Application of Low Noise and High Speed ​​Operational Amplifier in ADSL Analog Front End

In view of this situation, this article introduces a solution using the LMH6643 full-rail output chip, the LMH6672 line driver and the LMH6622 low-noise operational amplifier. This solution has the advantage of fully utilizing the performance of the ADSL baseband digital signal processor. See Figure 1 for the structural block diagram.

The output of the digital-to-analog converter in the figure can use a pair of amplifiers in the LMH6643 chip as differential buffer amplifiers to provide impedance matching, isolation and drive for the low-pass filter between the LMH6643 and the LMH6672. The circuit diagram of Figure 2 shows the SPICE model used. The voltage gain of the inverting buffer amplifier in the figure is defined as -RB1+/RB2+ and -RB1/RB2.

LMH6672 used as upstream line driver

Since the LMH6672 chip can output extremely high drive current with low distortion while only requiring one power supply, it can be used as an upstream DSL line driver. When used as a differential output driver, the LMH6672 chip can drive a 50Ω load to a swing of 16.8Vpp with a distortion rate of only -93dBc, which can support the highest upstream power and ensure that the ADSL line supports the highest transmission rate. Figure 3 is a typical line driver circuit diagram, in which the line driver can drive a 100Ω twisted pair cable through a transformer with a turns ratio of 1:2. The voltage gain of this non-inverting driver amplifier is determined by formula (1) or (2).

1 + 2×RF+/RG (1)

1 + 2×RF-/RG (2)

In the figure, capacitor CG is used to set the DC gain to 1V/V.

LMH6622 used as downstream low noise preamplifier

Because the LMH6622 has low noise and distortion, it can be used as a low-noise preamplifier for the downlink. The two advantages of low noise and low distortion ensure that the receiving path has a high dynamic range to meet the strict linearity and noise requirements of the ADSL standard. The LMH6622 chip in Figure 4 can realize the function of an inverting summing amplifier to provide the receiving preamplifier channel gain and eliminate the driver's echo signal. In order to eliminate the interference of the driver echo signal on the receiving channel, R1+ must be set to 2×R2+, and R1- must also be set to 2×R2-. In actual applications, since the signal matching is not ideal, the mixed suppression is about 12dB. The resistor value can be changed during simulation to test the true performance of the receiving circuit.

In addition to providing these design chips, National Semiconductor also provides a database that can be accessed online. Obviously, if you start from scratch to design the SPICE model of the operational amplifier for simulation, it is not only difficult but also time-consuming. Therefore, the company has specially established the OrCAD Capture database and the PSPICE model database to store information about high-speed operational amplifiers manufactured using advanced VIP10 process technology, including the performance model of the LMH series high-speed operational amplifier and the circuit diagram symbol of the operational amplifier.