AD9869-EBZ Analog Devices Inc, AD9869-EBZ Datasheet - Page 23

• Manufacturer Part Number: AD9869-EBZ
• Description: BOARD EVAL FOR AD9869
• Manufacturer: Analog Devices Inc
• Type: ADC + DAC, Codec, Front End for RFr

Specifications of AD9869-EBZ

• Contents: Board
• For Use With/related Products: AD9869
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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 R
IOUTP+ to IOUTP−) is equal to 2 × I × (R
and R
power being delivered to R
The TxDAC is capable of delivering up to 10 dBm peak power
to a load, R
current, users must increase V p-p across IOUTP+ and IOUTP−
by increasing one or more of the following parameters: R
(if possible), and/or the turns ratio, N, of the transformer. For
example, removing the R
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−.
L
Figure 22. TxDAC Output Directly via Center-Tap Transformer
= R
S
L
= 50 Ω, V p-p is equal to 0.5 V with 1 dBm of peak
. To increase the peak power for a fixed standing
0.1µF
TxDAC
0 TO –7.5dB
R
SET
S
from Figure 22 and applying a 2:1
L
and 1 dBm being dissipated in R
R
S
0 TO –12dB
IAMP
L
||R
S
1:1
). With I = 10 mA
IOUTN+
IOUTN–
R
L
S
L
, R
(and
L
S
Rev. 0 | Page 23 of 36
.
Optimum distortion performance can typically be achieved by
performing both of the following:
•
•
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).
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, I
drawn by the IAMP. In addition, because I
independent, a series resistor should be inserted between
AVDD and the center-tap transformer to provide provisions
such that the IAMP common-mode voltage, V
reduced since its optimum linearity performance is sensitive to
both the Tx signal’s peak-to-rms characteristics as well as the
IAMP V
power dissipated in the IAMP alone is as follows:
0.1µF
Limiting the peak positive V
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.
P
IAMP
TxDAC
0 TO –7.5dB
CM
= 2 × N × I × V
. Note that the V
R
SET
Figure 23. Current-Mode Operation
0 TO –12dB
CM
IAMP
CM
bias should not exceed 3.3 V. The
IOUTN+
IOUTN–
IOUTP+
and V
IOUT
POUT
0.1µF
IOUT
BIAS
PK
PK
IOUTP
PK
= N × I
= (IOUT
remains signal
AVDD
CM
BIAS
R
, can be
−
I
CM
BIAS
T:1
to 0.8 V to
, that is
PK
AD9869
)
= 2 × N × I
2
× T
2
R
× R
L
L
(3)