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TXPGA CONTROL
The AD9866 also contains a digital PGA in the Tx path
distributed between the TxDAC and IAMP. The TxPGA is used
to control the peak current from the TxDAC and IAMP over a
7.5 dB and 19.5 dB span, respectively, with 0.5 dB resolution. A
6-bit word is used to set the TxPGA attenuation according to
the mapping shown in Figure 58. The TxDAC gain mapping is
applicable only when Bit 0 of Register 0x0E is set, and only the
four LSBs of the 6-bit gain word are relevant.
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
–11
–12
–13
–14
–15
IAMPs IOUTN AND IOUTG
–16
OUTPUTS HAS 19.5dB RANGE
–17
–18
–19
–20
0
8
16
24
6-BIT DIGITAL CODE (Decimal Equivalent)
Figure 58. Digital Gain Mapping of TxPGA
TxDACs IOUTP OUTPUT
HAS 7.5dB RANGE
32
40
48
56
64
Rev. A | Page 27 of 48
The TxPGA register can be updated via the PGA[5:0] port or
SPI port. The first method should be considered for fast updates
of the TxPGA register. Its operation is similar to the description
in the RxPGA Control section. The SPI port allows direct update
and readback of the TxPGA register via Register 0x0A with an
update rate limited to 1.6 MSPS (SCLK = 32 MHz). Bit 6 of
Register 0x0A must be set for a read or write operation. Table 17
lists the SPI registers pertaining to the TxPGA. The TxPGA
control register default setting is for minimum attenuation
(0 dBFS) with the PGA[5:0] port disabled for Tx gain control.
Table 17. SPI Registers TxPGA Control
Address (Hex)
Bit
0x0A
(6)
(5:0)
0x0B
(6)
(5)
0x0E
(0)
AD9866
Description
Enable TxPGA update via SPI
TxPGA gain code
Select TxPGA via PGA[5:0]
Select RxPGA via PGA[5:0]
TxDAC output (IAMP disabled)
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