9.1 DSL Receive Channel Applications

Figure 35. ADSL Signal Description

The LMH6622 is a dual, wideband operational amplifier designed for use as a DSL line receiver. In the receive band of a Customer Premises Equipment (CPE) ADSL modem it is possible that as many as 255 Discrete Multi-Tone (DMT) QAM signals will be present, each with its own carrier frequency, modulation, and signal level. The ADSL standard requires a line referred noise power density of -140 dBm/Hz within the CPE receive band of 100 KHz to 1.1 MHz. The CPE driver output signal will leak into the receive path because of full duplex operation and the imperfections of the hybrid coupler circuit. The DSL analog front end must incorporate a receiver pre-amp which is both low noise and highly linear for ADSL-standard operation. The LMH6622 is designed for the twin performance parameters of low noise and high linearity.

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.

Figure 36. ADSL Receive Applications Circuit

The two R_S resistors are used to provide impedance matching through the 1:N transformer.

Equation 1.

where

• R_L is the impedance of the twisted pair line
• N is the turns ratio of the transformer

The resistors R₂ and R_F 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 R_F 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, V_OUT is found to be,

Equation 2.

The expression for V₁ and V₂ can be found by using superposition principle.

When V_S = 0,

Equation 3.

When V_A = 0,

Equation 4.

Therefore,

Equation 5.

And then,

Equation 6.

Setting R₁ = 2*R₂ to cancel unwanted driver signal in the receive path, then we have

Equation 7.

We can also find that,

Equation 8.

And then

Equation 9.

In conclusion, the peak-to-peak voltage to the ADC would be,

Equation 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:

Equation 11.

Noise power at each of the output of LMH6622:

Equation 12.

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

For a voltage feedback amplifier,

Equation 13.

Therefore, total output noise from the differential pre-amp is:

Equation 14.

The factor '2 ' appears here because of differential output.

9.3 Differential Analog-to-Digital Driver

Figure 37. Circuit for Differential A/D Driver

The LMH6622 is a low noise, low distortion high speed operational amplifier. The LMH6622 comes in either SOIC-8 or VSSOP-8 packages. Because two channels are available in each package the LMH6622 can be used as a high dynamic range differential amplifier for the purpose of driving a high speed analog-to-digital converter. Driving a 1 kΩ load, the differential amplifier of Figure 37 provides 20 dB gain, a flat frequency response up to 6 MHz, and harmonic distortion that is lower than 80 dBc. This circuit makes use of a transformer to convert a single-ended signal to a differential signal. The input resistor R_IN is chosen by the following equation,

Equation 15.

The gain of this differential amplifier can be adjusted by R_C and R_F,

Equation 16.

See Figure 38 and Figure 39 below for plots related to the discussion of Figure 37.

Figure 38. Frequency Response

Figure 39. Total Output Referred Noise Density

9.4 Typical Application

See Figure 40 for application circuit.

Figure 40. ADSL Receive Applications Circuit