WIDE BAND DIGITAL PROGRAMMABLE GAIN AMPLIFIER (LMH6720 ONLY)

As shown in Figure 6 and Figure 7 the LMH6720 can be

used to construct a digitally controlled programmable gain

amplifier. Each amplifier is configured to provide a digitally

selectable gain. To provide for accurate gain settings, 1% or

better tolerance is recommended on the feedback and gain

resistors. The gain provided by each digital code is arbitrary

through selection of the feedback and gain resistor values.

AMPLITUDE EQUALIZATION

Sending signals over coaxial cable greater than 50 meters in

length will attenuate high frequency signal components

much more than lower frequency components. An equalizer

can be made to pre emphasize the higher frequency com-

ponents so that the final signal has less distortion. This

process can be done at either end of the cable. The circuit in

Figure 8 shows a receiver with some additional components

in the feedback loop to equalize the incoming signal. The RC

networks peak the signal at higher frequencies. This peaking

is a piecewise linear approximation of the inverse of the

frequency response of the coaxial cable. Figure 9 shows the

effect of this equalization on a digital signal that has passed

through 150 meters of coaxial cable. Figure 10 shows a

Bode plot of the frequency response of the circuit in Figure 8

along with equations needed to design the pole and zero

frequencies. Figure 11 shows a network analyzer plot of an

LMH6714/6720/6722 with the following component values:

R

Ω

R1 = 450

Ω

C1 = 470pF

R2=91

Ω

C2 = 68pF

20056519

FIGURE 6. Wideband Digitally Controlled Programmable Gain Amplifier

Channel Switching

20056520

FIGURE 7. PGA Output

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