LT6232 datasheet(20/30 Pages) AD

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LTC6228/LTC6229

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 3.

Stray capacitances at the –IN and +IN pins should be

made as low as possible to reduce stability degradation.

For example, ground or supply planes on a PCB should

not encompass the areas just beneath the input pins.

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 0.88nV/√Hz is

equivalent to that of a 47Ω resistor at room temperature.

As with all BJT input amplifiers, lowering input referred

voltage noise is achieved by increasing the collector cur-

rent of the input differential pair, which increases the input

referred current noise.

Op amp input referred noise dominates the input referred

noise of the gain stage when

REQ << en2/4KT

Resistor noise dominates 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

To summarize, initially en dominates for low resistance val-

ues. As the resistance increased, resistor noise starts to

dominate, then on further increase current noise dominates.

With an input referred voltage noise spectral density of

0.88nV/Hz and an input referred current noise of 3pA/Hz

(bias cancellation disabled), it is easy to see that the gain

stage’s input referred noise is dominated by op amp volt-

age noise when REQ << 47Ω and by resistor noise when

55Ω << REQ << 1.8kΩ.

Above an REQ of 1.8kΩ, 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,

by reducing RF and RG, 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. The disadvantage,

however of making the resistors too large is that parasitic

capacitance can start to affect the gain at high frequen-

cies. Hence when designing a system using the LTC6228,

it is recommended that the resistor values be limited only

by the system noise requirements, with the caveat that the

effect of the impedances parasitic capacitances shouldn’t

affect the gain below the intended bandwidth. For exam-

ple, for a feedback resistor of 5k, a parasitic capacitor of

400fF will impact gain at frequencies above 79MHz.

6228 F03

LTC6228

RS1 en

Figure 3 shows the LTC6228 in a typical gain configuration.

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 = RS1 + RG||RF

opamp

voltage

noise

opamp

current

noise

resistor

thermal

noise

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