In some cases, the value for this series resistor may be zero as it depends upon the cmos driver characteristics. Highswing voltagemode driver termination is implemented with a combination of output driver transistors and series resistors to meet termination resistance levels 50. Using differential io lvds, sublvds, lvpecl in ice65 mobilefpgas 2. The termination resistor r is still selected to match the trace impedance z o, while the capacitor is selected by. Terminate out and out with identical termination on each for lowoutput distortion. Because the output biasing resistor is placed at the driver side before the coupling capacitors, the line termination resistors and input biasing must be provided at the receiver side. The max9121 inputs are high impedance and require an external termination resistor when used in a pointtopoint connection. V in differential input voltage must exceed the fpgas required minimum level. There are five disadvantages of the thevenin termination.
Output terminations for sit910290029107 lvpecl, lvds. Note that these re resistors can be placed near the pecl driver. Qan105 clock termination techniques and load matching. The capacitor value may be traded off to select a lower value below 200pf for low power. Ac termination of a line results in the lowest power drain, but also requires two parts. An5029 interfacing between pecl and lvds differential. Accouplingbetween differential lvpecl, lvds, hstl, and cml. Recently, lowvoltage differentialsignaling lvds logic has attained widespread popularity because of similar characteristics, but with lower amplitudes and lower power dissipation than ecl. Thevenin equivalentparallel termination r r section 4. This termination is not recommended for ttl or cmos circuits.
Thevenin termination also reduces the burden on the driver by supplying additional current to the load. A separate output driver power supply can be from 2. It uses a single resistor at the load end of the trace, as shown in figure 7 and, like the thevenin and ac methods, it acts by preventing signal. Calculations for pwb trace termination and reflection values. The reduced number of wires reduces system cost and in some cases. The max9180 lownoise lvds repeater is an example of this design, and shown in figure 5. Also, this type of termination pro vides good overshoot suppression. Output terminations for sit910290029107 lvpecl, lvds, cml, and hcsl differential drivers. The max9180 lownoise lvds repeater, figure 5, is an example of this design. If this configuration is not used, the dc voltage at the. Many of these high speed ad parts have lvds outputs without any internal back termination, so the ad driver current sources will run off into the rails and saturate on a long string of 1s or 0s without an external termination of some sort this sounds just like what you are seeing. For driver output levels that exceed the specified receiver input levels, custom termination techniques to reduce the common mode voltage level or the voltage swing can be used. The primary drawbacks to thevenin termination are the need for two resistors per line and the requirement for two supply voltages to be available near the termination.
Driver lvds receiver 150 q 150 q transmission line z0 100 q 0. The max9121max9122 are guaranteed to receive data at speeds up to 500mbps 250mhz over controlledimpedance media of approximately 100. Both pecl and lvds buffers implement differential lowvoltage signaling techniques, but with different swing and offset voltage levels. Some lvds devices have a failsafe circuit on their inputs. Lvds is defined for lowvoltage differential signal pointtopoint transmission. Lvds operates at low power and can run at very high speeds using inexpensive twistedpair copper cables. Output terminations for sit910290029103 lvpecl, lvds. The devices support a wide commonmode input range of 0. Ac termination adds a capacitive load to the driver and delay due to rc time constant, however, it consumes low power. As in previous cases, the accoupled capacitors may be used between the termination network and the receiver where needed. When interfacing the si533x device to an lvds signal, a 100. This document discusses termination and biasing schemes for lvds drivers and receivers with dc and. Thevenin termination ac termination is shown in the figure 4 below.
Dccoupling between differential lvpecl, lvds, hstl, and. Output terminations for differential oscillators sitime. Implementing pulldown resistors in a thevenin parallel termination resistor divider. Series back termination vbb bb driver receiver all media d1 d2 d1 d2 section 5. Ensure that output currents do not exceed the current limits as. The thevenin equation resistor terminates the transmission line z near the receiver. Design of a lowpower cmos lvds io interface circuit. Dccoupling between differential lvpecl, lvds, hstl, and cm 9 figure 11 is the most commonly used termination for lvds signals. Design of a lowpower cmos lvds io interface circuit 1102 fig. The disadvantage to this architecture is that the thevenin transformation results in requiring two 1% resistors, 82. A 5v pecl driver will provide a signal with too high of. Another solution is to terminate the lvpecl driver with a thevenin.
This way i might also include more than one load ic per signal. Its allowed however, to use an impedance matched transmitter as well, but not required for the signal rates covered by. This makes lvds desirable for parallel link data transmission. When a singleended signal is taken from the differential output, terminate both out and out. Simplified diagram of lvds driver and receiver connected via 100w differential impedance. Lvds, cml, ecldifferential interfaces with odd voltages. In most cases thevenin equivalent termination works well, but it can be sensitive to power. At the receiver side, it typically offers a 50 farend termination to provide a 400mv swing at the driver side. Quad lvds line receivers with integrated termination and. And8020d termination of ecl devices with ef emitter. Current only flows while the capacitor is charging.
It also assumes that the lvds receiver does not include onchiptermination. The power consumption in 150 ohm pull down resistors is less than thevenin equivalent termination. The values for r 1 and r 2 are based on the asymmetric characteristic of the driver during high and low levels of the logic, given that, a applying voltage division between r 1 and r 2 from the above. Current mode drivers use norton equivalent parallel. Output terminations for sit912091219122 and sit382822. As said, the lvds standard is based on a receiver side termination. The higher potential switching speeds of differential io allows data to be multiplexed onto a reduced number of wires at a much higher data rate per line. Dccoupled receiver common mode voltage shift by replacing the standard receiver termination with a custom termination network, the common. Parallel termination in parallel termination, a rc combination is placed at the load. It has several advantages that make it attractive to users.
Ac termination the ac termination, shown in figure 4, adds a series capacitor in the parallel leg. Accoupling between differential lvpecl, lvds, hstl, and. The value of the capacitor must be chosen carefully, usually smaller than 50pf. The equivalent 50 thevenin resistors of r1 and r2 are used to terminate the trace impedance as well as to set the commonmode voltage vcm 0. Inputs to the zl40200 are externally terminated to allow use of precision termination components and to allow full flexibility of input termination. For a pecl driver, each emitter follower output is properly terminated with a. So that means thevenin termination is not possible for 50 ohm or even 120 ohm impedance with this kind of logic ics. Scaa045 8 design and layout guidelines for the cdcvf2505 clock driver 3.
In most cases thevenin equivalent termination works well, but it can be sensitive to. The thevenin termination is most commonly used in pecl logic where the load impedance is 50 method 3. The application diagrams in the following figures allow the zl40212 to accept lvpecl, lvds, cml, hcsl and singleended inputs. In the circuit of figure 2, a thevenin resistor divider is used to generate the appropriate termination offset voltage and is a widely used termination approach for 3. An18 application note 69 as described for lvpecl to interface from pecl to lvds a thevenin equation should be applied. In thevenin termination, the parallel combination of thevenin resistors r 1 and r 2 matches the characteristic impedance, z 0, of the transmission line. Figure 3 when compared to the four resistor thevenin termination. Scaa059cmarch 2003revised october 2007 accoupling between differential lvpecl, lvds, hstl, and cml 5 submit documentation feedback. Lvds driver output structure lvds 1 is a highspeed digital interface suitable for many applications that require low power consumption and high noise immunity. This additional current helps the driver especially in a large voltageswing system, such as 5 and 3. The defining lvds specification can be found in reference 1, and references 2 and 3 should also prove useful. Lvpecl to lvds interface using a thevenin equivalent termination scheme.
If the output levels of the driver are within the xilinx specified receiver levels, the standard receiver termination can be used. The low signal swing yields low power consumption, at most 4ma are sent through the 100. Mathcad solution to an lvpecl to lvds matching problem. The max9121max9122 quad lowvoltage differential signaling lvds differential line receivers are ideal for applications requiring high data rates, low power, and low noise. The zl40200 can accept dc coupled lvpecl or lvds and ac coupled lvpecl, lvds, cml or hcsl input signals. Lowvoltage differential signaling, or lvds, also known as tiaeia644, is a technical standard that specifies electrical characteristics of a differential, serial communication protocol. The driver tends to be a currentmode driver, driving the balance interconnect cable to a load consisting of the termination resistor and the receiver. Design and layout guidelines for the cdcvf2505 clock. Lvds driver output structurelvds 1 is a highspeed digital interface suitable for many applications that require low. If an accoupled lvds link is attempted with a failsafe circuit, a thevenin termination of the inputs is. Current vs voltagemode driver signal integrity considerations min.
Planet analog advantages of accoupling for lvds signals. The capacitance c1 is used to create ac ground at the termination voltage. Lvds outputs use differential signals with low voltage. A pecl drivers differential output signal is more positive than is expected by the input circuit of an lvds receiver. Is there another driver ic or opamp that would be recommended for driving a properly terminated transmission line with a 150mhz 3,3v signal. Figure 6 lvds input dc coupled figure 7 lvds input ac coupled. The only termination necessary when interfacing a cmos driver to the si533x and si535655 is a source resistor rs placed near the driver to help match its output impedance to the transmission line impedance. Lvds to lvpecl in figure 11 is a combination of the more common 100. The difference between the input high and low levels must be greater than a minimum value to ensure that the receiver can distinguish between them. Dccoupling between differential lvpecl, lvds, hstl, and cml. If an accoupled lvds link is attempted with a failsafe circuit, a thevenin termination of the inputs is required. Cml to lvds rt presents a line termination, cc isolates the driver from the termination, r1rtr2 network provides an operating bias and also a 30mv failsafe bias for the lvds receiver.
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