Datasheet

e
2
TotalOutput
= 2 e
2
o
i
inv
= i
non-inv
= i
n
e
2
o
= A
2
n
[V
2
n
+ i
2
non-inv
R
+
2
+ 4kT R
+
] + i
2
inv
R
2
F
+ 4kT R
F
A
n
=
A
N
1 +
R
F
R
1
//R
2
R
F
=
2NR
2
V
S
V
OUT
2
R
F
=
4NR
2
V
S
V
OUT
T N S T 1 T N S
1 1 1
V V a n d V V V
2 N 2N
R
F
=
2R
2
V
T1
V
OUT
-
=
R
F
V
OUT
--
V
T1
V
A
V
A
2R
2
4R
2
2R
1
V
1
=
V
A
and
1
2
V
2
=
1
4
V
A
-
1
2
V
T1
-
V
1
=
0
and
V
2
=
1
2
V
T1
-
LMH6622
SNOS986D –DECEMBER 2001–REVISED MARCH 2013 www.ti.com
(4)
Therefore,
(5)
And then,
(6)
Setting R
1
= 2*R
2
to cancel unwanted driver signal in the receive path, then we have
(7)
We can also find that,
(8)
And then
(9)
In conclusion, the peak-to-peak voltage to the ADC would be,
(10)
RECEIVE CHANNEL NOISE CALCULATION
The circuit of Figure 37 also has the characteristic that it cancels noise power from the drive channel.
The noise gain of the receive pre-amp is found to be:
(11)
Noise power at each of the output of LMH6622:
(12)
where
V
n
Input referred voltage noise
i
n
Input referred current noise
i
non-inv
Input referred non-inverting current noise
i
inv
Input referred inverting current noise
k Boltzmann’s constant, K = 1.38 x 10
−23
T Resistor temperature in k
R
+
Source resistance at the non-inverting input to balance offset voltage, typically very small for this inverting
summing applications
For a voltage feedback amplifier,
(13)
Therefore, total output noise from the differential pre-amp is:
(14)
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