AND8020 datasheet(9/18 Pages) AD

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AND8020/D

http://onsemi.com

SECTION 3. THEVENIN EQUIVALENT PARALLEL TERMINATION

Although the single resistor termination to VTT conserves

power, it requires an additional supply voltage. An alternate

approach to using a VTT power supply is to use a resistor

divider network as shown in Figure 12 to develop a

Thevenin voltage, VTT, and provide a parallel impedance

matching

termination,

the

Thevenin

parallel

termination.

Figure 12. Thevenin Equivalent Parallel Termination

VEE

T−Line Z0

Driver

VCC

Receiver

Driver

Receiver

T−Line Z0

VEE

VCC

or Twisted Pair

VTT + VCC * 2.0V

+ VCC

) R2

(eq. 13)

Differential ECL outputs can be terminated as independent

complimentary single−ended lines. Both sides of a

differential pair must be terminated. Balanced, symmetrical

loading of each line must be preserved.

While a Thevenin Parallel technique dissipates more

termination power, it does not require the additional VTT

supply. This additional power is consumed entirely in the

external resistor divider network and thus will not change

the current being sourced by the device, hence it does not

alter the IC reliability or lifetime. As with standard parallel

termination, variance of VTT and VCC supplies must be

considered.

The Thevenin equivalent of the two resistors needs to be

equal to the characteristic impedance of the signal

transmission line. Calculated values for resistors R1 and R2

may be obtained from the following relationships.

+ Z0

VCC * VEE

VCC * VTT

(eq. 14)

+ R2

VCC * VTT

VTT

* VEE

(eq. 15)

Where:

VTT =VCC − 2.0 V

Z0 = Characteristic Impedance of the Signal

Transmission Line

For a typical VCC = 5.0 V PECL scheme, where VEE =

GND, VTT = 3.0 V, and Z0 = 50

+ 50 5 * 0

* 3 +

125

(eq. 16)

+ 125 5 * 3

* 0 +

83.3

(eq. 17)

and cross−checking for VTT:

VTT + 5

125

) 83.3 +

3.0 V

(eq. 18)

VTT + VCC * 2.0 V + 3.0 V

(eq. 19)

For the typical VCC = 3.3 V LVPECL scheme, where

VEE = GND, VTT = 1.3 V, and Z0 = 50

+ 50

3.3

* 0

3.3

* 1.3 +

82.5

(eq. 20)

+ 82.5 3.3 * 1.3

1.3

* 0

+ 126 W

(eq. 21)

and cross−checking for VTT:

VTT + 3.3

82.5

126

) 82.5 +

1.3 V

(eq. 22)

VTT + VCC * 2.0 V + 1.3 V

(eq. 23)

Table D. Thevenin Term Table

|VCC−VEE| = 5.0 V

|VCC−VEE| = 3.3 V

|VCC−VEE| = 2.5 V

125

127

250

62.5

117

175

178

115

350

87.5

125

188

190

123

375

93.8

133

200

203

132

400

100

150

225

229

149

450

112.5

100

167

250

100

253

165

100

500

125.5

120

200

300

120

305

198

120

600

150

250

375

150

381

248

150

750

187.5

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