MCP607-I/PG Microchip Technology, MCP607-I/PG Datasheet - Page 15

• Manufacturer Part Number: MCP607-I/PG
• Description: DUAL 25 UA, 2.5V OP AMP, -40C to +85C, 8-PDIP, TUBE
• Manufacturer: Microchip Technology

4.8.2
Sensors that produce an output current and have high
output impedance can be connected to a transimped-
ance 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
Figure 4-9
photodiode (D
across D
sensing.
As light impinges on D
a current to flow in the reverse bias direction of D
op amp’s negative feedback forces the voltage across
the D
into voltage. Capacitor C
helps
capacitance is large.
FIGURE 4-9:
mode) and Transimpedance Amplifier.
4.8.2.2
Figure 4-9
photodiode (D
(D
applications.
As light impinges on D
a current to flow in the reverse bias direction of D
Placing a negative bias on D
junction capacitance, which allows the circuit to
© 2008 Microchip Technology Inc.
1
Light
is reverse biased), which is used for high-speed
1
to be nearly 0V. Resistor R
stabilize
1
), which is used for precision photodiode
I
D1
shows a transimpedance amplifier with a
shows a transimpedance amplifier with a
PHOTODIODE AMPLIFIERS
Photo-Voltaic Mode
Photo-Conductive Mode
1
1
) biased in the Photo-voltaic mode (0V
) biased in the Photo-conductive mode
the
D
V
1
OUT
Photodiode (in Photo-voltaic
1
1
, charge is generated, causing
, charge is generated, causing
circuit
MCP606
2
=
limits the bandwidth and
I
V
1
D1
C
R
DD
significantly reduces its
2
2
R
when
2
2
converts the current
D
1
’s
V
junction
OUT
1
. The
1
.
operate at a much higher speed. This reverse bias also
increases the dark current and current noise, however.
Resistor R
C
when D
FIGURE 4-10:
Photo-conductive mode) and Transimpedance
Amplifier.
4.8.3
The two op amp instrumentation amplifier shown in
Figure 4-11
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
is typically at mid-supply (V
environment.
FIGURE 4-11:
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 com-
mon-mode rejection.
V
2
REF
limits the bandwidth and helps stabilize the circuit
V
V
Light
2
1
1
V OUT
’s junction capacitance is large.
2
I
R
D1
TWO OP AMP INSTRUMENTATION
AMPLIFIER
converts the current into voltage. Capacitor
serves the function of taking the difference
1
VB
=
(
V 1 V 2
D
MCP606/7/8/9
V
R
1
–
MCP607
OUT
2
Photodiode (in
Two op amp Instrumentation
R
½
V
) 1
G
⎛
⎜
⎝
B
MCP606
=
<
+
I
0
V
R
R 1
------
R 2
D1
DD
C
R
DD
2
2
2
R
+
/2) in a single-supply
2
2R 1
--------- -
R G
DS11177E-page 15
⎞
⎟
⎠
R
MCP607
+
1
V REF
½
V
OUT
V
OUT
REF
)