OPT201 PHOTODIODE-AMPLIFIER REJECTS AMBIENT LIGHT
by Mark Stitt and Wally Meinel (602) 746-7162
Many applications call for the measurement of 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 way to solve the
problem is to combine a photodiode-amplifier like the
OPT201 with a DC restoration circuit to reject low-fre-
quency background light signals.
The circuit for a photodiode amplifier with DC restoration is
shown in Figure 1. The circuit uses the OPT201 integrated
photodiode and amplifier and an external op amp for DC
restoration. The OPT201 combines a large 0.090 x 0.090
inch photodiode and high-performance transimpedance am-
plifier on a single chip. This combination eliminates the
problems commonly encountered in discrete designs such as
leakage current errors, noise pick-up, and gain peaking due
to stray capacitance.
The DC restoration circuit consists of a non-inverting inte-
grator driving the OPT201 transimpedance amplifier sum-
ming junction through a 100k鈩?resistor, R
3
. The current
through R
3
cancels the current from the photodiode at signal
frequencies below the pole frequency of the integrator to
drive the output of the OPT201 to 0V. The pole-frequency
of the integrator is set by R
2
and C
2
.
The component values shown in Figure 1 set the low-
frequency cutoff pole at 16Hz. Because of the long time
constant, it may take over a second for the OPT201 output
to come out of saturation when the circuit is first powered-
up.
A non-inverting integrator requires a matching pole. The
matching pole, set by R
1
and C
1
, prevents the OPT201 output
signals above the pole frequency from feeding directly back
into the summing junction of the OPT201. Matching of the
poles in not critical鈥斅?0% tolerance is adequate for most
applications.
The value used for R
3
depends on the amplitude of the
background light. With 10V output on A
1
, the 100k鈩?resis-
tor can provide 100碌A(chǔ) restoration current to the OPT201.
This is ten times the photodiode current that would other-
wise drive the OPT201 to 10V output when using the
internal 1M鈩?resistor. The DC restoration circuit can re-
move a background signal many times larger than the ac
signal of interest providing 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 increase
the noise gain of the transimpedance amplifier. Reducing R
3
to 10k鈩?would increase noise from 130碌Vrms to 650碌Vrms.
Values above 100k鈩?for R
3
will not substantially reduce
noise.
Integrator Pole Frequency
f
鈥?dB
=
1M
R
3
(2
鈰?蟺 鈰?/div>
R
2
鈰?/div>
C
2
)
C
2
0.1碌F
R
3
100k鈩?/div>
A
1
C
1
0.1碌F
R
2
1M鈩?/div>
2
1M鈩?/div>
40pF
R
1
1M鈩?/div>
4
75鈩?/div>
5
V
O
OPT201
8
FIGURE 1. Photodiode-Amplifier with DC Restoration Rejects Unwanted Background Light.
漏
1993 Burr-Brown Corporation
AB-061
Printed in U.S.A. November, 1993
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