MCP6V51-ESLASHMS datasheet(21/50 Pages) MICROCHIP

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DS20006136B-page 21

MCP6V51/2/4

4.0

APPLICATIONS

The MCP6V51/2/4 devices are designed for precision

applications with requirements for small packages and

low power. Their wide supply voltage range and low

quiescent current make the MCP6V51/2/4 devices

ideal for industrial applications.

4.1

Overview of Zero-Drift Operation

Figure 4-1 shows a simplified diagram of the

MCP6V5X zero-drift op amp. This diagram will be used

to explain how slow voltage errors are reduced in this

architecture (much better VOS, VOS/TA (TC1),

CMRR, PSRR, AOL and 1/f noise).

FIGURE 4-1:

Simplified Zero-Drift

Op Amp Functional Diagram.

4.1.1

BUILDING BLOCKS

The main amplifier is designed for high gain and

bandwidth, with a differential topology. Its main input

pair (+ and – pins at the top left) is used for the higher

frequency portion of the input signal. Its auxiliary input

pair (+ and – pins at the bottom left) is used for the

low-frequency portion of the input signal and corrects

the op amp’s input offset voltage. Both inputs are

added together internally.

The auxiliary amplifier, chopper input switches and

chopper output switches provide a high DC gain to the

input signal. DC errors are modulated to higher

frequencies, while white noise is modulated to low

frequency.

The low-pass filter reduces high-frequency content,

including harmonics of the chopping clock.

The output buffer drives external loads at the VOUT pin

(VREF is an internal reference voltage).

The oscillator runs at fOSC1 = 200 kHz. Its output is

divided by two, to produce the chopping clock rate of

fCHOP = 100 kHz.

The internal Power-on Reset (POR) starts the part in a

known good state, protecting against power supply

brown-outs.

The digital control block controls switching and POR

events.

4.1.2

CHOPPING ACTION

Figure 4-2 shows the amplifier connections for the first

phase of the chopping clock and Figure 4-3 shows the

connections for the second phase. Its slow voltage

errors alternate in polarity, making the average error

small.

FIGURE 4-2:

First Chopping Clock Phase;

Equivalent Amplifier Diagram.

FIGURE 4-3:

Second Chopping Clock

Phase; Equivalent Amplifier Diagram.

VIN+

VIN-

Main

Buffer

VOUT

VREF

Amp.

Output

Aux.

Amp.

Chopper

Input

Switches

Chopper

Output

Switches

Oscillator

Low-Pass

Filter

POR

Digital Control

–

VIN+

VIN-

Main

Amp.

Aux.

Amp.

Low-Pass

Filter

–

VIN+

VIN-

Main

Amp.

Aux.

Amp.

Low-Pass

Filter

–

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