MCP6286 datasheet(17/28 Pages) MICROCHIP

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DS22196A-page 17

MCP6286

4.6

Application Circuits

4.6.1

ACTIVE LOW-PASS FILTER

The MCP6286 op amp’s low input bias current makes

it possible for the designer to use larger resistors and

smaller

capacitors

for

active

low-pass

filter

applications. However, as the resistance increases, the

noise generated also increases. Parasitic capacitances

and the large value resistors could also modify the

frequency response. These trade-offs need to be

considered when selecting circuit elements.

Figure 4-6

and

Figure 4-7

show

low-pass,

second-order,

Butterworth

filters

with

cut-off

frequency of 10 Hz. The filter in Figure 4-6 has a

non-inverting gain of +1 V/V, and the filter in Figure 4-7

has an inverting gain of -1 V/V.

FIGURE 4-6:

Second-Order, Low-Pass

Butterworth Filter with Sallen-Key Topology.

FIGURE 4-7:

Second-Order, Low-Pass

Butterwork Filter with Multiple-Feedback

Topology.

4.6.2

PHOTO DETECTION

The MCP6286 op amps can be used to easily convert

the signal from a sensor that produces an output

current (such as a photo diode) into a voltage (a

transimpedance amplifier). This is implemented with a

single resistor (R2) in the feedback loop of the

amplifiers shown in Figure 4-8 and Figure 4-9. The

optional capacitor (C2) sometimes provides stability for

these circuits.

A photodiode configured in the Photovoltaic mode has

zero voltage potential placed across it (Figure 4-8). In

this mode, the light sensitivity and linearity is

maximized, making it best suited for precision

applications. The key amplifier specifications for this

application are: low input bias current, low noise,

common mode input voltage range (including ground),

and rail-to-rail output.

FIGURE 4-8:

Photovoltaic Mode Detector.

In contrast, a photodiode that is configured in the

Photoconductive mode has a reverse bias voltage

across the photo-sensing element (Figure 4-9). This

decreases the diode capacitance, which facilitates

high-speed

operation

(e.g.,

high-speed

digital

communications). The design trade-off is increased

diode leakage current and linearity errors. The op amp

needs to have a wide Gain Bandwidth Product

(GBWP).

FIGURE 4-9:

Photoconductive Mode

Detector.

VOUT

VIN

47 nF

382 k

Ω 641 kΩ

22 nF

G = +1 V/V

fP = 10 Hz

–

MCP6286

VOUT

VIN

VDD/2

G = -1 V/V

fP = 10 Hz

618 k

Ω 1.00 MΩ

8.2 nF

47 nF

–

MCP6286

Light

VOUT

VDD

ID1

VOUT = ID1*R2

–

MCP6286

Light

VOUT

VDD

ID1

VOUT = ID1*R2

VBIAS

VBIAS < 0V

–

MCP6286

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