Datasheet

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 (D
1
) biased in the Photo-voltaic mode (0V
across D
1
), which is used for precision photodiode
sensing.
As light impinges on D
1
, charge is generated, causing
a current to flow in the reverse bias direction of D
1
. The
op amp’s negative feedback forces the voltage across
the D
1
to be nearly 0V. Resistor R
2
converts the current
into voltage. Capacitor C
2
limits the bandwidth and
helps stabilize the circuit when D
1
’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 (D
1
) biased in the Photo-conductive mode
(D
1
is reverse biased), which is used for high-speed
applications.
As light impinges on D
1
, charge is generated, causing
a current to flow in the reverse bias direction of D
1
.
Placing a negative bias on D
1
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 R
2
converts the current into voltage. Capacitor
C
2
limits the bandwidth and helps stabilize the circuit
when D
1
’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 (V
REF
)
is typically at mid-supply (V
DD
/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
D1
R
2
=
R
2
D
1
V
OUT
Light
C
2
V
DD
I
D1
MCP606
V
OUT
I
D1
R
2
=
R
2
D
1
V
OUT
Light
C
2
V
DD
I
D1
VB
V
B
0 <
MCP606
V
OUT
V
1
V
2
–()1
R
1
R
2
------
2R
1
R
G
----------++
⎝⎠
⎜⎟
⎛⎞
V
REF
+=
R
2
R
1
V
OUT
V
2
V
REF
R
1
R
2
V
1
R
G
½
MCP607
½
MCP607