LMH6622 Datasheet, PDF (19/33 Pages) National Semiconductor (TI) – Dual Wideband, Low Noise, 160MHz, Operational Amplifiers

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LMH6622
SNOS986E – DECEMBER 2001 – REVISED JULY 2014
DSL Receive Channel Applications (continued)
Therefore,
V₁ =
1
2
V_A
and
V₂
=
-
1
4
V_A
-
1
2
V_T1
(5)
And then,
V_{OUT =} -R_F
V_A
-
V_A
V_T1
-
2R₁ 4R₂ 2R₂
(6)
Setting R₁ = 2*R₂ to cancel unwanted driver signal in the receive path, then we have
R_F
V_OUT = _2R2V_T1
(7)
We can also find that,
1
1
1
V_TN  V_S and V_T1  V_TN 
V_S
2
N
2N
(8)
And then
R_F
V_OUT = _4NR2 V_S
(9)
In conclusion, the peak-to-peak voltage to the ADC would be,
R_F
2 V_OUT = _2NR2 V_S
(10)
9.2 Receive Channel Noise Calculation
The circuit of Figure 36 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:
A_n
= 1 + ^RF
R₁ / R₂
(11)
Noise power at each of the output of LMH6622:
e^2o = A²ⁿ [V²ⁿ + i^2non-inv R₊₂ + 4kT R₊] + i^2inv R^2F + 4kT R_F A_n
where
•
•
•
•
•
•
•
V_n is the Input referred voltage noise
i_n is the Input referred current noise
i_non-inv is the Input referred non-inverting current noise
i_inv is the Input referred inverting current noise
k is the Boltzmann’s constant, K = 1.38 x 10^−23
T is the Resistor temperature in k
R₊ is the source resistance at the non-inverting input to balance offset voltage, typically very small for this
inverting summing applications
(12)
For a voltage feedback amplifier,
i_inv = i_non-inv = i_n
(13)
Therefore, total output noise from the differential pre-amp is:
e^2TotalOutput = 2 e^2o
(14)
The factor '2 ' appears here because of differential output.
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