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MCP651T Datasheet(PDF) 26 Page - Microchip Technology

Part No. MCP651T
Description  50 MHz, 6 mA Op Amps with mCal
Download  44 Pages
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

MCP651T Datasheet(HTML) 26 Page - Microchip Technology

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MCP651/2/5
DS22146A-page 26
© 2009 Microchip Technology Inc.
It is also possible to add a capacitor (CF) in parallel with
RF to compensate for the de-stabilizing effect of CG.
This makes it possible to use larger values of RF. The
conditions
for
stability
are
summarized
in
Equation 4-10.
EQUATION 4-10:
4.5
Power Supply
With this family of operational amplifiers, the power
supply pin (VDD 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. Surface mount,
multilayer ceramic capacitors, or their equivalent,
should be used.
These op amps require a bulk capacitor (i.e., 2.2 µF or
larger) within 50 mm to provide large, slow currents.
Tantalum capacitors, or their equivalent, may be a good
choice. This bulk capacitor can be shared with other
nearby analog parts as long as crosstalk through the
supplies does not prove to be a problem.
4.6
High Speed PCB Layout
These op amps are fast enough that a little extra care
in the PCB (Printed Circuit Board) layout can make a
significant difference in performance. Good PC board
layout
techniques
will
help
you
achieve
the
performance shown in the specifications and Typical
Performance Curves; it will also help you minimize
EMC (Electro-Magnetic Compatibility) issues.
Use a solid ground plane. Connect the bypass local
capacitor(s) to this plane with minimal length traces.
This cuts down inductive and capacitive crosstalk.
Separate digital from analog, low speed from high
speed, and low power from high power. This will reduce
interference.
Keep sensitive traces short and straight. Separate
them from interfering components and traces. This is
especially important for high frequency (low rise time)
signals.
Sometimes, it helps to place guard traces next to victim
traces. They should be on both sides of the victim
trace, and as close as possible. Connect guard traces
to ground plane at both ends, and in the middle for long
traces.
Use coax cables, or low inductance wiring, to route
signal and power to and from the PCB. Mutual and self
inductance of power wires is often a cause of crosstalk
and unusual behavior.
4.7
Typical Applications
4.7.1
POWER DRIVER WITH HIGH GAIN
Figure 4-13 shows a power driver with high gain
(1 + R2/R1). The MCP651/2/5 op amp’s short circuit
current makes it possible to drive significant loads. The
calibrated input offset voltage supports accurate
response at high gains. R3 should be small, and equal
to R1||R2, in order to minimize the bias current induced
offset.
FIGURE 4-13:
Power Driver.
4.7.2
OPTICAL DETECTOR AMPLIFIER
Figure 4-14 shows a transimpedance amplifier, using
the MCP651 op amp, in a photo detector circuit. The
photo detector is a capacitive current source. The op
amp’s input common mode capacitance (5 pF, typical)
acts in parallel with CD. RF provides enough gain to
produce 10 mV at VOUT. CF stabilizes the gain and lim-
its the transimpedance bandwidth to about 1.1 MHz.
RF’s parasitic capacitance (e.g., 0.2 pF for a 0805
SMD) acts in parallel with CF.
FIGURE 4-14:
Transimpedance Amplifier
for an Optical Detector.
fF fGBWP 2GN2
()
, GN1 GN2
<
We need:
GN1
1RF RG
+
=
GN2
1CG CF
+
=
fF
12
πR
FCF
()
=
fZ
fF GN1 GN2
()
=
Given:
fF fGBWP 4GN1
()
, GN1 GN2
>
R1
R2
MCP65X
VIN
VDD/2
VOUT
R3
RL
Photo
Detector
CD
CF
RF
VDD/2
MCP651
30pF
100 k
Ω
1.5 pF
ID
100 nA
VOUT


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