MCP6231-ESLASHOT datasheet(9/20 Pages) MICROCHIP

9 / 20 page

 2004 Microchip Technology Inc.

DS21881B-page 9

MCP6231/2

4.0

APPLICATION CIRCUITS

4.1

Matching the Impedance at the

Inputs

To minimize the effect of input bias current in an

amplifier circuit (this is important for very high source-

impedance applications, such as pH meters and

transimpedance amplifiers), the impedance at both

inverting and non-inverting inputs needs to be

matched. This is done by choosing the circuit resistor

values so that the total resistance at each input is the

same. Figure 4-1 shows a summing amplifier circuit.

FIGURE 4-1:

Summing Amplifier Circuit.

To match the inputs, set all voltage sources to ground

and calculate the total resistance at the input nodes. In

this summing amplifier circuit, the resistance at the

inverting input is calculated by setting VIN1, VIN2 and

VOUT to ground. In this case, RG1, RG2 and RF are in

parallel. The total resistance at the inverting input is:

At the non-inverting input, VDD is the only voltage

source. When VDD is set to ground, both Rx and Ry are

in parallel. The total resistance at the non-inverting

input is:

To minimize output offset voltage and increase circuit

accuracy, the resistor values need to meet the

conditions:

4.2

Compensating for the Parasitic

Capacitance

In analog circuit design, the PCB parasitic capacitance

can compromise the circuit behavior; Figure 4-2 shows

a typical scenario. If the input of an amplifier sees

parasitic capacitance of several picofarad (CPARA,

which includes the common mode capacitance of 6 pF,

typical), and large RF and RG, the frequency response

of the circuit will include a zero. This parasitic zero

introduces gain peaking and can cause circuit

instability.

FIGURE 4-2:

Effect of Parasitic

Capacitance at the Input.

One solution is to use smaller resistor values to push

the zero to a higher frequency. Another solution is to

compensate by introducing a pole at the point at which

the zero occurs. This can be done by adding CF in

parallel with the feedback resistor (RF). CF needs to be

selected so that the ratio CPARA:CF is equal to the ratio

of RF:RG.

MCP623X

VOUT

VIN2

–

VIN1

RG2

RG1

VDD

VIN-

---------

------





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

Where:

RVIN– = total resistance at the inverting input

VIN+

------





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

Where:

RVIN+ = total resistance at the inverting

input

VIN+

VIN

VOUT

VDC

–

VAC

CPARA

PARA

-------

•

MCP623X

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