MCP6V06T datasheet(22/44 Pages) MICROCHIP

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MCP6V06/7/8

DS22093B-page 22

4.1.2

AUTO-ZEROING ACTION

Figure 4-2 shows the connections between amplifiers

during the Normal Mode of operation (

φ1). The hold

capacitor (CH) corrects the Null Amplifier’s input offset.

Since the Null Amplifier has very high gain, it

dominates the signal seen by the Main Amplifier. This

greatly reduces the impact of the Main Amplifier’s input

offset voltage on overall performance. Essentially, the

Null Amplifier and Main Amplifier behave as a regular

op amp with very high gain (AOL) and very low offset

voltage (VOS).

FIGURE 4-2:

Normal Mode of Operation (

1); Equivalent Amplifier Diagram.

Figure 4-3 shows the connections between amplifiers

during the Auto-zeroing Mode of operation (

φ2). The

signal goes directly through the Main Amplifier, and the

flywheel capacitor (CFW) maintains a constant correc-

tion on the Main Amplifier’s offset.

The Null Amplifier uses its own high open loop gain to

drive the voltage across CH to the point where its input

offset voltage is almost zero. Because the principal

input is connected to VIN+, the auto-zeroing action

corrects the offset at the current common mode input

voltage (VCM) and supply voltage (VDD). This makes

the DC CMRR and PSRR very high also.

Since these corrections happen every 50 µs, or so, we

also minimize slow errors, including offset drift with

temperature (

ΔVOS/ΔTA), 1/f noise, and input offset

aging.

FIGURE 4-3:

Auto-zeroing Mode of Operation (

2); Equivalent Diagram.

4.1.3

INTERMODULATION DISTORTION

(IMD)

The MCP6V06/7/8 op amps will show intermodulation

distortion (IMD), products when an AC signal is

present.

The signal and clock can be decomposed into sine

wave tones (Fourier series components). These tones

interact with the auto-zeroing circuitry’s non-linear

response to produce IMD tones at sum and difference

frequencies. IMD distortion tones are generated about

all of the square wave clock’s harmonics. See

Figure 2-37 and Figure 2-38.

VIN+

VIN–

Main

Output

VOUT

VREF

Amp.

Buffer

Null

Amp.

CFW

VIN+

VIN–

Main

Output

VOUT

VREF

Amp.

Buffer

Null

Amp.

CFW

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