another article  , an AD converter and bus interface logic.
- The transceiver includes a direct conversion receiver (actually a homodyne), a load modulation circuit for transmission of data out of the system, control logic, supply regulators and references.
2. System description
- The following is a description of the transceiver chip and the aspects relating to communication between it and the other parts of the system.
- Since the main focus of this article is on the way in which partitioning of functions can solve some of the problems in an implanted system of this type, the sensor chip will only be described inasmuch as it relates to the top-level design of the system.
- The principle of partitioning applies to any transducer type that is relevant in an implanted system, so the internals of the sensor chip will not be emphasized.
- Figure 2 shows atoplevel diagram of the system.
2.2. The power supply
- The transceiver chip provides the supply voltage for the other chips in the system.
- Two strands out of four in the Cooper cable have been assigned to Vss and VD,.
- Because of the helically wound construction of the cable, and the Pt-Ir composition, the impedance is relatively high at all frequencies.
- To provide a stable supply voltage for the outlying chips, it is therefore necessary to add decoupling capacitors to the supply lines at the transducer ends.
- A stimulator chip can require relatively large current pulses from the supply while delivering a stimulus.
- The target bit rate for the system was 50kbiv's, and the receiver is designed to handle up to 10Okbit/s. Data is transmitted out of the system by load modulation.
- The reflected impedance seen by the external transmitter is varied by changing the load seen by the secondary LC circuit.
- A switch is connected between the terminals of the antenna, and by closing the switch, a maximum change in the reflected impedance is obtained.
- This simple scheme has the disadvantage of stopping power transfer to the system during load modulation, and other load modulation circuits have been designed to avoid this  .
- The duty cycle of the switch closure is however so low in their case that the reduction in average power transfer is small.
2.4. Interchip communication
- The line drivers are class AB circuits which have a quiescent current consumption of 5pA each, and can slew the line voltages with a 50 pA current.
- This type of driver was chosen instead of faster types because this is more than sufficient for the purpose, and by limiting the slewing currents the supply transients are reduced.
- The high and low voltages on the signal lines are 1.OV and 0.3V respectively instead of the full supply range, again to reduce supply transients and power consumption.
3. Measurement results
- The data link to the transceiver chip was tested by using a Class D transmitter driving the inductive link, with a data rate of 50kbitfs and 20% ASK modulation and Manchester encoding.
- The data transfer functions according to the specifications, and higher data rates can easily be supported with minor modifications.
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Cites background or methods from "A distributed transducer system for..."
...However, the magnetic coupling signal of the inductive coupling may be several orders of magnitude larger than these recorded biological signals (Gudnason et al 2001)....
...The impedance reflection technique, utilizing a LSK modulation method, is generally adopted in implantable biomicrosystem applications (Smith et al 1998, Gudnason et al 2001, Lanmüller et al 1999)....
Cites background from "A distributed transducer system for..."
...Some systems provide stimulation and sensing in different devices ....