MCP606/7/8/9
DS11177F-page 18
2009 Microchip Technology Inc.
4.8.2
PHOTODIODE AMPLIFIERS
Sensors that produce an output current and have high
output
impedance
can
be
connected
to
a
transimpedance
amplifier.
The
transimpedance
amplifier converts the current into voltage. Photodiodes
are one sensor that produce an output current.
The key op amp characteristics that are needed for
these circuits are: low input offset voltage, low input
bias current, high input impedance and an input
common mode range that includes ground. The low
input offset voltage and low input bias current support
a very low voltage drop across the photodiode; this
gives
the
best
photodiode
linearity.
Since
the
photodiode is biased at ground, the op amp’s input
needs to function well both above and below ground.
4.8.2.1
Photo-Voltaic Mode
Figure 4-9 shows a transimpedance amplifier with a
photodiode (D1) biased in the Photo-voltaic mode (0V
across D1), which is used for precision photodiode
sensing.
As light impinges on D1, charge is generated, causing
a current to flow in the reverse bias direction of D1. The
op amp’s negative feedback forces the voltage across
the D1 to be nearly 0V. Resistor R2 converts the current
into voltage. Capacitor C2 limits the bandwidth and
helps
stabilize
the
circuit
when
D1’s junction
capacitance is large.
FIGURE 4-9:
Photodiode (in Photo-voltaic
mode) and Transimpedance Amplifier.
4.8.2.2
Photo-Conductive Mode
Figure 4-9 shows a transimpedance amplifier with a
photodiode (D1) biased in the Photo-conductive mode
(D1 is reverse biased), which is used for high-speed
applications.
As light impinges on D1, charge is generated, causing
a current to flow in the reverse bias direction of D1.
Placing a negative bias on D1 significantly reduces its
junction capacitance, which allows the circuit to
operate at a much higher speed. This reverse bias also
increases the dark current and current noise, however.
Resistor R2 converts the current into voltage. Capacitor
C2 limits the bandwidth and helps stabilize the circuit
when D1’s junction capacitance is large.
FIGURE 4-10:
Photodiode (in Photo-
conductive mode) and Transimpedance
Amplifier.
4.8.3
TWO OP AMP INSTRUMENTATION
AMPLIFIER
The two op amp instrumentation amplifier shown in
Figure 4-11 serves the function of taking the difference
of two input voltages, level-shifting it and gaining it to
the output. This configuration is best suited for higher
gains (i.e., gain > 3 V/V). The reference voltage (VREF)
is typically at mid-supply (VDD/2) in a single-supply
environment.
FIGURE 4-11:
Two Op Amp
Instrumentation Amplifier.
The key specifications that make the MCP606/7/8/9
family appropriate for this application circuit are low
input bias current, low offset voltage and high
common-mode rejection.
V
OUT
I
D1R2
=
R2
D1
VOUT
Light
C2
VDD
ID1
MCP606
V
OUT
I
D1R2
=
R2
D1
VOUT
Light
C2
VDD
ID1
VB
V
B
0
<
MCP606
V
OUT
V
1
V
2
–
() 1
R
1
R
2
------
2R
1
R
G
----------
++
V
REF
+
=
R2
R1
VOUT
V2
VREF
R1
R2
V1
RG
MCP607
MCP607