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LTC6417

6417f

applicaTions inForMaTion

This can be avoided by following a few precautions when

putting the LTC6417 in sleep mode:

• Donotforcetheoutputsbelowtheinputs,thiswillturn

the output stages on.

• EitherfloatCLHIortieittoVCC. This will allow a wider

signalrangeattheinputsbeforetheclampsareactivated.

• MaintaintheinputsbelowCLHIor2.5Vwhicheveris

lower, otherwise the input clamps will be activated.

• DonotshortVCM or the outputs to GND, in either case

the output clamps will turn on. Current drawn from the

supply can be as high as 180mA.

• Floattheoutputsifpossible.Theoutputswillbepulled

down by internal resistors to VCM.

Heeding these precautions will protect the LTC6417 as

well any part it is driving, while maintaining a low current

consumption in sleep mode.

Noise and Noise Figure

TheLTC6417’sdifferentialinputreferredvoltageandcurrent

noisedensitiesare 1.5nV/√Hzand4.3pA/√Hz,respectively.

Before presenting a noise model, the circuit with the

transformer in Figure 10 will be simplified. In Figure 11,

the circuit is redrawn with the source impedance reflected

to the secondary side of the transformer. The source

impedance is multiplied by the impedance ratio m of the

transformer. In Figure 12, noise sources associated with

the amplifier and resistors have been added. Based on

this noise model of the LTC6417, the total output noise

power excluding the noise contribution of the source is:

e 2

no = e

ni + ini • REQ

(

)2 +i2

RT • R

= e2ni + ini •REQ

(

)2 + 4kT

•R2EQ

where REQ = mRS||RT is the equivalent impedance seen

at the input of the LTC6417. The output noise power due

to the noise of source resistance is given by:

no(mRS) = i

mRS • R

4kT

mRS

•R2EQ

Noise figure (NF) is calculated from the ratio of these

noise powers:

= 10log 1+

e2no

e2no(mRS)













Figure 10. LTC6417 with a Transformer

6417 F10

LTC6417

1:m

TRANSFORMER

Figure 11. Source Resistance Referred to the Secondary

6417 F11

LTC6417

mRS

Figure 12. LTC6417 Simplified Noise Model

6417 F12

i2ni

eni2

eno2

LTC6417

mRS

i2mRS

i2RT

LTC6409 Datasheet(PDF) 23 Page - Linear Technology

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