MCP6486UT-ESLASHOT datasheet(17/46 Pages) MICROCHIP

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DS20006679A-page 17

MCP6486/6R/6U/7/9

4.6

PCB Surface Leakage

In applications where low input bias current is critical,

the 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

. A 5V difference would

cause 5 pA of current to flow, which is greater than the

MCP6486/6R/6U/7/9’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 4-6.

FIGURE 4-6:

Example Guard Ring Layout

for Inverting Gain.

Noninverting Gain and Unity-Gain Buffer:

a) Connect the noninverting 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 noninverting

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.

4.7

Unused Op Amps

An unused op amp in a dual (MCP6487) or quad

(MCP6489) package should be configured as shown in

Figure 4-7. 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; the op amp buffers

that reference voltage. Circuit B uses the minimum

number of components.

FIGURE 4-7:

Unused Op Amps.

4.8

Electromagnetic Interference

Rejection Ratio (EMIRR)

Definitions

The electromagnetic interference (EMI) is the

disturbance that affects an electrical circuit due to

either electromagnetic induction or electromagnetic

radiation emitted from an external source.

The parameter which describes the EMI robustness of

an op amp is the Electromagnetic Interference

Rejection Ratio (EMIRR). It quantitatively describes the

effect that a Radio Frequency (RF) interfering signal

has on op amp performance. Internal passive filters

make EMIRR better, compared with older parts. This

means that, with good PCB layout techniques, the

EMC performance should be better.

EMIRR is defined in Equation 4-1.

EQUATION 4-1:

Guard Ring

VIN–VIN+

VSS

¼ MCP6489 (A)

¼ MCP6489 (B)

REF



VREF

VDD

EMIRR dB



20 log



VRF

V

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





Where:

VRF = Peak Amplitude of

RF Interfering Signal (VPK)

VOS = Input Offset Voltage Shift (V)

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