LTC5599 datasheet(21/42 Pages) LINER

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LTC5599

5599f

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applicaTions inForMaTion

generation but for gain and offset setting only, because

there are no reconstruction filters between the DACs and

the mixer core, and there is only indirect access between

the DAC values and the register settings. The following

functions are implemented in this way:

• Coarse digital gain control with 1dB steps

• Fine digital gain control with 0.1dB steps

• Gain-temperature correction

• DC offset adjustment in the I-channel

• DC offset adjustment in the Q-channel

• I/Q gain balance control

• Disable Q-channel

• Continuous variable gain control

Coarse Digital Gain Control (DG) with 1dB Steps

(Register 0x01)

Twenty digital gain positions 1dB apart are implemented

by hardwiring a corresponding DAC code for all four

DACs. The coarse digital gain is set by writing to the five

least-significant bits in register 0x01, see Table 10 and 11.

The gain is the highest for code 00000 (code 0 = 0dB, DG

= 0) and the lowest for code 10011 (code 19 = –19dB,

DG = –19). Note that the gain 0dB set by the digital gain

control is not the same as the voltage gain of the part.

The remaining 12 codes (decimal 20 to 31) are reserved.

The digital gain in dB equals minus the decimal value writ-

ten into the 5 least-significant bits of the gain register. The

formula relating the modulator gain G(in V/V) relative to

the maximum conversion gain therefore equals:

G(V/V) = 10(DG/20)

Fine Digital Gain Control(FDG) with 0.1dB Steps and

Gain-Temperature Correction (Register 0x07)

Sixteen digital gain positions about 0.1dB apart can be set

directlyusingthefourleast-significantbitsinregister0x07

combined with bit 2 = 1 in register 0x08 (TEMPCORR = 1).

For coarse digital gain settings code 9 and higher some or

more subsequent codes of the fine digital gain positions

may be the same due to the limited resolution of the 11-

bit DACs. The main purpose of these 0.1dB gain steps is

to implement an automatic gain/temperature correction

which can be activated by setting TEMPCORR = 1. In that

case, the input of the fine digital gain control will be the

on-chipthermometer.Theon-chipthermometergenerates

a 4-bit digital code with code 0 corresponding to –30°C

and code 15 corresponding to 120°C and 10°C spacing

between the codes. The on-chip thermometer output code

can be updated continuous (by clearing TEMPUPDT, bit 7

in register 0x01, see Table 10) or can be updated by bring-

ing the external pin TTCK from low to high (and setting

TEMPUPTD = 1). In case of continuous update the code

willbeanasynchronousupdatewheneverthetemperature

crosses a certain threshold. In some cases it is desired to

prevent a gain update to happen in the middle of a data

frame. In that case, the gain/temperature update can be

synchronized using the TTCK pin for example at the begin-

ning or end of a data frame. The on-chip temperature can

be read back by reading register 0x1F (TEMP[3:0]).The

decimal value of TEMP[3:0] is given by:

TEMP[3:0] = round(T/10) + 3

with T the actual on-chip temperature in °C. It’s accuracy

is about ±10°C. TEMP[3:0] defaults to 7 after an EN low

to high transition with TEMPUPDT = 1. Switching from

TEMPUPDT = 0 to TEMPTUPDT = 1, TEMP[3:0] indicates

the temperature during the last time TTCK went from low

to high. Note that the actual on-chip temperature cannot

be read if TEMPCORR = 1 or when TEMPUPDT = 1 without

toggling TTCK.

Analog Gain Control

The LTC5599 supports analog control of the conversion

gain through a voltage applied to VCTRL (pin 1). The gain

can be controlled downward from the digital gain setting

(DG) programmed in register 0x01. In order to minimize

distortion in the RF output signal the AGCTRL bit (bit 6 in

not used, VCTRL should be connected to VCC and AGCTRL

set to 0; this saves about 2.58mA of supply current. The

typical usable gain control range is from 0.9V to 3.3V.

Setting VCTRL to a voltage lower than VCC with AGCTRL

= 0 significantly impairs the linearity of the RF output

signal and lowers the VCTRL response time. A simplified

schematic is shown in Figure 1.

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