MCP601 datasheet(12/28 Pages) MICROCHIP

12 / 28 page

MCP601/2/3/4

DS21314F-page 12

 2004 Microchip Technology Inc.

FIGURE 3-3:

Recommended RISO values

for capacitive loads.

Once you’ve selected RISO for your circuit, double-

check the resulting frequency response peaking and

step response overshoot in your circuit. Evaluation on

the bench and simulations with the MCP601/2/3/4

SPICE macro model are very helpful. Modify RISO’s

value until the response is reasonable.

3.5

Supply Bypass

With this family of op amps, the power supply pin (VDD

for single-supply) should have a local bypass capacitor

(i.e., 0.01 µF to 0.1 µF) within 2 mm for good high-fre-

quency performance. It also needs a bulk capacitor

(i.e., 1 µF or larger) within 100 mm to provide large,

slow currents. This bulk capacitor can be shared with

other parts.

3.6

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 MCP601/2/3/4 family’s bias current at +25°C

(1 pA, typ.).

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 3-4.

FIGURE 3-4:

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).

3.7

Typical Applications

3.7.1

ANALOG FILTERS

Figure 3-5 and Figure 3-6 show low-pass, second-

order, Butterworth filters with a cutoff frequency of

10 Hz. The filter in Figure 3-5 has a non-inverting gain

of +1 V/V, and the filter in Figure 3-6 has an inverting

gain of -1 V/V.

FIGURE 3-5:

Second-Order, Low-Pass

Sallen-Key Filter.

FIGURE 3-6:

Second-Order, Low-Pass

Multiple-Feedback Filter.

The MCP601/2/3/4 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 FilterLab® software.

100

1,000

100

1,000

10,000

Normalized Load Capacitance; CL / GN (F)

10p

100p

10n

100

GN = +1

GN +2

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

618 k

Ω 1.00 MΩ

8.2 nF

47 nF

MCP60X

–

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