datasheetbank_Logo
Integrated circuits, Transistor, Semiconductors Search and Datasheet PDF Download Site

AD9869 View Datasheet(PDF) - Analog Devices

Part Name
Description
View to exact match
AD9869 Datasheet PDF : 36 Pages
First Prev 21 22 23 24 25 26 27 28 29 30 Next Last
AD9869
TxDAC AND IAMP ARCHITECTURE
The Tx path contains a TxDAC with a current amplifier, IAMP.
The TxDAC reconstructs the output of the interpolation filter
and sources a differential current output that can be directed to
an external load or fed into the IAMP for further amplification.
The TxDAC and IAMP peak current outputs are digitally
programmable over a 0 dB to −7.5 dB and 0 dB to −19.5 dB
range, respectively, in 0.5 dB increments. Note that this assumes
default register settings for Register 0x10 and Register 0x11.
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. For power-sensitive applications requiring the
highest Tx power efficiency, the TxDAC 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.
Most applications requiring higher peak signal powers (up to
17 dBm) should use the IAMP. The IAMP can be configured
as a current source for loads having a well-defined impedance
(50 Ω or 75 Ω systems).
Figure 21 shows the equivalent schematic of the TxDAC and
IAMP. The TxDAC provides a differential current output
appearing at IOUTP+ and IOUTP−. The TxDAC can also be
modeled as a differential current source generating a signal-
dependent ac current, when ΔIS has a peak current of I along
with two dc current sources, sourcing a standing current equal
to I. The full-scale output current, IOUTP_FS, is equal to the
sum of these standing current sources (IOUTP_FS = 2 × I).
RSET 0.1µF
REFADJ
REFIO
I + ΔI IOUTP+
I – ΔI IOUTP–
TxDAC
I
I
±ΔIS
IOFF1
IOFF1
xN
xN
IAMP
Figure 21. Equivalent Schematic of TxDAC and IAMP
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 IOUTP_FS 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
connected to the IOUTP pins, but it can be directed to the
IAMP by clearing Bit 0 of Register 0x0E. As a result, the
IOUTP pins must remain completely open if the IAMP is to
be used. The IAMP consists of programmable current mirrors
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. The maximum peak current per output
is 100 mA and occurs when the TxDAC standing current, I, is
set for 12.5 mA (IOUTP_FS = 25 mA).
Because the current mirrors consist of NMOS devices, they sink
current. Therefore, each output pin requires a dc current path to
a positive supply. The voltage output of each output pin is
allowed to swing between 0.5 V and 3.9 V. Lastly, both the
standing current, I, and the ac current, ΔIS, from the TxDAC are
amplified by the gain factor (N) with the total standing current
drawn from the positive supply being equal to
2 × (N) × I
(2)
Programmable current sources, IOFF1 via Register 0x12, can be
used to improve the linearity performance under certain
conditions by increasing their signal-to-standing current ratios.
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 dBFS 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 current consumption.
Table 18. SPI Registers for TxDAC and IAMP
Address (Hex) Bit Description
0x0E
0 TxDAC output.
0x10
7 Enable current mirror gain settings.
2:0 Primary path NMOS gain of 0 to 4
with ∆ = 1.
0x12
2:0 IOFF1 standing current.
Rev. 0 | Page 22 of 36
 

Share Link: 

datasheetbank.com [ Privacy Policy ] [ Request Datasheet ] [ Contact Us ]