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Anal. Chem. 7903, 55.407-408
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AE
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B
A
Figure 4. Results with battery-powered coulostat, 5 mV vertical and 50 ns horizontal per division: (A) same as Figure 3. (B) photovoltage response of a TiO, electrode to bandgap light.
6N135 isolator is relatively fast but has very low supply current since it contains no amplifier. Its switching time is 2 ps with a 100-kn load resistor. This switching time as well as the solid-state switch's settling time must be accounted for when triggering the light source. A delay time ( 10 ps) should be present between triggering the optoisolator and the actual occurrence of the light pulse. This delay may be inherent in the light source's firing circuitry or may be externally generated electronically. The optoisolator must be mounted directly onto the front panel connector where the trigger input for the cell switch enten3 the box. This keeps the lead length and resulting noise generation as small as possible. The signal and ground leads to the external voltmeter used to measure the potential applied before the flash must both be switched out prior to the flash to reduce noise. Typical coulostatic-flush results are shown in Figures 3 and 4. Figure 3 shows the noise response with the conventionally powered coulostat (light blocked from the electrode), whereas N
Figure 4A shows the noise response with the battery-powered coulostat. The photovoltage response of a TiOz electrode to bandgap light is shown in Figure 4B using the bactery powered coulostat. The noise level shown in Figure 3 is comparable to that observed previously ( 2 ) when comparable sound grounding and shielding practices were also followed. The noise level shown in Figure 4 is nearly 2 orders of magnitude lower. Thus, the battery-powered coulostat does allow measurements of photovoltage transients in the nanoseconds time domain with
