MCP6021 datasheet(20/42 Pages) MICROCHIP

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MCP6021/1R/2/3/4

DS21685D-page 20

4.9

PCB Surface Leakage

In applications where low input bias current is critical,

PCB (printed circuit board) 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

MCP6021/1R/2/3/4 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.

Figure 4-11 shows an example of this type of layout.

FIGURE 4-11:

Example Guard Ring

Layout.

Non-inverting Gain and Unity-Gain Buffer.

Connect the guard ring to the inverting input

pin (VIN–); this biases the guard ring to the

common mode input voltage.

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

input with a wire that does not touch the

PCB surface.

Inverting (Figure 4-11) and Transimpedance

Gain 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’s input (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

4.10

High Speed PCB Layout

Due to their speed capabilities, a little extra care in the

PCB (Printed Circuit Board) layout can make a

significant difference in the performance of these op

amps. Good PC board layout techniques will help you

achieve the performance shown in Section 1.0 “Elec-

trical Characteristics” and Section 2.0 “Typical Per-

formance Curves”, while also helping you minimize

EMC (Electro-Magnetic Compatibility) issues.

Use a solid ground plane and 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. Separating

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 the guard

trace 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.

4.11

Typical Applications

4.11.1

A/D CONVERTER DRIVER AND

ANTI-ALIASING FILTER

Figure 4-12 shows a third-order Butterworth filter that

can be used as an A/D converter driver. It has a band-

width of 20 kHz and a reasonable step response. It will

work well for conversion rates of 80 ksps and greater (it

has 29 dB attenuation at 60 kHz).

FIGURE 4-12:

A/D Converter Driver and

Anti-aliasing Filter with a 20 kHz Cutoff

Frequency.

This filter can easily be adjusted to another bandwidth

by multiplying all capacitors by the same factor.

Alternatively, the resistors can all be scaled by another

common factor to adjust the bandwidth.

Guard Ring

VIN–VIN+

14.7 k

Ω 33.2 kΩ

1.0 nF

100 pF

MCP602X

8.45 k

1.2 nF

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