Photodiode signal conditioner

Posted on Aug 7, 2012

The photodiode specified responds linearly to light intensity over a 100 dB range. Digitizing the diodes linearly amplified output would require an A-D converter with 17 bits of range. This requirement can be eliminated by logarithmically compressing the diode's output in the signal conditioning circuity. Al and Q4 convert the diode's photocurrent to voltage output with a logarithmic transfer function. A2 provides offsetting and additional gain. A3 and its associated components form a temperature control loop which maintains Q4 at constant temperature (all transistors in this circuit are part of a CA3096 monolithic array).

Photodiode signal conditioner
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The 0.033 µ¥ value at A3's compensation pins gives good loop damping if the circuit is built using the array's transistors in the location shown. Because of the array die's small size, response is quick and clean. A full-scale step requires only 250 ms to settle to final value. To use this circuit, first set the thermal control loop. To do this, ground Q3's base and set the 2 k pot so A3's negative input voltage is 55 mV above its positive input. This places the servo's setpoint at about 50°C (25°C ambient + (2.2 mV/°C 25°C rise = 55 mV = 50°C). Unground Q3's base and the array will come to temperature. Next, place the photodiode in a completely dark environment and adjust the "dark trim" so A2's output is 0 V. Finally, apply or electrically simulate 1 mW of light and set the ''full-scale" trim for 10 V out. Once adjusted, this circuit responds logarithmically to light inputs from lOnW to ImW with an accuracy limited by the diode's 1% error.

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