DS90LV048A

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SNLS045B – JULY 1999 – REVISED APRIL 2013

APPLICATION INFORMATION

General application guidelines and hints for LVDS drivers and receivers may be found in the following application

notes: LVDS Owner's Manual (lit #550062-002), AN-808 (SNLA028), AN-977 (SNLA166), AN-971 (SNLA165),

AN-916 (SNLA219), AN-805 (SNOA233), AN-903 (SNLA034). The latest applications material is available on the

web at: www.ti.com

LVDS drivers and receivers are intended to be primarily used in an uncomplicated point-to-point configuration as

is shown in Figure 5. This configuration provides a clean signaling environment for the fast edge rates of the

drivers. The receiver is connected to the driver through a balanced media which may be a standard twisted pair

cable, a parallel pair cable, or simply PCB traces. Typically, the characteristic impedance of the media is in the

range of 100

Ω. A termination resistor of 100Ω (selected to match the media), and is located as close to the

receiver input pins as possible. The termination resistor converts the driver output (current mode) into a voltage

that is detected by the receiver. Other configurations are possible such as a multi-receiver configuration, but the

effects of a mid-stream connector(s), cable stub(s), and other impedance discontinuities as well as ground

shifting, noise margin limits, and total termination loading must be taken into account.

The DS90LV048A differential line receiver is capable of detecting signals as low as 100mV, over a ±1V common-

mode range centered around +1.2V. This is related to the driver offset voltage which is typically +1.2V. The

driven signal is centered around this voltage and may shift ±1V around this center point. The ±1V shifting may be

the result of a ground potential difference between the driver's ground reference and the receiver's ground

reference, the common-mode effects of coupled noise, or a combination of the two. The AC parameters of both

receiver input pins are optimized for a recommended operating input voltage range of 0V to +2.4V (measured

turn on the ESD protection circuitry which will clamp the bus voltages.

The DS90LV048A has a flow-through pinout that allows for easy PCB layout. The LVDS signals on one side of

the device easily allows for matching electrical lengths of the differential pair trace lines between the driver and

the receiver as well as allowing the trace lines to be close together to couple noise as common-mode. Noise

isolation is achieved with the LVDS signals on one side of the device and the TTL signals on the other side.

Power Decoupling Recommendations

Bypass capacitors must be used on power pins. Use high frequency ceramic (surface mount is recommended)

0.1

μF and 0.001μF capacitors in parallel at the power supply pin with the smallest value capacitor closest to the

device supply pin. Additional scattered capacitors over the printed circuit board will improve decoupling. Multiple

vias should be used to connect the decoupling capacitors to the power planes. A 10

μF (35V) or greater solid

tantalum capacitor should be connected at the power entry point on the printed circuit board between the supply

and ground.

PC Board considerations

Use at least 4 PCB layers (top to bottom); LVDS signals, ground, power, TTL signals.

Isolate TTL signals from LVDS signals, otherwise the TTL may couple onto the LVDS lines. It is best to put TTL

and LVDS signals on different layers which are isolated by a power/ground plane(s)

Keep drivers and receivers as close to the (LVDS port side) connectors as possible.

Differential Traces

Use controlled impedance traces which match the differential impedance of your transmission medium (ie. cable)

and termination resistor. Run the differential pair trace lines as close together as possible as soon as they leave

the IC (stubs should be < 10mm long). This will help eliminate reflections and ensure noise is coupled as

common-mode. In fact, we have seen that differential signals which are 1mm apart radiate far less noise than

traces 3mm apart since magnetic field cancellation is much better with the closer traces. In addition, noise

induced on the differential lines is much more likely to appear as common-mode which is rejected by the

receiver.

Copyright © 1999–2013, Texas Instruments Incorporated

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