ADA4051-2

Rev. 0 | Page 15 of 20

THEORY OF OPERATION

The ADA4051-2 micropower chopper operational amplifier

features a novel patent-pending technique that suppresses

offset-related ripple in a chopper amplifier. It nulls out the

amplifier’s initial offset in the dc domain that otherwise

becomes a ripple at the overall output, instead of filtering

the ripple in the ac domain.

Auto-zeroing and chopping are widely used for a high precision

CMOS amplifier to achieve low offset, low offset drift, and no

1/f noise. Auto-zeroing and chopping both have pros and cons.

Auto-zeroing gets more in-band noise due to aliasing intro-

duced by sampling. Chopping has offset-related ripple, because

it modulates the initial offset associated with the amplifier up to

its chopping frequency.

To accomplish the best noise vs. power trade-off, the chopping

technique is the right approach to design a low offset amplifier.

It is preferable to suppress the offset-related ripple in a chopper

amplifier in the amplifier itself, which otherwise must be

eliminated by an extra off-chip post filter.

Figure 56 shows the block diagram design of the ADA4051-2

chopper amplifier, employing a local feedback loop called auto

correction feedback (ACFB). The main signal path contains

an input chopping switch network (CHOP1), a first transcon-

ductance amplifier (Gm1), an output chopping switch network

(CHOP2), a second transconductance amplifier (Gm2), and a

third transconductance amplifier (Gm3). CHOP1 and CHOP2

operate at 40 kHz of chopping frequency to modulate the initial

offset and 1/f noise from Gm1 up to the chopping frequency. A

fourth transconductance amplifier (Gm4) in the ACFB senses

the modulated ripple at the output of CHOP2, caused by the

initial offset voltage of Gm1. Then, the ripple is demodulated

down to a dc domain through a third chopping switch network

(CHOP3), operating with the same chopping clock as CHOP1

and CHOP2. Finally, a null transconductance amplifier (Gm5)

tries to null out any dc component at the output of Gm1, which

would otherwise appear in the overall output as ripple.

A switched capacitor notch filter (NF) functions to selectively

suppress the undesired offset-related ripple, without disturbing

the desired input signal from the overall input. The desired

input dc signal appears as a dc signal at CHOP2’s output. Then,

it is modulated up to the chopping frequency by CHOP3 and

filtered out by the NF. Therefore, it does not create any feed-

back and does not disturb the desired input signal. The NF is

synchronized with the chopping clock to perfectly filter out the

modulated component. In the same manner, the offset of Gm5

is filtered out by the combination of CHOP3 and the NF,

enabling accurate ripple sensing at the output of CHOP2.

In parallel with the high dc gain path, a feedforward transcon-

ductance amplifier (Gm6) is added to bypass the phase shift

introduced by the ACFB at the chopping frequency. The Gm6

is designed to have the same transconductance as the Gm1 to

avoid the pole-zero doublets. Such design avoids any instability

introduced by the ACFB in the overall feedback loop.

Gm6 (= Gm1)

C2

C1

Gm3

C3

NF

Gm1

Gm2

CHOP1

CHOP2

Gm4

Gm5

CHOP3

OUT

+IN

–IN

Figure 56. ADA4051-2 Chopper Amplifier Block Diagram