AD9866 Analog Devices, AD9866 Datasheet - Page 30

• Manufacturer Part Number: AD9866
• Manufacturer: Analog Devices

Specifications of AD9866

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 80MSPS
• Interface: Nibble
• Analog Input Type: Diff-Uni
• Ain Range: 6.3 V p-p,8 mV p-p
• Adc Architecture: Pipelined
• Pkg Type: CSP

AD9866
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.
Table 19. SPI Registers for TxDAC and IAMP
Address (Hex)
0x0E
0x10
0x11
0x12
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 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 7.5 dB range in 0.5 dB increments, and
the IAMP providing 12 dB range in 6 dB increments. As a result,
the IAMP’s composite 19.5 dB span is valid only if Register 0x10
remains at its default setting of 0x44. 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 63 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
IOUT+ to IOUT−) is equal to 2 × I × (R
and R
power being delivered to R
L
= R
S
= 50 Ω, V p-p is equal to 0.5 V with 1 dBm of peak
Bit
(0)
(7)
(6:4)
(3)
(2:0)
(7)
(6:4)
(3)
(2:0)
(6:4)
(2:0)
L
Description
TxDAC output
Enable current mirror gain settings
Secondary path first stage gain of 0
to 4 with ∆ = 1
Not used
Primary path NMOS gain of 0 to 4
with ∆ = 1
Don’t care
Secondary path second stage gain of
0 to 1.5 with ∆ = 0.25
Not used
Secondary path third stage gain of 0
to 5 with ∆ = 1
IOFF2, secondary path standing
current
IOFF1, primary path standing current
and 1 dBm being dissipated in R
L
//R
S
). With I = 10 mA
L
(and
Rev. B | Page 30 of 48
S
.
The TxDAC is capable of delivering up to 10 dBm peak power
to a load, R
current, one must increase V p-p across IOUTP+ and IOUTP−
by increasing one or more of the following parameters: R
possible), and/or the turns ratio, N, of transformer. For exam-
ple, the removal of R
transformer in the previous example 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−. See Figure 27 through Figure 38 for performance
plots on the TxDAC’s ac performance. Optimum distortion
performance can typically be achieved by:
•
•
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 as
shown in Figure 64 for loads remaining relatively constant. In
this mode, the primary path mirrors should be used to deliver
the signal-dependent current to the load via a center-tapped
transformer, because it provides the best linearity performance.
Because the mirrors exhibit a high output impedance, they can
be easily back-terminated (if required).
For peak signal currents (IOUT
path mirror gain should be used for optimum distortion
performance and power efficiency. The primary path’s gain
should be set to 4, with the secondary path’s gain stages set to 0
(Register 0x10 = 0x84). The TxDAC’s standing current, I, can be
set between 2.5 mA and 12.5 mA with the IOUTP outputs left
open. The IOUTN outputs should be connected to the transformer,
with the IOUTG (and IOUTP)
Limiting the peak positive V
avoid onset of TxDAC’s output compression. (TxDAC’s
voltage compliance is around 1.2 V.)
Limiting V p-p seen at IOUTP+ and IOUTP− to less
than 1.6 V.
Figure 63. TxDAC Output Directly via Center-Tap Transformer
0.1μF
L
. To increase the peak power for a fixed standing
TxDAC
0 TO –7.5dB
R
S
SET
and the use of a 2:1 impedance ratio
PK
R
S
up to 50 mA), only the primary
0 TO –12dB
IOUTP+
IAMP
and V
1:1
IOUTG–
IOUTN+
IOUTG+
IOUTN–
IOUTP
−
R
to 0.8 V to
L
S
, R
L
(if