MCP6001 datasheet(10/28 Pages) MICROCHIP

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MCP6001/2/4

DS21733F-page 10

4.6

Application Circuits

4.6.1

UNITY-GAIN BUFFER

The rail-to-rail input and output capability of the

MCP6001/2/4 op amp is ideal for unity-gain buffer

applications. The low quiescent current and wide

bandwidth makes the device suitable for a buffer

configuration in an instrumentation amplifier circuit, as

shown in Figure 4-6.

FIGURE 4-6:

Instrumentation Amplifier

with Unity-Gain Buffer Inputs.

4.6.2

ACTIVE LOW-PASS FILTER

The MCP6001/2/4 op amp’s low input bias current

makes it possible for the designer to use larger resis-

tors 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 fre-

quency 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 possi-

ble to have the op amp bandwidth 10X higher than the

cutoff frequency, thus having a design that is more

sensitive to component tolerances.

Figure 4-7 shows a second-order Butterworth filter with

100 kHz cutoff frequency and a gain of +1 V/V; the op

amp bandwidth is only 10X higher than the cutoff

frequency. The component values were selected using

Microchip’s FilterLab® software.

FIGURE 4-7:

Active Second-Order

Low- Pass Filter.

4.6.3

PEAK DETECTOR

The MCP6001/2/4 op amp has a high input impedance,

rail-to-rail input/output and low input bias current, which

makes this device suitable for peak detector applica-

tions. Figure 4-8 shows a peak detector circuit with

clear and sample switches. The peak-detection cycle

uses a clock (CLK), as shown in Figure 4-8.

At the rising edge of CLK, Sample Switch closes to

begin sampling. The peak voltage stored on C1 is sam-

pled to C2 for a sample time defined by tSAMP. At the

end of the sample time (falling edge of Sample Signal),

Clear Signal goes high and closes the Clear Switch.

When the Clear Switch closes, C1 discharges through

R1 for a time defined by tCLEAR. At the end of the clear

time (falling edge of Clear Signal), op amp A begins to

store the peak value of VIN on C1 for a time defined by

tDETECT.

In order to define tSAMP and tCLEAR, it is necessary to

determine the capacitor charging and discharging

period. The capacitor charging time is limited by the

amplifier source current, while the discharging time (

τ)

is defined using R1 (

τ = R

1C1). tDETECT is the time that

the input signal is sampled on C1 and is dependent on

the input voltage change frequency.

The op amp output current limit, and the size of the

storage capacitors (both C1 and C2), could create slew-

ing limitations as the input voltage (VIN) increases.

Current through a capacitor is dependent on the size of

the capacitor and the rate of voltage change. From this

relationship, the rate of voltage change or the slew rate

can be determined. For example, with an op amp short-

circuit current of ISC = 25 mA and a load capacitor of

C1 = 0.1 µF, then:

EQUATION 4-1:

VIN1

MCP6002

VIN2

MCP6002

VREF

MCP6001

VOUT

–

1/2

OUT

IN2

IN1

–

()

------

•

REF

R1 = 20 kΩ

R2 = 10 kΩ

14.3 k

MCP6002

VOUT

53.6 k

100 pF

VIN

33 pF

–

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

--------

25mA

0.1

μF

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

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

250mV

μs

⁄

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

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