MAX44206 datasheet(21/29 Pages) MAXIM

21 / 29 page

A terminating resistor is inserted to correct for impedance

mismatch between the source and input. The gain resis-

tor mismatch across feedback networks is created due to

the parallel combination of RT and RS. So, to balance out

the gain resistor mismatch on the other input, insert RB

such that:

−

Effects of Input Resistor Mismatch

If there is a mismatch between the feedback resistor (RF)

pair and gain resistor (RG) pair, there will be a small delta

in the feedback factor across the input pins. This delta in

the feedback factor is a source of common-mode error.

To apply an AC CMRR test without a differential input

signal, the common-mode rejection is proportional to

the resistor mismatch. Using 0.1% or better resistors will

mitigate most of the problems and will yield good CMRR

performance.

Noise Calculations

The MAX44206 offers input voltage and current noise

densities of 3.1nV/√Hz and 1.5pA/√Hz, respectively. From

Figure 7, the total output noise is a combination of noise

generated by the amplifier and the feedback and gain

resistors. The total output noise generated by both the

amplifier and the feedback components is given by the

equation:

nRG

nRF

)]

2 (i

R )

2 (e

)

2 (e

)

× +

+×

+ ×

ent is total output noise of the circuit shown in Figure 7

en is the input voltage-noise density

in is the input current-noise density

enRG is the noise voltage density contributed by the gain

resistor RG

enRF is the noise voltage density contributed by the feed-

back resistor RF

Resistor Noise = 4k T R

× × × ×∆

in nV/√Hz

T is absolute temperature in Kelvin

k is Boltzmann constant: k = 1.38 x 10-23 in joules/Kelvin

R is resistance in ohms and ∆f is frequency range in Hertz

The MAX44206 input-referred voltage noise contributes

the equivalent noise of a 600Ω resistor. For low noise,

keep the source and feedback resistance at or below this

value, i.e. RS + RG//RF ≤ 600Ω. At combinations of below

600Ω, amplifier noise is dominant, but in the region 600Ω

to 10kΩ, the noise is dominated by resistor thermal noise.

Any larger resistances beyond that, the noise current mul-

tiplied by the total resistance dominated the noise.

Figure 6. Compensation for Source Impedance

Figure 7. Fully Differential Amplifier

OUT-

OUT+

VS+

VS-

OUT+

OUT-

VS+

VS-

MAX44206

VOCM

IN-

IN+

RB = RS//RT

RINM

VINM

SIGNAL

GENERATOR

VS+

VS-

OUT+

OUT-

+5V

-5V

MAX44206

VOCM

INM

INP

VOCM

OUT-

OUT+

IN-

IN+

MAX44206

180MHz, Low-Noise, Low-Distortion, Fully

Differential Op Amp/ADC Driver

www.maximintegrated.com

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