stimulator.jpg

Time has not been kind to my isolated stimulator design (above) which has accumulated some additional circuitry over the past few years. The original specifications called for a circuit that would receive a program voltage on the non-isolated side and then drive a load with the same voltage off the isolated side using batteries (not pictured) and return the actual drive voltage and current back to the non-isolated side. The requirement for isolation was pretty relaxed and it was assumed that the load’s common mode potential would be on a few volts from the non-isolated ground.

It was then decided that we needed a voltage-programmed, current-output stimulator, so the same design was slightly modified to have a current follower as the output stage. Due to electrochemical properties of the load, we wanted to clamp the potential between the stimulating electrodes (in current-output mode) to some safety window, so one of the extra circuits was added for this function. As all real electrodes, our electrodes could also polarize, so a small (<1uA) over days could cause some problems with poor performing electrodes. This was a problem due to the single-ended nature of the isolation system. To address this issue, an effective high-pass circuit was carefully added to the isolated side with a time constant of minutes to return the output current to zero, which lead to another small PCB added.

I decided that it was finally time to redesign the stimulator since the kludges were beginning to cause the whole device to fail and were a problem to troubleshoot. The points that I wanted to address specifically are:

  • What happens to the isolated output when the non-isolated side loses power (safety!)
  • To design a true DC stimulator (no high-pass) while automatically adjusting for offset current
  • To introduce full optical isolation
  • To provide a warning to the non-isolated side when the isolated side suffers battery failure

The first issue is a big one. Since I am expecting a program voltage which can be positive and negative, the operation of a linear optocoupler requires that the driving LED have some fixed intensity for zero program voltage, with higher and lower intensities for higher and lower program voltage respectively (can be opposite). This means that a power failure on the non-isolated side would shut off the LED completely and would cause the perceived program voltage on the isolated side to rail to one of the supplies, thereby providing maximum stimulation. Looking through several other application notes [1] [2] [3], I did not see a clear solution to this issue. However, using a pair of optocouplers in a differential mode should take care of the problem. That is, still require that both driving LEDs are biased to a certain intensity for zero program voltage, however, only use one to transmit the program voltage and use the other to transmit effective ground. The voltages (or currents) generated by the photovoltaic cells on the isolated side can then be subtracted, and with a little tuning, should provide proper isolation during normal operation and the difference should go to zero when the non-isolated side loses power. Using this differential scheme should also provide a means to control for small offset voltages and should remove the need for a built-in high-pass filter.

The question of full optical isolation is also a tricky one. In the current scheme, the isolated side measures current and voltage and feeds them directly into CMOS inputs of voltage followers on the non-isolated side. This would work fine if the isolated and non-isolated sides are at the same potential, however, any difference creates non-linear current paths through the ESD suppression mechanisms that are built into the CMOS op-amps that we use. I am thinking of using a two or three optocouplers to send the voltage and current signals back to the non-isolated side, however, I will have to work out if I can use the ground from the above paragraph somehow to remove any offsets in their transmission and work to limit the drive current to minimize power usage. The Clare LOC110 (the optocoupler we are using) can easily take 25-50mA across the driving LED, which is more than the rest of the isolated circuitry.

Finally, there needs to be an indicator for battery failure on the isolated side. It can be argued that proper tracking of the stimulation voltage and current would be a good indication of battery condition, but this will not work if the program voltage stays at zero (i.e., if we request zero volts, we will get zero volts out even if the batteries are dead). A compounding problem is that the isolated side is driven by a pair of 1.5V batteries (in series) to provide a total of 3V and it should be remembered that the LEDs in the optocouplers require a minimum of about 1V to operate. Hooking them up to the battery power (one for + and one for -) would be quite a wasteful solution as it would draw a substantial amount of current to operate, especially when the batteries are full. My present design idea is to use a pair of optocouplers, each in series with a depletion-mode FET connected to a resistor bridge. This way, when the bridge voltage drops below a certain value, the depletion-mode FETs will conduct and the LED will turn on, warning of battery failure.

I am hoping to get this design finished before August and fully expect another post in a year with new kludges, perhaps bluetooth or something.

 
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