Electronic Design

Optically isolated precision bipolar current source

One of the most frequently requested instruments in the field of in-vivo and in-vitro neurophysiology research is the galvanically isolated neural stimulator. Applications in this field require an accurate, voltage-programmable, bipolar current source capable of driving microamp to milliamp currents into isolated electrodes with a voltage compliance in the range of volts to tens of volts. Commercial products of this type exist, but none of them are precise enough for quantitative studies without resorting to time-consuming unit-specific calibration.

This stimulator described here combines a dual op amp with two multichannel optoisolators one dual and one quad) to control a floating, battery-powered H-bridge output stage (see the figure). The bridge topology has the advantages of generating both output current polarities from a single battery (B1), and using a single output-current-monitoring optoisolator (E1/Q1) for both output polarities with an associated improvement in output symmetry.

To understand circuit operation, consider first a positive-polarity input: VIN > 0. To sink the resulting input current and thus maintain input balance, A1-pin1 will slew negative until IE2 = VIN/R1. The E2/Q2 coupled gain will drive IQ2 > 0 and a consequent negative excursion appears at A2-pin6. A2-pin7 will therefore slew positive, taking the common node of LEDs E3 and E5 with it.

The negative voltage present at A1-pin1 will force Q7 off and thus steer current into E3 and E4, turning on H-bridge phototransistors Q3 and Q4. The resulting electrode current is sourced from B1 and must therefore pass through E1, closing a current regulating feedback loop around A2 via the E1/Q1 coupled gain. A2 will then adjust the E3-E4 drive current until IQ1 = IQ2. In doing so, because of the good tracking that exists over wide ranges of photocurrent between channels in a multichannel optoisolator like the PS2501 series, A2 accurately forces IE1 = IE2 (the adjustment of R1 taking care of any a-priori mismatch between the coupled gains of the two channels) and thus:

IE1 = VIN/5000

Operation for negative (VIN < 0) control inputs is similar, except A1-pin1 will, of course, slew positive until IE2 = −VIN/R1. This will turn Q7 on and steer the A2 output current to E5-E6 instead of E3-E4. Consequently, bridge output polarity will reverse. However, the output current magnitude is still sensed by E1/Q1, and thus the same R1 adjustment will serve to establish:

IE1 = −VIN/5000

What results is precise correspondence between VIN and output current over several orders of magnitude. To improve the reliability of the overall operation, Q8 and visible LED E7 watch for excess current drive from A2 to the H-bridge. E7 will thus give a warning when the voltage compliance of the H-bridge is exceeded. This will occur when the electrodes develop excessive impedance or when B1 needs to be replaced. When the drop across R2 is > 0.7 V, indicating that the bridge is nearing or in output saturation, Q8 will turn on and light E7.

Defective electrodes are a more common cause of voltage compliance faults than battery failure due to the fact that B1’s service life will typically approximate the shelf life of a 9-V, 500-mAhr battery. For applications requiring more output-voltage compliance than a single 9-V battery can provide, advantage can be taken of the 80-VCEO rating of the PS2501 phototransistors by simply adding additional batteries in series with B1.

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