MCP6484-ESLASHST datasheet(17/50 Pages) MICROCHIP

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DS20002322C-page 17

MCP6481/2/4

4.5

Unused Op Amps

An unused op amp in a quad package (MCP6484)

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

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, and 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.

4.6

PCB Surface Leakage

In applications where low-input bias current is critical,

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

. A 5V difference would cause 5 pA of current

to flow, which is greater than the MCP6481/2/4 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.

Non-Inverting Gain and Unity-Gain Buffer:

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

input with a wire that does not touch the

PCB surface.

b.Connect the guard ring to the inverting input

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

Common mode input voltage.

Inverting

Gain

and

Transimpedance

Gain

Amplifiers (convert current to voltage, such as

photo detectors):

a.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).

b.Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

VDD

VREF

REF

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



¼ MCP6484 (A)

¼ MCP6484 (B)

Guard Ring

VIN–VIN+

VSS

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