AD9865BCPRL datasheet(32/48 Pages) AD

32 / 48 page

AD9865

Rev. A | Page 32 of 48

currents. The gain of the secondary path, G, and the TxDAC’s

standing current, I, can be set using the following equation:

IOUTPK + 3 mA = G × I

(6)

The voltage output driver exhibits a high output impedance if

the bias currents for the npn transistors are removed. This

feature is advantageous in half-duplex applications (for

example, power lines) in which the Tx output driver must go

into a high impedance state while in Rx mode. If the AD9865 is

configured for the half-duplex mode (MODE = 0), the IAMP,

TxDAC, and interpolation filter are automatically powered

down after a Tx burst (via TXEN), thus placing the Tx driver

into a high impedance state while reducing its power

consumption.

IAMP CURRENT CONSUMPTION CONSIDERATIONS

The Tx path’s analog current consumption is an important

consideration when determining its contribution to the overall

on-chip power dissipation. This is especially the case in full-

duplex applications, where the power dissipation can exceed the

maximum limit of 1.66 W, if the IAMP’s IOUTPK is set to high.

The analog current consumption includes the TxDAC’s analog

supply (Pin 43) along with the standing current from the IAMP’s

outputs. Equation 2 and Equation 5 can be used to calculate the

power dissipated in the IAMP for the current and voltage mode

configuration. Figure 66 shows the current consumption for the

TxDAC and IAMP as a function of the TxDAC’s standing current,

I, when only the IOUTN outputs are used. Figure 67 shows the

current consumption for the TxDAC and IAMP as a function of

the TxDAC’s standing current, I, when the IOUTN and IOUTG

outputs are used. Both figures are with the default current mirror

gain settings of N = 4 and G = 12.

I (mA)

345

678

100

IAMPN OUTPUT

TxDACs AVDD

Figure 66. Current Consumption of TxDAC and IAMP in Current-Mode

Operation with IOUTN Only (Default IAMP Settings)

I (mA)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

100

110

120

130

140

150

IOUTN OUTPUT

IOUTG OUTPUT

TxDAC AVDD

Figure 67. Current Consumption of TxDAC and IAMP in Current-Mode

Operation with IOUTN Only (Default IAMP Settings)

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