ACC16 Coupler for the Microstimulator System

Tech Note: TN0249
Product: MS16, MS4
Version: All
Date Added: 2007-03-30


The MS16 and MS4 stimulus isolators may generate a DC bias current (up to 0.2 uAmp in the 100 uAmp range and up to 2 uAmp in the 1 mAmp range) on any stimulation channel even during a quiescent state. While this may not have significant short-term effects, over time, it may cause unintended tissue damage.


This problem primarily affects researchers using electrodes with impedances of more than 100 kOhms.


To resolve this issue, TDT supplies an ACC16 AC coupler (pictured below) with each MS16/MS4.

Each channel of the ACC16 coupler includes an RC circuit with a one microfarad capacitor in parallel with a 1 MOhm resistor. The coupler acts as a 1.6 Hz high pass filter, eliminating the DC bias current. It also acts as a voltage divider, decreasing the voltage and thus the current delivered through the electrode. The actual current delivered through the ACC16 depends on the ratio of the coupler impedance to the impedance of the electrode in use.

To calculate a correction factor for actual current delivered:

  1. Determine the impedance of your stimulating electrode.

  2. Calculate the following equation:

    Correction = 1/(1000000/(ElectrodeImp+1000000))

    Correction = (Electrode Imp +1,000,000)/1,000,000

  3. In your circuit, scale the current output by this value.

    In the example circuit above:

    1. The value for "correction" represents the results of the calculation above.

    2. The value for "amp" represents the desired amplitude of the stimulus signal.

    3. The values for the "Limit" component should be set based on the actual limits of your systems. The MS16 is available in 100 uAmp and 1 milliAmp versions. In either case, when using the ACC16 you will NOT be able to deliver the MAXIMUM current.


      The scale factor for the Float2Int component above is configured for use with the RX7 processor. When using RZ5, use a scale factor of 1.7394e+006.

If an ACC16 was not provided with your MS16/MS4, contact TDT to request one.