MAX44205 datasheet(21/29 Pages) MAXIM

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Applications Information

The fully differential op amp is shown in Figure 3 for refer-

ence. Fully differential op amps provide a lot of advantag-

es, including rejecting common-mode noise coupled to the

input, the output, and from the power supply. The effective

output swing is increased by a factor of two as the outputs

are equal in amplitude and 180° apart in phase.

For example, by applying a fully differential input signal of

1VP-P across INP and INN on Figure 1 there is a 2VP-P

differential output voltage swing. Another advantage of

having fully differential outputs is that even order harmon-

ics will be suppressed at the output.

Input Impedance Mismatch Due to Source

Impedance

The impedance looking into the IN+ and IN- nodes of

Figure 5 depends on how the inputs are driven. For a

fully differential input signal, i.e., VINP = -VINM, the input

impedance looking into inputs is shown in Figure 6.

RINP = RINM = RG

For a single-ended input signal, since the inputs are not

balanced, the input impedance actually increases relative

to the fully differential case. The input impedance looking

into either input is:

INP

INM

]

R )

 

−×

 

 

Apart from the single-ended input and differential input

signal cases, an input signal source from a nonzero

source impedance may cause imbalance between feed-

back resistor networks for single-ended input driving case

as shown in the Figure 7. A terminating resistor RT as

shown in Figure 7 is used to impedance match to the

source such that:

INM

−

Potential Difference Between

Supply Voltage Pins

Figure 4. Explaining Potential Difference Between Supply

Voltage Pins

2.7V TO 13.2V

VS+

GND

VS-

2.7V TO 13.2V

≥ 0V

VS+

VS-

Figure 5. Showing Fully Differential Architecture

VS+

VCLPH

VCLPL

VS-

OUT+

OUT-

VS+

VS-

MAX44205

VOCM

IN-

IN+

VOCM

OUT-

OUT+

DIFFERENTIAL STRUCTURE AT INPUT, OUTPUT REJECTS COUPLED

NOISE AT THE INPUT, OUTPUT AND AT THE POWER SUPPLY

Figure 6. Fully Differential Amplifier

VS+

VCLPH

VCLPL

VS-

OUT+

OUT-

+5V

-5V

MAX44205

VOCM

INM

INP

VOCM

OUT-

OUT+

IN-

IN+

MAX44205

180MHz, Low-Noise, Low-Distortion,

Fully Differential Op Amp/SAR ADC Driver

www.maximintegrated.com

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