MCP606 Microchip Technology, MCP606 Datasheet - Page 14

• Manufacturer Part Number: MCP606
• Description: 2.5V to 5.5V Micropower CMOS Op Amps
• Manufacturer: Microchip Technology

MCP606/607/608/609
3.6
3.6.1
The MCP606/607/608/609 amplifiers can be used to
sense the output current on the low side of a battery
using the circuit in Figure 3-10. In this circuit, the cur-
rent from the power supply (minus the current required
to power the MCP606) flows through a ten ohm resistor
from the rest of the circuit in the system. This current
is converted to a voltage through the sense resistor and
gained by the resistors around the amplifier. Since the
input bias current and offset voltage of the MCP606 is
low, there is very little error generated by the amplifier.
Additionally, the amplifier is capable of swinging below
ground and the quiescent current is very low. These
four specifications make this amplifier appropriate for
this type of circuit.
FIGURE 3-10: Low Side Battery Current Sensing
3.6.2
Any amplifier from this family of operational amplifiers
can be used to convert an output current signal from a
sensor into a voltage. A sensor that fits this description
is a photodetector as shown in Figure 3-11. This type of
circuit is implemented with a single resistor and an
optional capacitor in the feedback loop of the amplifier.
As light impinges on the photo diode, charge is gener-
ated, causing a current to flow in the reverse bias direc-
tion of the photodetector.
Two circuits are shown in Figure 3-11. The top circuit is
designed to provide precision sensing from the photo-
detector. In this circuit the voltage across the detector
is nearly zero and equal to the offset voltage of the
amplifier. With this configuration, current that appears
across the resistor, R
excitation on the photodetector. The photosensing cir-
cuit on the bottom of Figure 3-11 is designed for higher
DS11177B-page 14
+2.5 to 5.5V
Typical Applications
LOW SIDE BATTERY CURRENT SENSING
PREAMPLIFIER FOR PHOTO DETECTION
CIRCUIT
V
DD
I
----------------------------------------------
LOAD
V
SS
1
2
, is primarily a result of the light
+
R
---------- -
R IN
R
SENSE
F
5k
10
R
IN
MCP606
50k
R
V
V
F
DD
SS
To load
speed sensing. This is done by reverse biasing the
photodetector, which reduces the parasitic capacitance
of the diode.
The key specifications that influence the accuracy of
these circuits are low offset voltage, low input bias cur-
rent, high input impedance and an input common mode
range below ground. The low input offset voltage and
low input bias current provide an environment where
there is minimal voltage placed across the photodetec-
tor, consequently the linearity of the photodetector is
not compromised. Given that the MCP606/607/608/
609 amplifiers are specified for a ±250 V(max) offset
voltage and input bias currents in the pico ampere
region they are ideal for these circuits. Additionally,
these two circuits will only work if the common-mode
range of the amplifier includes zero, which is the case
with the MCP606/607/608/609 amplifiers.
FIGURE 3-11: Pre-Amplifier for Photo
*Bypass Capacitor, 1 F
Light
Light
D
V
1
BIAS
D
1
I
D1
Detection Circuit
I
D1
V
OUT
2000 Microchip Technology Inc.
MCP606
MCP606
= R
C
R
2
2
2
V
R
V
I
DD
D1
2
DD
*
*
V
OUT
V
OUT