AD9869_15 Datasheet, PDF (23/36 Pages) Analog Devices – Broadband Modem Mixed-Signal Front End

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.
1:1
R_S
R_L
0.1µF
R_SET
TxDAC
IOUTN+
IAMP
0 TO –7.5dB
0 TO –12dB _IOUTN–
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−.
AD9869
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).
0.1µF
R_SET
AVDD
R_CM
0.1µF
I_BIAS = 2 × N × I
TxDAC
IOUTN+
T:1
IAMP
IOUT_PK
R_L
0 TO –7.5dB
0 TO –12dB ^IOUTN–
IOUT_PK = N × I
POUT_PK = (IOUT_PK)² × T² × R_L
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)
Rev. A | Page 23 of 36