LT1028 datasheet(18/28 Pages) AD

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LTC6226/LTC6227

Rev 0

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APPLICATIONS INFORMATION

Figure 3.

For single supply applications, it is recommended that high

quality 0.1µF||1000pF ceramic bypass capacitors be placed

directly between each V+ pin and its closest V– pin with

short connections. The V– pins (including the Exposed Pad)

should be tied directly to a low impedance ground plane

with minimal routing. For dual (split) power supplies, it is

recommended that additional high quality 0.1µF||1000pF

ceramic capacitors be used to bypass V+ pins to ground

and V– pins to ground, again with minimal routing.

Noise Considerations

The ultralow input referred voltage noise of of 1nV/√Hz

is equivalent to that of an 60Ω resistor. As with all BJT

input amplifiers, lowering input referred noise is achieved

by increasing the collector current of the input differential

pair, which increases the input referred current noise.

Figure 3 shows the LTC6226 in a typical gain configuration.

Resistor noise dominated the input referred noise of the

gain stage when

REQ >> en2/4kT and REQ << 4kT/in2

Op amp input referred current noise dominates the input

referred noise when

REQ >> 4kT/in2

With an input referred voltage noise spectral density of

1nV/Hz and an input referred current noise of 2.4pA/Hz,

it is easy to see that the gain stage’s input referred noise

is dominated by op amp voltage noise when REQ << 60Ω

and by resistor noise when

60Ω << REQ << 2.9kΩ.

Above an REQ of 2.9kΩ, input referred current noise

dominates.

Distortion/Noise Trade-Off

As evident from the previous section, gain stage noise

can be reduced by reducing REQ. However, reducing REQ

has its disadvantages. In addition to increasing power

dissipation in the presence of large output signals, the use

of smaller resistors for a given gain results in increased

distortion, because the internal nonlinearities of the op

amp worsen with increasing load current. In addition,

smaller resistors decrease op amp gain and hence can

affect bandwidth. Hence when designing a system using

the LTC6226/LTC6227, it is recommended that the resis-

tor values be limited only by the system noise require-

ments with the caveat that the effect of the impedances'

parasitic capacitances shouldn’t affect the gain below the

intended bandwidth. For example, for a feedback resistor

of 5kΩ, a parasitic capacitor of 400fF will impact gain at

frequencies above 79MHz.

62267 F03

RS1

LTC6226

RS2 en

As can be seen, the input referred noise spectral density

of the gain stage (eT) can be calculated by the following

equations:

eT2 = en2 + in2REQ2 + 4kTREQ

Where

REQ = RS2 + RS1||RF, k is the Boltzmann constant and

T is the temperature (in Kelvin).

Op amp input referred noise dominates the input referred

noise of the gain stage when

REQ << en2/4kT

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