THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

25

POST OFFICE BOX 655303

APPLICATION INFORMATION

general configurations (continued)

If even more attenuation is needed, a multiple-pole filter is required. The Sallen-Key filter can be used for this

task. For best results, the THS7001 and THS7002 preamplifier should have a bandwidth that is 8 to 10 times

the filter frequency bandwidth. Failure to do this can result in phase shift of the amplifier.

VI

C2

R2

R1

C1

RF

RG

R1 = R2 = R

C1 = C2 = C

Q = Peaking Factor

(Butterworth Q = 0.707)

(

=

1

Q

2 –

)

RG

RF

_

+

f

–3dB +

1

2

pRC

Figure 64. 2-Pole Low-Pass Sallen-Key Filter

ADSL

The ADSL receive band consists of up to 255 separate carrier frequencies each with its own modulation and

amplitude level. With such an implementation, it is imperative that signals received off the telephone line have

as high a signal-to-noise ratio (SNR) as possible. This is because of the numerous sources of interference on

the line. The best way to accomplish this high SNR is to have a low-noise preamplifier on the front-end. It is also

important to have the lowest distortion possible to help minimize against interference within the ADSL carriers.

The THS7001 and THS7002 was designed with these two priorities in mind.

By taking advantage of the superb characteristics of the complimentary bipolar process (BICOM), the THS7001

and THS7002 offers extremely low noise and distortion while maintaining a high bandwidth. There are some

aspects that help minimize distortion in any amplifier. The first is to extend the bandwidth of the amplifier as high

as possible without peaking. This allows the amplifier to eliminate any nonlinearities in the output signal. Another

thing that helps to minimize distortion is to increase the load impedance seen by the amplifier, thereby reducing

the currents in the output stage. This will help keep the output transistors in their linear amplification range and

will also reduce the heating effects.

One central-office side terminal circuit implementation, shown in Figure 65, uses a 1:2 transformer ratio. While

creating a power and output voltage advantage for the line drivers, the 1:2 transformer ratio reduces the SNR

for the received signals. The ADSL standard, ANSI T1.413, stipulates a noise power spectral density of –140

dBm/Hz, which is equivalent to 31.6 nV/

√Hz for a 100 Ω system. Although many amplifiers can reach this level

of performance, actual ADSL system testing has indicated that the noise power spectral density may typically

be

≤ –150 dBm/Hz, or ≤ 10 nV/√Hz. With a transformer ratio of 1:2, this number reduces to less than 5 nV/√Hz.

The THS7002 preamplifiers, with an equivalent input noise of 1.7 nV/

√Hz, is an excellent choice for this

application. Coupled with a very low 0.9 pA/

√Hz equivalent input current noise and low value resistors, the

THS7001 and THS7002 will ensure that the received signal SNR will be as high as possible.