LMH6628 Datasheet, PDF (9/11 Pages) National Semiconductor (TI) – Dual Wideband, Low Noise, Voltage Feedback Op Amp

Application Section (Continued)
The circuit gain is +1 and the delay is determined by the
following equations.
(1)
1 dφ
T_d = _{360 df ;}
(2)
where T_d is the delay of the op amp at A_V = +1.
The LMH6628 provides a typical delay of 2.8ns at its −3dB
point.
FULL DUPLEX DIGITAL OR ANALOG TRANSMISSION
Simultaneous transmission and reception of analog or digital
signals over a single coaxial cable or twisted-pair line can
reduce cabling requirements. The LMH6628’s wide band-
width and high common-mode rejection in a differential am-
plifier configuration allows full duplex transmission of video,
telephone, control and audio signals.
In the circuit shown in Figure 3, one of the LMH6628’s amps
is used as a "driver" and the other as a difference "receiver"
amplifier. The output impedance of the "driver" is essentially
zero. The two R’s are chosen to match the characteristic
impedance of the transmission line. The "driver" op amp gain
can be selected for unity or greater.
Receiver amplifier A₂ (B₂) is connected across R and forms
differential amplifier for the signals transmitted by driver A₂
(B₂). If R_F equals R_G, receiver A₂ (B₁) will then reject the
signals from driver A₁ (B₁) and pass the signals from driver
B₁ (A₁).
FIGURE 4.
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POSITIVE PEAK DETECTOR
The LMH6628’s dual amplifiers can be used to implement a
unity-gain peak detector circuit as shown in Figure 5.
FIGURE 3.
The output of the receiver amplifier will be:
20038503
20038505
FIGURE 5.
The acquisition speed of this circuit is limited by the dynamic
resistance of the diode when charging C_hold. A plot of the
circuit’s performance is shown in Figure 6 with a 1MHz
sinusoidal input.
(3)
Care must be given to layout and component placement to
maintain a high frequency common-mode rejection. The plot
of Figure 4 shows the simultaneous reception of signals
transmitted at 1MHz and 10MHz.
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