Posted on Feb 23, 2013

Most circuits that provide an electrical stimulus for research subjects are constant-voltage designs; this circuit is a constant-current design. Stimulator circuits must be isolated for two reasons: to ensure safety and to minimize interference. Isolated stimulators are essentially two-terminal devices; output currents can flow only between tbe two output terminals and can at no tiroe flow through any other path, such as the power ground.

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The circuit"s bandwidth ranges from 50 Hz to 5kHz when a ±1 V sinusoidal input drives tbe circuit. Output loads can range from a short circuit to 100 K!l and have as much as 0.033 JLF of parallel capacitance. The transformer and associated circuitry conveniently connect to tbe main circuit via a cable. Note: This circuit is not approved for use on human beings. Op amps, IC1 and IC2, buffer and set the gain of the circuit, respectively. You adjust trimmer R1 so that R2, a 10-tum pot, yields output currents ranging from 0 to 1 mA/ViN· IC3 is a power op amp. Its output drives the primary of a transformer that has a current gain of0.1, or a voltage gain of 10. Operating from a + 15 V supply, the transformer therefore has a voltage compliance of ±150 V. The circuit senses not only the current supplied to tbe transformer but also tbe current in tbe transformer"s secondary. IC7, a fully isolated, medical-grade amplifier, provides tbe isolated feedback signal because the op amp has its own built -in isolation transformer. Trimmer R3 sets the feedback gain precisely at 27 Kohm nominal.

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