MCP6V27 datasheet(21/46 Pages) MICROCHIP

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DS25007A-page 21

MCP6V27

4.2

Other Functional Blocks

4.2.1

RAIL-TO-RAIL INPUTS

The input stage of the MCP6V27 op amp uses two

differential CMOS input stages in parallel. One

operates at low common mode input voltage (VCM,

which is approximately equal to VIN+ and VIN– in

normal operation) and the other at high VCM. With this

topology, the input operates with VCM up to VDD +0.2V,

and down to VSS – 0.15V, at +25°C (see Figure 2-18).

The input offset voltage (VOS) is measured at

VCM =VSS – 0.15V and VDD + 0.2V to ensure proper

operation.

The transition between the input stages occurs when

VCM ≈ VDD –1.2V (see Figure 2-7 and Figure 2-8). For

the best distortion and gain linearity, with non-inverting

gains, avoid this region of operation.

4.2.1.1

Phase Reversal

The input devices are designed to not exhibit phase

inversion when the input pins exceed the supply

voltages.

Figure 2-43 shows

input voltage

exceeding both supplies with no phase inversion.

4.2.1.2

Input Voltage Limits

In order to prevent damage and/or improper operation

of these amplifiers, the circuit must limit the voltages at

the input pins (see Section 1.1, Absolute Maximum

Ratings †). This requirement is independent of the cur-

rent limits discussed later on.

The ESD protection on the inputs can be depicted as

shown in Figure 4-4. This structure was chosen to

protect the input transistors against many (but not all)

over-voltage conditions, and to minimize input bias cur-

rent (IB).

FIGURE 4-4:

Simplified Analog Input ESD

Structures.

The input ESD diodes clamp the inputs when they try

to go more than one diode drop below VSS. They also

clamp any voltages that are well above VDD; their

breakdown voltage is high enough to allow normal

operation, but not low enough to protect against slow

over-voltage (beyond VDD) events. Very fast ESD

events (that meet the spec) are limited so that damage

does not occur.

In some applications, it may be necessary to prevent

excessive voltages from reaching the op amp inputs;

Figure 4-5 shows one approach to protecting these

inputs. D1 and D2 may be small signal silicon diodes,

Schottky diodes for lower clamping voltages or diode-

connected FETs for low leakage.

FIGURE 4-5:

Protecting the Analog Inputs

Against High Voltages.

4.2.1.3

Input Current Limits

In order to prevent damage and/or improper operation

of these amplifiers, the circuit must limit the currents

into the input pins (see Section 1.1, Absolute Maxi-

mum Ratings †). This requirement is independent of

the voltage limits previously discussed.

Figure 4-6 shows one approach to protecting these

inputs. The resistors R1 and R2 limit the possible cur-

rent in or out of the input pins (and into D1 and D2). The

diode currents will dump onto VDD.

FIGURE 4-6:

Protecting the Analog Inputs

Against High Currents.

Bond

Pad

Bond

Pad

Bond

Pad

VDD

VIN+

VSS

Input

Stage

Bond

Pad

VIN–

VDD

VOUT

½MCP6V27

VDD

min(R1,R2)>

VSS –min(V1,V2)

2mA

VOUT

min(R1,R2)>

max(V1,V2)– VDD

2mA

½ MCP6V27

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