AD9868BCPZ datasheet(21/37 Pages) AD

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AD9868

Rev. 0 | Page 20 of 36

TxPGA CONTROL

The AD9868 also contains a digital PGA in the Tx path distri-

buted 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 17. The TxDAC gain mapping is

applicable only when Bit 0 of Register 0x0E is set, and only

when the 4 LSBs of the 6-bit gain word are relevant.

6-BIT DIGITAL CODE (Decimal Equivalent)

–20

–16

–18

–14

–12

–10

–8

–6

–2

–4

–1

–17

–19

–15

–13

–11

–9

–7

–3

–5

TxDACs IOUTP OUTPUT

HAS 7.5dB RANGE

IAMPs IOUTN OUTPUT

HAS 19.5dB RANGE

Figure 17. Digital Gain Mapping of TxPGA

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 R

xPGA 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 16 lists the SPI registers pertaining to the TxPGA. The

TxPGA control register default setting is for minimum attenua-

tion (0 dBFS) with the PGA[5:0] port disabled for Tx gain control.

Table 16. SPI Registers TxPGA Control

Address (Hex)

Bit

Description

0x0A

Enable TxPGA update via SPI.

5:0

TxPGA gain code.

0x0B

Select TxPGA via PGA[5:0].

Select RxPGA via PGA[5:0].

0x0E

TxDAC output (IAMP disabled).

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