MCP6021 datasheet(14/28 Pages) MICROCHIP

14 / 28 page

MCP6021/2/3/4

DS21685B-page 14

 2003 Microchip Technology Inc.

Inverting (Figure 3-7) and Transimpedance

Gain Amplifiers (convert current to voltage, such

as photo detectors).

Connect the guard ring to the non-inverting

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).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

Non-inverting Gain and Unity-Gain Buffer

Connect the guard ring to the inverting input

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

common mode input voltage.

Connect the non-inverting pin (VIN+) to the

input with a wire that does not touch the

PCB surface.

3.8

High-Speed PCB Layout

Due to their speed capabilities, a little extra care in the

PCB (Printed Circuit Board) layout can make a

significant difference in the performance of these op

amps. Good PC board layout techniques will help you

achieve the performance shown in the Electrical

Characteristics and Typical Performance Curves, while

also helping you minimize EMC (Electro-Magnetic

Compatibility) issues.

Use a solid ground plane and connect the bypass local

capacitor(s) to this plane with minimal length traces.

This cuts down inductive and capacitive crosstalk.

Separate digital from analog, low-speed from high-

speed and low power from high power. This will reduce

interference.

Keep sensitive traces short and straight. Separating

them from interfering components and traces. This is

especially important for high-frequency (low rise-time)

signals.

Sometimes it helps to place guard traces next to victim

traces. They should be on both sides of the victim

trace, and as close as possible. Connect the guard

trace to ground plane at both ends, and in the middle

for long traces.

Use coax cables (or low inductance wiring) to route

signal and power to and from the PCB.

3.9

Typical Applications

3.9.1

A/D CONVERTER DRIVER AND

ANTI-ALIASING FILTER

Figure 3-8 shows a third-order Butterworth filter that

can be used as an A/D converter driver. It has a band-

width of 20 kHz and a reasonable step response. It will

work well for conversion rates of 80 ksps and greater (it

has 29 dB attenuation at 60 kHz).

FIGURE 3-8:

A/D converter driver and

anti-aliasing filter with a 20 kHz cutoff frequency.

This filter can easily be adjusted to another bandwidth

by multiplying all capacitors by the same factor.

Alternatively, the resistors can all be scaled by another

common factor to adjust the bandwidth.

3.9.2

OPTICAL DETECTOR AMPLIFIER

Figure 3-9 shows the MCP6021 op amp used as a

transimpedance amplifier in a photo detector circuit.

The photo detector looks like a capacitive current

source, so the 100 k

Ω resistor gains the input signal to

a reasonable level. The 5.6 pF capacitor stabilizes this

circuit and produces a flat frequency response with a

bandwidth of 370 kHz.

FIGURE 3-9:

Transimpedance amplifier

for an optical detector.

14.7 k

Ω 33.2 kΩ

1.0 nF

100 pF

MCP602X

8.45 k

Ω

1.2 nF

Photo

Detector

100 pF

5.6 pF

100 k

Ω

VDD/2

MCP6021

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