MCP602-ESLASHP datasheet(14/34 Pages) MICROCHIP

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MCP601/1R/2/3/4

DS21314G-page 14

4.6

Unused Op Amps

An unused op amp in a quad package (MCP604)

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.

4.7

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. This is greater

than the MCP601/1R/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.

An example of this type of layout is shown in

Figure 4-7.

FIGURE 4-7:

Example Guard Ring layout.

Connect the guard ring to the inverting input pin

(VIN–) for non-inverting gain amplifiers, includ-

ing unity-gain buffers. This biases the guard ring

to the common mode input voltage.

Connect the guard ring to the non-inverting input

pin (VIN+) for inverting gain amplifiers and

transimpedance amplifiers (converts current to

voltage, such as photo detectors). This biases

the guard ring to the same reference voltage as

the op amp (e.g., VDD/2 or ground).

4.8

Typical Applications

4.8.1

ANALOG FILTERS

Figure 4-8 and Figure 4-9 show low-pass, second-

order, Butterworth filters with a cutoff frequency of

10 Hz. The filter in Figure 4-8 has a non-inverting gain

of +1 V/V, and the filter in Figure 4-9 has an inverting

gain of -1 V/V.

FIGURE 4-8:

Second-Order, Low-Pass

Sallen-Key Filter.

FIGURE 4-9:

Second-Order, Low-Pass

Multiple-Feedback Filter.

The MCP601/1R/2/3/4 family of op amps have low

input bias current, which allows the designer to select

larger resistor values and smaller capacitor values for

these filters. This helps produce a compact PCB layout.

These filters, and others, can be designed using

Microchip’s Design Aids; see

Section 5.2 “FilterLab®

Software” and Section 5.3 “Mindi™ Simulatior

Tool”.

VDD

¼ MCP604 (A)

¼ MCP604 (B)

VDD

VREF

REF

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

⋅

Guard Ring

VIN– VIN+

VOUT

VIN

47 nF

382 k

Ω 641 kΩ

22 nF

G = +1 V/V

fP = 10 Hz

MCP60X

–

VOUT

VIN

VDD/2

G = -1 V/V

fP = 10 Hz

618 k

Ω 1.00 MΩ

8.2 nF

47 nF

MCP60X

–

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