AD9865
clearing Bit 0 of Register 0x0E. As a result, the IOUTP pins
Applications demanding the highest spectral performance
and/or lowest power consumption can use the TxDAC output
directly. The TxDAC is capable of delivering a peak signal
power-up to 10 dBm while maintaining respectable linearity
performance, as shown in Figure 27 through Figure 38. For
power-sensitive applications requiring the highest Tx power
efficiency, the TxDAC’s full-scale current output can be reduced
to as low as 2 mA, and its load resistors sized to provide a
suitable voltage swing that can be amplified by a low-power op-
amp-based driver.
must remain completely open, if the IAMP is to be used. The
IAMP contains two sets of current mirrors that are used to
replicate the TxDAC’s current output with a selectable gain. The
first set of current mirrors is designated as the primary path,
providing a gain factor of N that is programmable from 0 to 4 in
steps of 1 via Bits 2:0 of Register 0x10 with a default setting of
N = 4. Bit 7 of this register must be set to overwrite the default
settings of this register. This differential path exhibits the best
linearity performance (see Figure 42) and is available at the
IOUTN+ and IOUTN− pins. The maximum peak current per
output is 100 mA and occurs when the TxDAC’s standing
current, I, is set for 12.5 mA (IOUTFS = 25 mA).
Most applications requiring higher peak signal powers (up to
23 dBm) should consider using the IAMP. The IAMP can be
configured as a current source for loads having a well defined
impedance (50 Ω or 75 Ω systems), or a voltage source (with the
addition of a pair of npn transistors) for poorly defined loads
having varying impedance (such as power lines).
The second set of current mirrors is designated as the secon-
dary path providing a gain factor of G that is programmable
from 0 to 36 via Bits 6:4 of Register 0x10 and Bits 6:0 of Register
0x11 with a default setting of G = 12. This differential path is
intended to be used in the voltage mode configuration to bias
the external npn transistors, because it exhibits degraded
linearity performance (see Figure 43) relative to the primary
path. It is capable of sinking up to 180 mA of peak current into
either its IOUTG+ or IOUTG− pins. The secondary path
actually consists of three gain stages (G1, G2, and G3), which
are individually programmable as shown in Table 19. While
many permutations may exist to provide a fixed gain of G, the
linearity performance of a secondary path remains relatively
independent of the various individual gain settings that are
possible to achieve a particular overall gain factor.
Figure 62 shows the equivalent schematic of the TxDAC and
IAMP. The TxDAC provides a differential current output
appearing at IOUTP+ and IOUTP−. It can be modeled as a
differential current source generating a signal-dependent ac
current, when ∆IS has a peak current of I along with two dc
current sources, sourcing a standing current equal to I. The full-
scale output current, IOUTFS, is equal to the sum of these
standing current sources (IOUTFS = 2 × I).
TxDAC
Both sets of mirrors sink current, because they originate from
NMOS devices. Therefore, each output pin requires a dc current
path to a positive supply. Although the voltage output of each
output pin can swing between 0.5 V and 7 V, optimum ac per-
formance is typically achieved by limiting the ac voltage swing
with a dc bias voltage set between 4 to 5 V. Lastly, both the
standing current, I, and the ac current, ∆IS, from the TxDAC are
amplified by the gain factor (N and G) with the total standing
current drawn from the positive supply being equal to
I
I
±∆I
S
REFADJ
REFIO
R
0.1µF
SET
I
I
OFF2
OFF2
I
I
OFF1
OFF1
IOUTP+
IOUTP–
I + ∆I
I – ∆I
xN
xN
xG
xG
IAMP
2 × (N = G) × I
Figure 62. Equivalent Schematic of TxDAC and IAMP
Programmable current sources IOFF1 and IOFF2 via Register 0x12
can be used to improve the primary and secondary path
mirrors’ linearity performance under certain conditions by
increasing their signal-to-standing current ratio. This feature
provides a marginal improvement in distortion performance
under large signal conditions when the peak ac current of the
reconstructed waveform frequently approaches the dc standing
current within the TxDAC (0 to −1 dBFS sine wave) causing the
internal mirrors to turn off. However, the improvement in
distortion performance diminishes as the crest factor (peak-to-
rms ratio) of the ac signal increases. Most applications can
disable these current sources (set to 0 mA via Register 0x12) to
reduce the IAMP’s current consumption.
The value of I is determined by the RSET value at the REFADJ
pin along with the Tx path’s digital attenuation setting. With
0 dB attenuation, the value of I is
I = 16 × (1.23/RSET
)
(1)
For example, an RSET value of 1.96 kΩ results in I equal to
10.0 mA with IOUTFS equal to 20.0 mA. Note that the REFIO
pin provides a nominal band gap reference voltage of 1.23 V
and should be decoupled to analog ground via a 0.1 µF
capacitor.
The differential current output of the TxDAC is always con-
nected to the IOUTP pins, but can be directed to the IAMP by
Rev. A | Page 29 of 48
ADI [ ADI ]
