Datasheet

LTC6417
23
6417f
applicaTions inForMaTion
This can be avoided by following a few precautions when
putting the LTC6417 in sleep mode:
• Donotforcetheoutputsbelowtheinputs,thiswillturn
the output stages on.
• EitherfloatCLHIortieittoV
CC
. This will allow a wider
signal range at the inputs before the clamps are activated.
• MaintaintheinputsbelowCLHIor2.5Vwhicheveris
lower, otherwise the input clamps will be activated.
• DonotshortV
CM
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.
• Floattheoutputsifpossible.Theoutputswillbepulled
down by internal resistors to V
CM
.
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
The LTC6417’s differential input referred voltage and current
noise densities are 1.5nV/√Hz and 4.3pA/√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
2
ni
+ i
ni
•R
EQ
( )
2
+i
2
R
T
•R
2
EQ
= e
2
ni
+ i
ni
•R
EQ
( )
2
+
4kT
R
T
•R
2
EQ
where R
EQ
= mR
S
||R
T
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:
e
2
no(mR
S
)
= i
2
mR
S
•R
2
EQ
=
4kT
mR
S
•R
2
EQ
Noise figure (NF) is calculated from the ratio of these
noise powers:
NF = 10log 1+
e
2
no
e
2
no(mR
S
)








Figure 10. LTC6417 with a Transformer
6417 F10
R
T
LTC6417
R
S
1:m
TRANSFORMER
Figure 11. Source Resistance Referred to the Secondary
6417 F11
R
T
LTC6417
mR
S
Figure 12. LTC6417 Simplified Noise Model
6417 F12
R
T
i
2
ni
e
ni
2
e
no
2
LTC6417
mR
S
i
2
mRS
i
2
RT