AN9524: EL5108 Video Buffer Forms Differential Line Driver/Receiver

EL5108 Video Buffer Forms Differential Line
Driver/Receiver
®
October 19, 2004
Application Note
The EL5108 Video Buffer can be used to implement
differential line drivers and receivers with a minimum of
external components (see Figure 2). Common mode
rejection is set by the internal matched thin film resistors
which are pin strapped to set the various closed loop gains
that are required.
AN9524.1
The oscilloscope photograph illustrates the common mode
rejection of the receiver. VIN is a 10MHz, 1.5VP-P sine wave.
The ground reference of U1 is driven by a 1MHz, 1.5VP-P
common mode signal. The combined signal, seen on the top
trace is measured at RX-. VOUT, on the bottom trace, is a
faithful reproduction of VIN with the common mode signal
removed.
VIN is terminated into 75Ω and drives U1 and U2 amplifiers.
U1 has a gain of -1 while U2 has a gain of +1. These
amplifiers create a differential signal with a gain of 2. Series
75Ω resistors provide impedance matching to the
transmission line.
R X-
The 150Ω termination resistor on the receive side of the
transmission line provides proper impedance matching and
attenuation for a gain of one at the receive input. U3 and U4
perform differential to single ended conversion and provide
common mode rejection.
VOUT
U3 is configured in a gain of +2. U2B subtracts out common
mode signals and applies a gain of +2 to differential signals.
When VOUT is terminated into 75Ω, the overall gain from
VIN to VOUT is unity. Because of the gain of +2 in U3, the
peak voltage at the receiver may not exceed 1.5V.
VIN = 10MHz, 1.5VP-P
VCM = 1MHz, 1.5VP-P ON GROUND OF U1 WITH RESPECT TO U3
FIGURE 1. COMMON MODE REJECTION
VIN
75Ω
+
-
100Ω
+
U1
75Ω
-
TX
+
-
U3
RX
150Ω
+
+
U2
+
-
+
75Ω
U4
75Ω
+ VOUT
- 75Ω
FIGURE 2. DIFFERENTIAL VIDEO LINE DRIVER/RECEIVER
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CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 9524
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