MCP6L91 Microchip Technology Inc., MCP6L91 Datasheet - Page 12

• Manufacturer Part Number: MCP6L91
• Description: 10 Mhz, 850 ?a Op Amps
• Manufacturer: Microchip Technology Inc.

MCP6L91/1R/2/4
4.4
With this family of operational amplifiers, the power
supply pin (V
bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm
for good high frequency performance. It also needs a
bulk capacitor (i.e., 1 µF or larger) within 100 mm to
provide large, slow currents. This bulk capacitor can be
shared with other nearby analog parts.
4.5
An unused op amp in a quad package (e.g., MCP6L94)
should be configured as shown in
circuits prevent the output from toggling and causing
crosstalk. Circuit 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 4-3:
4.6
In applications where low input bias current is critical,
PCB (printed circuit board) 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 10
cause 5 pA of current to flow; this is greater than this
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.
Figure 4-4
DS22141A-page 12
V
¼ MCP6L94 (A)
DD
R
R
V
1
2
Supply Bypass
Unused Op Amps
PCB Surface Leakage
REF
is an example of this type of layout.
DD
=
V
for single supply) should have a local
V
DD
DD
⋅
------------------
R
Unused Op Amps.
1
R
+
2
R
V
12
2
REF
Ω. A 5V difference would
¼ MCP6L94 (B)
Figure
V
DD
4-3. These
FIGURE 4-4:
1.
2.
4.7
4.7.1
The MCP6L91/1R/2/4 op amp’s low input noise and
good output current drive make it possible to design
low noise filters. Reducing the resistors’ values also
reduces the noise and increases the frequency at
which parasitic capacitances affect the response.
These trade-offs need to be considered when selecting
circuit elements.
Figure 4-5
1 kHz bandwidth, 0.2 dB ripple and a gain of +1 V/V.
The component values were selected using Micro-
chip’s FilterLab
value by increasing C
FIGURE 4-5:
V
IN
Inverting Amplifiers
pedance Gain Amplifiers (convert current to
voltage, such as photo detectors).
a)
b)
Non-inverting Gain and Unity-Gain Buffer.
a)
b)
3.01 kΩ 6.81 kΩ
120 nF
Guard Ring
R
Connect the guard ring to the non-inverting
input pin (V
to the same reference voltage as the op
amp’s input (e.g., V
Connect the inverting pin (V
with a wire that does not touch the PCB sur-
face.
Connect the guard ring to the inverting input
pin (V
common mode input voltage.
input with a wire that does not touch the
PCB surface.
Application Circuit
Connect the non-inverting pin (V
1
C
shows a third-order Chebyshev filter with a
ACTIVE LOW-PASS FILTER
1
IN
®
12 nF
–); this biases the guard ring to the
software. Resistor R
R
2
C
IN
2
3
Example guard ring layout.
+); this biases the guard ring
Chebyshev Filter.
in FilterLab.
© 2009 Microchip Technology Inc.
V
MCP6L91
(Figure
IN
– V
DD
IN
/2 or ground).
+
4-4) and Transim-
9.31 kΩ
IN
27 nF
3
–) to the input
R
was reduced in
3
C
3
IN
+) to the
V
OUT