SPT104 Просмотр технического описания (PDF) - Signal Processing Technologies

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SPT104

DC TO 1.1GHz LINEAR AMPLIFIER

Signal Processing Technologies 

PC Board Layout Considerations

Proper layout of printed circuit boards is important to

achieve optimum performance of a circuit operating in the

1GHz frequency range. Use of microstripline is recom-

mended for all signal-carrying paths and low resistance, low

inductance signal return and bypass paths should be used.

To keep the impedance of these paths low, use as much

ground plane as possible. Ground plane also serves to in-

crease the flow of heat out of the package.

The SPT104 has three types of connections: signal paths

(input and output), DC inputs (supplies and offset adjust),

and grounds. 50Ω microstrip is recommended for connec-

tion to the input (pin 4) and output (pin 11). Microstrip on a

doublesided PC board consists of a ground plane on one

side of the board and a constant-width signal-carrying trace

on the other side of the board. For 1/16" G10 or FR-4 PC

board material, a 0.1" wide trace will have a 50Ω character-

istic impedance. The ground plane beneath the signal trace

must extend at least one trace width on either side of the

trace. Also, all traces (including ground) should be kept at

least one trace width from the signal-carrying traces.

To keep power supply noise and oscillations from appear-

ing at the amplifier output, all supply pins should be capaci-

tively bypassed to ground. The power supply pins (1 and 2)

are the inputs to a pair of voltage regulators whose outputs

are at pins 13 and 14. It is recommended that 0.01µF or

larger ceramic capacitors be connected from pins 1, 2, 13

and 14 to ground, within 0.2" of the pins. A 1µF or larger

solid tantalum capacitor to ground is required within 3" of

pins 1 and 2, and for good low frequency performance, solid

tantalum capacitors of at least 15µF should be connected

from pins 13 and 14 to ground within 3" of the pins. Use

0.025" or wider traces for the supply lines. The offset adjust

pin (12) also requires bypassing; a 0.01µF or larger ceramic

capacitor to ground within 0.2" of the pin is recommended.

Grounding is the final layout consideration. Pins 3 and 5-10

should all be connected to a ground plane which should

cover as much of one side of the board around the amplifier

as possible.

Reducing DC Offset

DC offset of the SPT104 may be adjusted by applying a DC

voltage to the amplifier’s offset adjust pin (12). The simplest

method is shown in Figure 1. Using this method of offset

adjust it is possible to vary the output offset by approxi-

mately ±400mV. This simple adjustment has no effect on

the offset drift characteristics of the SPT104.

Figure 1: Basic Circuit

If lower offset and offset drift are required, a low frequency

op amp may be used in conjunction with the SPT104 in a

composite configuration. The suggested circuit appears in

Figure 2. Its method of operation is to compare an attenu-

ated version of the output signal to the input signal and ap-

ply a correcting voltage at the offset adjust pin. A compensa-

tion capacitor CS reduces the bandwidth of the op amp

correction circuit to limit the op amp’s effect on the SPT104

to frequencies below f45, the frequency at which the op amp

has 45dB of open loop gain. Using an LM108, f45 is about

7Hz with CS = 0.1µF. Thus the op amp can correct DC and

low frequency errors below f45, without affecting SPT104

performance above f45. Also note that the noise perfor-

mance of the op amp will dominate below f45.

Figure 2: Composite Amplifier

With an LM108 op amp in this composite configuration, in-

put offset is typically 2mV and drift is 15mV/°C. At frequen-

cies well below f45, the composite gain is equal to (1 +

49.9k/(Ra + Rb)) and the output impedance is very low. As

SPT

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SPT104

9/30/99

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