MCP6281-ESLASHP datasheet(14/36 Pages) MICROCHIP

14 / 36 page

MCP6281/1R/2/3/4/5

DS21811E-page 14

4.6

Supply Bypass

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. 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.

4.7

Unused Op Amps

An unused op amp in a quad package (MCP6284)

should be configured as shown in Figure 4-6. These

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.

4.8

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

MCP6281/1R/2/3/4/5 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.

For

Inverting

Gain

and

Transimpedance

Amplifiers (convert current to voltage, such as

photo detectors):

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).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

Non-inverting Gain and Unity-Gain Buffer:

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

VDD

¼ MCP6284 (A)

¼ MCP6284 (B)

VDD

VREF

REF

------------------

⋅

Guard Ring

VSS

VIN–VIN+

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