MCP6L91RTESLASHMS datasheet(14/40 Pages) MICROCHIP

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MCP6L91/1R/2/4

DS20002141C-page 14

 2009-2019 Microchip Technology Inc.

4.4

Supply Bypass

With this family of operational amplifiers, 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-frequency 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 nearby analog parts.

4.5

Unused Op Amps

An unused op amp in a quad package (e.g., MCP6L94)

should be configured as shown in Figure 4-3. 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

and operates as a comparator, but it may draw more

current.

FIGURE 4-3:

Unused Op Amps.

4.6

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

. A 5V difference would

cause 5 pA of current to flow; this is greater than this

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 4-4 is an example of this type of layout.

FIGURE 4-4:

Example Guard Ring Layout.

Inverting Amplifiers (Figure 4-4) and Trans-

Impedance 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’s input (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.

Noninverting Gain and Unity Gain Buffer.

a) Connect the guard ring to the inverting input

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

Common-mode input voltage.

b) Connect the noninverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

4.7

Application Circuit

4.7.1

ACTIVE LOW-PASS FILTER

The MCP6L91/1R/2/4 op amp’s low input noise and

good output current drive make it possible to design

low noise filters. Reducing the resistors’ values also

reduces the noise and increases the frequency at

which parasitic capacitances affect the response.

These trade-offs need to be considered when selecting

circuit elements.

Figure 4-5 shows a third-order Chebyshev filter with a

1 kHz bandwidth, 0.2 dB ripple and a gain of +1 V/V.

The

component

values

were

selected

using

Microchip’s FilterLab® software. Resistor R3 was

reduced in value by increasing C3 in FilterLab.

FIGURE 4-5:

Chebyshev Filter.

VDD

¼ MCP6L94 (A)

¼ MCP6L94 (B)

VDD

VREF

VDD

R1 R2

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



–

Guard Ring

VIN-VIN+

VIN

VOUT

3.01 k

 6.81 k

MCP6L91

120 nF

9.31 k



27 nF

12 nF

–

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