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APPLICATION BULLETIN
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PHOTODIODE MONITORING WITH OP AMPS
With their low-input currents, FET input op amps are uni-
versally used in monitoring photodetectors, the most com-
mon of which are photodiodes. There are a variety of
amplifier connections for this purpose and the choice is
based on linearity, offset, noise and bandwidth consider-
ations. These same factors influence the selection of the
amplifier with newer devices offering very low-input cur-
rents, low noise and high speed.
Photodetectors are the bridge between a basic physical
indicator and electronics resulting in the largest single usage
of FET op amps. As a measure of physical conditions, light
is secondary to temperature and pressure until the measure-
ment is made remotely with no direct contact to the moni-
tored object. Then, the signals of a CAT scanner, star-
tracking instrument or electron microscope depend on light
for the final link to signal processing. Photodiodes have
made that link economical and expanded usage to detector
arrays that employ more than 1000 light sensors. Focus then
turns to accurate conversion of the photodiode output to a
linearly related electrical signal. As always, this is a contest
between speed and resolution with noise as a basic limiting
element. Central to the contest is the seemingly simple
current-to-voltage converter which displays surprising mul-
tidimensional constraints and suggests alternative configura-
tions for many optimizations.
CURRENT-TO-VOLTAGE
The energy transmitted by light to a photodiode can be
measured as either a voltage or current output. For a voltage
response, the diode must be monitored from a high imped-
ance that does not draw significant signal current. That
condition is provided by Figure 1a. Here, the photodiode is
in series with the input of an op amp where ideally zero
current flows. That op amp has feedback set by R
1
and R
2
to
establish amplification of the voltage diode just as if it was
an offset voltage of the amplifier. While appealing to more
common op amp thinking, this voltage mode is nonlinear.
The response has a logarithmic relationship to the light
energy received since the sensitivity of the diode varies with
its voltage.
Constant voltage for a fixed sensitivity suggests current
output instead and that response is linearly related to the
incident light energy. A monitor of that current must have
zero input impedance to respond with no voltage across the
diode. Zero impedance is the role of an op amp virtual
ground as high-amplifier loop gain removes voltage swing
漏
from the input. That is the key to the basic current-to-voltage
converter connection of Figure 1b. It provides an input
resistance of R
1
/A where A is the open-loop gain of the op
amp. Even though R
1
is generally very large, the resulting
input resistance remains negligible in comparison to the
output resistance of photodiodes.
(a)
R
2
100k鈩?/div>
R
1
100k鈩?/div>
D
1
A
1
e
O
e
O
= (1 + R
2
/R
1
) (KT/q) In (1 + I
P
/I
S
)
A
1
: OPA128
D
1
: HP5082-4204
(b)
R
1
100M鈩?/div>
I
P
D
1
A
1
e
O
R
1
100M鈩?/div>
0.1碌F
e
O
= I
P
R
1
A
1
: OPA111
D
1
: HP5082-4204
FIGURE 1a. Photodiode Output Can be Monitored as a
Voltage; or, 1b, as a Current.
Diode current is not accepted by the input of the op amp as
its presence stimulates the high amplifier gain to receive that
current through the feedback resistor, R
1
. To do so, the
amplifier develops an output voltage equal to the diode
current times the feedback resistance, R
1
. For that current-to-
voltage gain to be high, R
1
is made as large as other
constraints will permit. At higher resistance levels, that
resistor begins to develop significant thermal DC voltage
1
AB-075
1994 Burr-Brown Corporation
Printed in U.S.A. January, 1995
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