MCP621 datasheet(26/44 Pages) MICROCHIP

26 / 44 page

MCP621/2/5

DS22188A-page 26

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 MCP621/2/5 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 MCP621 op amp, in a photo detector circuit. The

photo detector is a capacitive current source. The op

amp’s input common mode capacitance (9 pF, typical)

and differential mode capacitance (2 pF, typical) act in

parallel with CD. RF provides enough gain to produce

10 mV at VOUT. CF stabilizes the gain and limits

the transimpedance bandwidth to about 0.51 MHz.

RF’s parasitic capacitance (e.g., 0.15 pF for a

0603 SMD) acts in parallel with CF.

FIGURE 4-14:

Transimpedance Amplifier

for an Optical Detector.

GBWP

()

⁄

, G

≤

We need:

F RG

⁄

G CF

⁄

πR

FCF

()

⁄

F GN1 GN2

⁄

()

Given:

GBWP

()

⁄

, G

≤

VIN

VDD/2

VOUT

MCP62X

Photo

Detector

VDD/2

30pF

100 k

3pF

100 nA

VOUT

MCP621

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