MCP6021/1R/2/3/4
To use the internal mid-supply reference for an
inverting gain circuit, connect the V
REF
pin to the
non-inverting input, as shown in Figure 4-8. The
capacitor C
B
helps reduce power supply noise on the
output.
R
G
V
IN
R
F
V
OUT
4.9
PCB Surface Leakage
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
12
Ω.
A 5V difference would
cause 5 pA of current to flow, which is greater than the
MCP6021/1R/2/3/4 family’s bias current at +25°C
(1 pA, typ).
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 4-10 shows an example of this type of layout.
Guard Ring
V
IN
– V
IN
+
V
REF
C
B
FIGURE 4-8:
Inverting gain circuit using
V
REF
(MCP6021 and MCP6023 only).
If you don’t need the mid-supply reference, leave the
V
REF
pin open.
4.7
Supply Bypass
With this family of operational amplifiers, the power
supply pin (V
DD
for single supply) should have a local
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 nearby analog parts.
FIGURE 4-10:
Layout.
1.
Example Guard Ring
4.8
Unused Op Amps
2.
An unused op amp in a quad package (MCP6024)
should be configured as shown in Figure 4-9. These
circuits prevent the output from toggling and causing
crosstalk. Circuit A can use any reference voltage
between the supplies, provides a buffered DC voltage,
and minimizes the supply current draw of the unused
op amp. Circuit B uses the minimum number of compo-
nents and operates as a comparator; it may draw more
current.
¼ MCP6144 (A)
V
DD
R
R
V
DD
¼ MCP6144 (B)
V
DD
Non-inverting Gain and Unity-Gain Buffer.
a) Connect the guard ring to the inverting input
pin (V
IN
–); this biases the guard ring to the
common mode input voltage.
b) Connect the non-inverting pin (V
IN
+) to the
input with a wire that does not touch the
PCB surface.
Inverting (Figure 4-10) and Transimpedance
Gain Amplifiers (convert current to voltage, such
as photo detectors).
a) Connect the guard ring to the non-inverting
input pin (V
IN
+). This biases the guard ring
to the same reference voltage as the op
amp’s input (e.g., V
DD
/2 or ground).
b) Connect the inverting pin (V
IN
–) to the input
with a wire that does not touch the PCB
surface.
4.10
High Speed PCB Layout
Due to their speed capabilities, a little extra care in the
PCB (Printed Circuit Board) layout can make a
significant difference in the performance of these op
amps. Good PC board layout techniques will help you
achieve the performance shown in
Section 1.0 “Elec-
trical Characteristics”
and
Section 2.0 “Typical Per-
formance Curves”,
while also helping you minimize
EMC (Electro-Magnetic Compatibility) issues.
Use a solid ground plane and connect the bypass local
capacitor(s) to this plane with minimal length traces.
This cuts down inductive and capacitive crosstalk.
FIGURE 4-9:
Unused Op Amps.
©
2006 Microchip Technology Inc.
DS21685C-page 15
MICROCHIP [ MICROCHIP TECHNOLOGY ]
