AD9869BCPZ datasheet(24/37 Pages) AD

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AD9869

Rev. A | Page 23 of 36

Tx PROGRAMMABLE GAIN CONTROL

TxPGA functionality is also available to set the peak output

current from the TxDAC or IAMP. The TxDAC and IAMP are

digitally programmable via the PGA[5:0] port or SPI over a 0 dB

to −7.5 dB range and 0 dB to −19.5 dB range, respectively, in

0.5 dB increments.

The TxPGA can be considered as two cascaded attenuators with

the TxDAC providing a 7.5 dB range in 0.5 dB increments, and

the IAMP providing a 12 dB range in 6 dB increments. As a result,

the IAMP composite 19.5 dB span is valid only if Register 0x10

remains at its default setting of 0x04. Modifying this register

setting corrupts the LUT and results in an invalid gain mapping.

TxDAC OUTPUT OPERATION

The differential current output of the TxDAC is available at the

IOUTP+ and IOUTP− pins, and the IAMP should be disabled

by setting Bit 0 of Register 0x0E. Any load connected to these

pins must be ground referenced to provide a dc path for the

current sources. Figure 22 shows the outputs of the TxDAC

driving a doubly terminated 1:1 transformer with its center tap

tied to ground. The peak-to-peak voltage, V p-p, across RL (and

IOUTP+ to IOUTP−) is equal to 2 × I × (RL||RS). With I = 10 mA

and RL = RS = 50 Ω, V p-p is equal to 0.5 V with 1 dBm of peak

power being delivered to RL and 1 dBm being dissipated in RS.

IOUTN–

IOUTN+

0 TO –7.5dB

0 TO –12dB

IAMP

RSET

0.1µF

1:1

TxDAC

Figure 22. TxDAC Output Directly via Center-Tap Transformer

The TxDAC is capable of delivering up to 10 dBm peak power

to a load, RL. To increase the peak power for a fixed standing

current, users must increase V p-p across IOUTP+ and IOUTP−

by increasing one or more of the following parameters: RS, RL

(if possible), and/or the turns ratio, N, of the transformer. For

example, removing the RS from Figure 22 and applying a 2:1

impedance ratio transformer results in 10 dBm of peak power

capabilities to the load. Note that increasing the power output

capabilities of the TxDAC reduces the distortion performance

due to the higher voltage swings seen at IOUTP+ and IOUTP−.

Optimum distortion performance can typically be achieved by

performing both of the following:

•

Limiting the peak positive VIOUTP+ and VIOUTP− to 0.8 V to

avoid onset of TxDAC output compression (TxDAC

voltage compliance is around 1.2 V).

•

Limiting V p-p seen at IOUTP+ and IOUTP− to less

than 1.6 V.

Applications demanding higher output voltage swings and

power drive capabilities can benefit from using the IAMP.

IAMP CURRENT-MODE OPERATION

The IAMP can be configured for the current-mode operation

(see Figure 23) for loads remaining relatively constant. In this

mode, the IAMP delivers the signal-dependent current to the

load via a center-tap transformer. Because the mirrors exhibit a

high output impedance, they can be easily back-terminated (if

required).

TxDAC

IOUTN–

IOUTN+

0 TO –7.5dB

0 TO –12dB

IAMP

RSET

0.1µF

IBIAS = 2 × N × I

IOUTPK

T:1

IOUTPK = N × I

POUTPK = (IOUTPK)2 × T2 × RL

RCM

AVDD

Figure 23. Current-Mode Operation

The IAMP gain, N, can be set between 0 and 4, while the

TxDAC standing current, I, can be set between 2 mA and

12.5 mA (with the IOUTP outputs left open). The IOUTN

outputs should be connected to the transformer, which needs to

be specified to handle the dc standing current, IBIAS, that is

drawn by the IAMP. In addition, because IBIAS remains signal

independent, a series resistor should be inserted between

AVDD and the center-tap transformer to provide provisions

such that the IAMP common-mode voltage, VCM, can be

reduced since its optimum linearity performance is sensitive to

both the Tx signal’s peak-to-rms characteristics as well as the

IAMP VCM. Note that the VCM bias should not exceed 3.3 V. The

power dissipated in the IAMP alone is as follows:

PIAMP = 2 × N × I × VCM

(3)

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