Data Sheet

MAX74810

analog.com

Rev. 0

18 of 23

THEORY OF OPERATION

The MAX74810 is a dual, ultralow noise, high voltage, zero drift, rail-to-rail output operational amplifier. It features a

chopping technique that offers an ultralow input offset voltage of 7µV and an input offset voltage drift of 30nV/°C

maximum. Offset voltage errors due to common-mode voltage swings and power supply variations are also

corrected by the chopping technique, resulting in a superb typical CMRR figure of 144dB and a PSRR figure of 160dB

at a 50V supply voltage.

The MAX74810 has a wide operating voltage range from ±2.25V (or 4.5V) to ±25V (or 50V). It is a single supply amplifier,

where its input voltage range includes the lower supply rail. It also offers low voltage noise density of 5.8nV/√Hz (at

f = 1kHz, AV= 100) and reduced 1/f noise component. These features are ideal for the amplification of low level signals

in high precision applications. A few examples of such applications are weigh scales, high precision current sensing,

high voltage buffers, and signal conditioning for temperature sensors, among others.

Figure 51 shows the MAX74810 architecture block diagram. The architecture consists of an input EMI filter and clamp

circuitry, three gain stages (Gm1, Gm2, and Gm3), input and output chopping networks (CHOPIN and CHOPOUT), a clock

generator, offset and ripple correction loop circuitry, frequency compensation capacitors (C1, C2, and C3), and

thermal shutdown circuitry.

An EMI filter and clamp circuit are implemented at the input front-end to protect the internal circuitry against

electrostatic discharge (ESD) stresses and high voltage transients.

CHOPIN and CHOPOUT are controlled by a clock generator and operate at 4.8MHz. The input baseband signal is initially

modulated by CHOPIN. Next, CHOPOUT demodulates the input signal and modulates the millivolt level input offset

voltage and 1/f noise of the input transconductance amplifier, Gm1, to the chopping frequency at 4.8MHz. The

chopping networks remove the low frequency errors, but, in return, the networks introduce chopping artifacts at the

chopping frequency. Therefore, an offset and ripple correction loop, operating at 800kHz, is used. This frequency is

the switching frequency of the amplifier. This circuitry reduces chopping artifacts, allowing the MAX74810 to have a

high chopping frequency with minimal artifacts.

The thermal shutdown circuit shuts down the circuit when the die is overheated (see the Thermal Shutdown section

for more information).

Figure 51. MAX74810 Architecture Block Diagram

On-Chip Input EMI Filter and Clamp Circuit

Figure 52 shows the input EMI filter and clamp circuit. The MAX74810 has internal ESD protection diodes (D1, D2, D3,

and D4) connected between the inputs and each supply rail. These diodes protect the input transistors in the event

of electrostatic discharge and are reverse-biased during normal operation. This protection scheme allows voltages

as high as approximately 300mV beyond the rails to be applied at the input of either terminal without causing

permanent damage. See Table 3 in the Absolute Maximum Ratings section for more information.

Gm1

Gm2

Gm3

EMI

FILTER

AND

CLAMP

THERMAL

SHUTDOWN

OFFSET

AND RIPPLE

CORRECTION

LOOP

+IN x

–IN x

CHOPIN

CHOPOUT

C3

OUT x

C2

C1

4.8MHz CLOCKS

800kHz CLOCKS

CLOCK

GENERATOR