THS7001 Datasheet, PDF (27/32 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

THS7001, THS7002
70-MHz PROGRAMMABLE-GAIN AMPLIFIERS
SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999
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.
Preamp
V0+ ^R1
R2
C2
RECEIVER
BLOCK
C1
THS6062
+
_
RF
RG
OPTIONAL CIRCUIT
TO DSP
G0 G1 G2
_
+
PGA
3.3 k
3.3 k
3.3 k
+5 V
VH
To
CODEC
VL
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.
D Ground planes—It is highly recommended that a ground plane be used on the board to provide all
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.
D Proper power supply decoupling—Use a 6.8-µF tantalum capacitor in parallel with a 0.1-µF ceramic
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.
D Sockets—Sockets are not recommended for high-speed operational amplifiers. The additional lead
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.
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