MCP616-ISLASHMS datasheet(18/38 Pages) MICROCHIP

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MCP616/7/8/9

DS21613C-page 18

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

MCP616/7/8/9

family’s

bias

current

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 is shown below in Figure 4-8.

FIGURE 4-8:

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 (con-

vert current to voltage, such as photo detectors)

amplifiers:

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.

4.9

Application Circuits

4.9.1

HIGH GAIN PRE-AMPLIFIER

The MCP616/7/8/9 op amps are well suited to

amplifying small signals produced by low-impedance

sources/sensors. The low offset voltage, low offset

current and low noise fit well in this role. Figure 4-9

shows a typical pre-amplifier connected to a low-

impedance source (VS and RS).

FIGURE 4-9:

High Gain Pre-amplifier.

For the best noise and offset performance, the source

resistance RS needs to be less than 15 kΩ. The DC

resistances at the inputs are equal to minimize the

offset voltage caused by the input bias currents

(Section 4.2 “DC Offsets”). In this circuit, the DC gain

is 10 V/V, which will give a typical bandwidth of 19 kHz.

4.9.2

TWO OP AMP INSTRUMENTATION

AMPLIFIER

The two-op amp instrumentation amplifier shown in

Figure 4-10 serves the function of taking the difference

of two input voltages, level-shifting it and gaining it to

the output. This configuration is best suited for higher

gains (i.e., gain > 3 V/V). The reference voltage (VREF)

is typically at mid-supply (VDD/2) in a single-supply

environment.

FIGURE 4-10:

Two-Op Amp

Instrumentation Amplifier.

The key specifications that make the MCP616/7/8/9

family appropriate for this application circuit are low

input bias current, low offset voltage and high common-

mode rejection.

Guard Ring

VIN–VIN+ VSS

MCP616

11.0 k

100 k

10 k

VDD/2

VOUT

OUT

–

() 1

------

----------

⎝⎠

⎜⎟

⎛⎞

REF

MCP617

VOUT

VREF

MCP617

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