Electronic Design

Photodiode-Amp Nulls Ambient Light

Many applications use a photodiode to measure a light signal in the presence of ambient background light. Sometimes, the photodiode can be optically shielded from background light to eliminate unwanted signals. Another solution is to use a photodiode-amplifier with a dc-restoration circuit to reject low-frequency background light signals.

The circuit shown represents the latter method (see the figure). It consists of a Burr-Brown OPT201 integrated photodiode and amplifier, and an external op amp for dc restoration. The OPT201 combines a large 0.090-by-0.090-in. photodiode and a high-performance transimpedance amplifier on one chip. This composite eliminates the problems common among discrete designs, including leakage current errors, noise pickup, and gain peaking due to stray capacitance. The dc-restoration circuit consists of a noninverting integrator driving the transimpedance-amplifier summing junction through a 100-kΩ resistor, R3. The current through R3 cancels the current from the photodiode at signal frequencies below integrator's pole frequency to drive the output of the photodiode amplifier to 0 V. The pole frequency is set by R2 and C2:

f-3db = (1 MΩ) / (2p R2 C2 R3)

The 1-MΩ, 0.1-mF values shown in the figure for R2 and C2 set the low-frequency cutoff pole at 16 Hz. Because of the long time constant, it may take more than one second for the circuit to come out of saturation when first powered up.

A noninverting integrator requires a matching pole. The matching pole, set by R1 and C1, prevents photodiode-amplifier output signals above the pole frequency from feeding directly back into the summing junction of the transimpedance amplifier. Matching the poles isn't critical—± 30% tolerance is adequate for most applications.

The value used for R3 depends on the amplitude of the background light. With a 10-V output on A1, the 100-k resistor can provide a 100-mA restoration current. This represents ten times the photodiode current that would otherwise drive the photodiode amplifier into saturation when using the internal 1-MΩ resistor. The dc-restoration circuit can remove a background signal many times larger than the ac signal of interest, thus supplying the increased signal-to-noise level critical in many applications.

Reducing the value of R3 will increase the dc restoration range, but will also raise the noise gain of the transimpedance amplifier. Dropping R3 to 10 kΩ would increase noise from 130mV rms to 650 mV rms. Values above 100 kΩ for R3 will not substantially reduce noise.

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