MCP6484-ESLASHST datasheet(18/50 Pages) MICROCHIP

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MCP6481/2/4

DS20002322C-page 18

 2012-2013 Microchip Technology Inc.

4.7

Application Circuits

4.7.1

PHOTO DETECTION

The MCP6481/2/4 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, 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). However, the reverse bias voltage

also increased diode leakage current and caused

linearity errors.

FIGURE 4-9:

Photoconductive Mode

Detector.

4.7.2

ACTIVE LOW PASS FILTER

The MCP6481/2/4 op amps’ low-input bias current

makes it possible for the designer to use larger

resistors and smaller capacitors for active low-pass

filter

applications.

However,

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.

Usually, the op amp bandwidth is 100x the filter cutoff

frequency (or higher) for good performance. It is

possible to have the op amp bandwidth 10x higher than

the cutoff frequency, thus having a design that is more

sensitive to component tolerances.

Figure 4-10 and Figure 4-11 show low-pass, second-

order, Butterworth filters with a cutoff frequency of

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

of +1 V/V, and the filter in Figure 4-11 has an inverting

gain of -1 V/V.

FIGURE 4-10:

Second-Order, Low-Pass

Butterworth Filter with Sallen-Key Topology.

FIGURE 4-11:

Second-Order, Low-Pass

Butterworth Filter with Multiple-Feedback

Topology.

Light

VOUT

VDD

ID1

VOUT = ID1*R2

–

MCP648X

Light

VOUT

VDD

ID1

VOUT = ID1*R2

VBIAS

VBIAS < 0V

–

MCP648X

VOUT

VIN

47 nF

768 k

 1.27 M

22 nF

fP = 10 Hz, G = +1 V/V

–

MCP648X

VOUT

VIN

VDD/2

fP = 10 Hz, G = -1 V/V

618 k



618 k

 1.00 M

8.2 nF

47 nF

–

MCP648X

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