2009 Microchip Technology Inc.
DS11177F-page 17
MCP606/7/8/9
4.6
Unused Op Amps
An unused op amp in a quad package (MCP609)
circuits prevent the output from toggling and causing
crosstalk. Circuits A sets the op amp at its minimum
noise gain. The resistor divider produces any desired
reference voltage within the output voltage range of the
op amp; the op amp buffers that reference voltage.
Circuit B uses the minimum number of components
and operates as a comparator, but it may draw more
current.
FIGURE 4-6:
Unused Op Amps.
4.7
PCB Surface Leakage
In applications where low input bias current is critical,
Printed Circuit Board (PCB) surface-leakage effects
need to be considered. Surface leakage is caused by
humidity, dust or other contamination on the board.
Under low humidity conditions, a typical resistance
between nearby traces is 1012
Ω. A 5V difference would
cause 5 pA of current to flow, which is greater than the
MCP606/7/8/9 family’s bias current at +25°C (1 pA,
typical).
The easiest way to reduce surface leakage is to use a
guard ring around sensitive pins (or traces). The guard
ring is biased at the same voltage as the sensitive pin.
An example of this type of layout is shown in
Figure 4-7.FIGURE 4-7:
Example Guard Ring Layout
for Inverting Gain.
1.
Non-inverting Gain and Unity-gain Buffer:
a)
Connect the non-inverting pin (VIN+) to the
input with a wire that does not touch the
PCB surface.
b)
Connect the guard ring to the inverting input
pin (VIN–). This biases the guard ring to the
common mode input voltage.
2.
Inverting
Gain
and
Transimpedance
Gain
(convert current to voltage, such as photo
detectors) amplifiers:
a)
Connect the guard ring to the non-inverting
input pin (VIN+). This biases the guard ring
to the same reference voltage as the op
amp (e.g., VDD/2 or ground).
b)
Connect the inverting pin (VIN–) to the input
with a wire that does not touch the PCB
surface.
4.8
Application Circuits
4.8.1
LOW-SIDE BATTERY CURRENT
SENSOR
The MCP606/7/8/9 op amps can be used to sense the
load current on the low-side of a battery using the
circuit in
Figure 4-8. In this circuit, the current from the
power supply (minus the current required to power the
MCP606) flows through a sense resistor (RSEN), which
converts it to voltage. This is gained by the the amplifier
and resistors, RG and RF.Since the non-inverting input
of the amplifier is at the load’s negative supply (VLM),
the gain from RSEN to VOUT is RF/RG.
FIGURE 4-8:
Low Side Battery Current
Sensor.
Since the input bias current and input offset voltage of
the MCP606 are low, and the input is capable of
swinging below ground, there is very little error
generated by the amplifier. The quiescent current is
very low, which helps conserve battery power. The
rail-to-rail output makes it possible to read very low
currents.
VDD
R1
R2
VDD
VREF
V
REF
V
DD
R
2
R
1
R
2
+
-------------------
=
MCP609 (A)
MCP609 (B)
Guard Ring
VSS
VIN-VIN+
V
OUT
V
LM
I
+
L
R
SEN
R
F
R
G
()
=
RF
To Load
2.5V
RG
5k
Ω
50 k
Ω
To Load
VOUT
RSEN
10
Ω
(VLM)
(VLP)
IL
to
6.0V
MCP606