MCP6231 datasheet(9/28 Pages) MICROCHIP

9 / 28 page

DS21881C-page 9

MCP6231/2/4

FIGURE 4-4:

Recommended RISO Values

for Capacitive Loads.

After selecting RISO for your circuit, double-check the

resulting

frequency

response

peaking

and

step

response overshoot. Evaluation on the bench and

simulations with the MCP6231/2/4 SPICE macro

model are very helpful. Modify RISO’s value until the

response is reasonable.

4.4

Supply Bypass

With this op amp, 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 can use 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 (MCP6234)

should be configured as shown in Figure 4-5. Both

circuits prevent the output from toggling and causing

crosstalk. Circuit A can use any reference voltage

between the supplies, provides a buffered DC voltage

and minimizes the supply current draw of the unused

amp.

Circuit

minimizes

the

number

components, but may draw a little more supply current

for the unused op amp.

FIGURE 4-5:

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

Ω. A 5V difference would

cause 5 pA of current to flow, which is greater than the

MCP6231/2/4 family’s bias current at 25°C (1 pA, typ.).

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.

An example of this type of layout is shown in

Figure 4-6.

FIGURE 4-6:

Example Guard Ring Layout

for Inverting Gain.

Non-inverting Gain and Unity-Gain Buffer:

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

input with a wire that does not touch the

PCB surface.

Connect the guard ring to the inverting input

pin (VIN–). This biases the guard ring to the

common mode input voltage.

Inverting Gain and Transimpedance 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 (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

with a wire that does not touch the PCB

surface.

100

1,000

10,000

100

1000

10000

Normalized Load Capacitance; CL/GN (F)

GN = 1 V/V

GN = 2 V/V

GN 4 V/V

10p

100p

10n

10k

100

VDD

¼ MCP6234 (A)

¼ MCP6234 (B)

Guard Ring

VIN–VIN+

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