THS7001, THS7002

70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999

27

POST OFFICE BOX 655303

APPLICATION INFORMATION

ADSL (continued)

Typically, the outputs of the preamplifiers are carried into a CODEC, which incorporates an analog-to-digital

converter (ADC). The problem with this setup is that it only uses fixed gain elements. But, when the client is close

to the central office, the gain must be set to receive a high-level signal; or for the opposite, set to receive a

low-level signal. To solve this problem, a programmable-gain amplifier (PGA) should be used. The THS7001

and THS7002 PGAs allow the gain of the receiver signals to be varied from –22 dB to 20 dB. By allowing the

gains to be controlled with a TTL-compatible signal, it is very easy to integrate the THS7001 and THS7002 into

any system.

By having the preamplifier output separate from the PGA input, inserting more amplifiers into the system can

be accomplished easily. The functionality of the amplifier is typically as an active fixed gain filter. This is shown

in Figure 66.

PGA

To

CODEC

_

+

_

+

V0+ R1

R2

C1

C2

RG

RF

G0 G1 G2

TO DSP

3.3 k

3.3 k

3.3 k

THS6062

VL

+5 V

VH

RECEIVER

BLOCK

OPTIONAL CIRCUIT

Preamp

Figure 66. Typical PGA Setup (One Channel)

circuit layout considerations

In order to achieve the levels of high-frequency performance of the THS7001 and THS7002, it is essential that

proper printed-circuit board high-frequency design techniques be followed. A general set of guidelines is given

below. In addition, a THS7001 and THS7002 evaluation board is available to use as a guide for layout or for

evaluating the device performance.

components with a low inductive ground connection. However, in the areas of the amplifier inputs and

output, the ground plane can be removed to minimize the stray capacitance.

capacitor on each supply terminal. It may be possible to share the tantalum among several amplifiers

depending on the application, but a 0.1-

µF ceramic capacitor should always be used on the supply terminal

of every amplifier. In addition, the 0.1-

µF capacitor should be placed as close as possible to the supply

terminal. As this distance increases, the inductance in the connecting trace makes the capacitor less

effective. The designer should strive for distances of less than 0.1 inches between the device power

terminals and the ceramic capacitors.

inductance in the socket pins will often lead to stability problems. Surface-mount packages soldered directly

to the printed-circuit board is the best implementation.