LTC5599IUFPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5599IUFPBF' PDF : 42 Pages View PDF
LTC5599
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)
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-chip thermometer. The on-chip thermometer generates
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
will be an asynchronous update whenever the temperature
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:
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.
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.
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
directly using the four least-significant bits in register 0x07
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
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
register 0x01) should be set to 1. If analog gain control is
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.
5599f
For more information www.linear.com/LTC5599
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