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| LMH6622 Datasheet, PDF (18/33 Pages) National Semiconductor (TI) – Dual Wideband, Low Noise, 160MHz, Operational Amplifiers | |||
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LMH6622
SNOS986E â DECEMBER 2001 â REVISED JULY 2014
www.ti.com
DSL Receive Channel Applications (continued)
Applications ranging from +5 V to +12 V or ±2.5 V to ±6 V are fully supported by the LMH6622. In Figure 36, the
LMH6622 is used as an inverting summing amplifier to provide both received pre-amp channel gain and driver
output signal cancellation, that is, the function of a hybrid coupler.
LMH6672
DAC +
DAC -
IN OUT
IN OUT
+VA/2
-VA/2
V1
1/2RS
1/2RL
1:N
++
VT1
VTN
V2
-
-
VS
1/2RS
1/2RL
RF
R1
ADC +
+VOUT
R2
-
+
R+
ADC -
LMH6622
-VOUT
+
-
RF
R+
R1
R2
Figure 36. ADSL Receive Applications Circuit
The two RS resistors are used to provide impedance matching through the 1:N transformer.
RL
RS = N2
where
⢠RL is the impedance of the twisted pair line
⢠N is the turns ratio of the transformer
(1)
The resistors R2 and RF are used to set the receive gain of the pre-amp. The receive gain is selected to meet the
ADC full-scale requirement of a DSL chipset.
Resistor R1 and R2 along with RF are used to achieve cancellation of the output driver signal at the output of the
receiver.
Since the LMH6622 is configured as an inverting summing amplifier, VOUT is found to be,
VOUT = -RF
V1
R1
+
V2
R2
(2)
The expression for V1 and V2 can be found by using superposition principle.
When VS = 0,
V1 =
1
2
VA
and
V2
=
-
1
4
VA
(3)
When VA = 0,
V1 = 0
and
V2
=
-
1
2
VT1
(4)
18
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