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) / (2
p R
2
C
2 R
3) The 1-MΩ, 0.1-
mF values shown in the figure
for R
2 and C
2 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 R
3 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 R
3 will increase the dc restoration range,
but will also raise the noise gain of the transimpedance amplifier. Dropping
R
3 to 10 kΩ would increase noise from 130
mV rms
to 650
mV rms. Values above 100 kΩ for R
3
will not substantially reduce noise.
