AD9869BCPZ datasheet(23/37 Pages) AD

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AD9869

Rev. A | Page 22 of 36

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).

±∆IS

TxDAC

REFADJ

REFIO

IOUTP+

IOUTP–

I + ∆I

I – ∆I

IOFF1

RSET 0.1µF

IOFF1

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

Table 18. SPI Registers for TxDAC and IAMP

Address (Hex)

Bit

Description

0x0E

TxDAC output.

0x10

Enable current mirror gain settings.

2:0

Primary path NMOS gain of 0 to 4

with ∆ = 1.

0x12

2:0

IOFF1 standing current.

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