LTC1666_15 Datasheet, PDF (12/24 Pages) Linear Technology – 12-Bit, 14-Bit, 16-Bit, 50Msps DACs

LTC1666/LTC1667/LTC1668
APPLICATIO S I FOR ATIO
Adjusting the Full-Scale Output
In Figure 2, a serial interfaced DAC is used to set I_OUTFS.
The LTC1661 is a dual 10-bit V_OUT DAC with a buffered
voltage output that swings from 0V to V_REF.
5V
REF
1/2 LTC1661
0.1µF
R_SET
1.9k
2.5V
REFERENCE
I_REFIN
LTC1666/
LTC1667/
LTC1668
+
–
1666/7/8 F03
Figure 2. Adjusting the Full-Scale Current of
the LTC1666/LTC1667/LTC1668 with a DAC
DAC Transfer Function
The LTC1666/LTC1667/LTC1668 use straight binary digital
coding. The complementary current outputs, I_{OUT A} and I_OUT
B^{, sink current from 0 to I}OUTFS^{. For I}OUTFS ^{= 10mA (nomi-}
nal), I_{OUT A} swings from 0mA when all bits are low (e.g.,
Code␣ = 0) to 10mA when all bits are high (e.g., Code = 65535
for LTC1668) (decimal representation). I_{OUT B} is comple-
mentary to I_{OUT A}. I_{OUT A} and I_{OUT B} are given by the following
formulas:
LTC1666:
I_{OUT A} = I_OUTFS • (DAC Code/4096)
(2)
I_{OUT B} = I_OUTFS • (4095 – DAC Code)/4096
(3)
LTC1667:
I_{OUT A} = I_OUTFS • (DAC Code/16384)
(4)
I_{OUT B} = I_OUTFS • (16383 – DAC Code)/16384 (5)
LTC1668:
I_{OUT A} = I_OUTFS • (DAC Code/65536)
(6)
I_{OUT B} = I_OUTFS • (65535 – DAC Code)/65536 (7)
In typical applications, the LTC1666/LTC1667/LTC1668
differential output currents either drive a resistive load
directly or drive an equivalent resistive load through a
transformer, or as the feedback resistor of an I-to-V
converter. The voltage outputs generated by the I_{OUT A} and
I_{OUT B} output currents are then:
V_{OUT A} = I_{OUT A} • R_LOAD
(8)
V_{OUT B} = I_{OUT B} • R_LOAD
(9)
The differential voltage is:
V_DIFF = V_{OUT A} – V_{OUT B}
(10)
= (I_{OUT A} – I_{OUT B}) • (R_LOAD)
Substituting the values found earlier for I_{OUT A}, I_{OUT B} and
I_OUTFS (LTC1668):
V_DIFF = {2 • DAC Code – 65535)/65536} • 8 •
(R_LOAD/R_SET) • (V_REF)
(11)
From these equations some of the advantages of differen-
tial mode operation can be seen. First, any common mode
noise or error on I_{OUT A} and I_{OUT B} is cancelled. Second, the
signal power is twice as large as in the single-ended case.
Third, any errors and noise that multiply times I_{OUT A} and
I_{OUT B}, such as reference or I_OUTFS noise, cancel near
midscale, where AC signal waveforms tend to spend the
most time. Fourth, this transfer function is bipolar; e.g. the
output swings positive and negative around a zero output
at mid-scale input, which is more convenient for AC
applications.
Note that the term (R_LOAD/R_SET) appears in both the
differential and single-ended transfer functions. This means
that the Gain Error of the DAC depends on the ratio of
R_LOAD to R_SET, and the Gain Error tempco is affected by the
temperature tracking of R_LOAD with R_SET. Note also that
the absolute tempco of R_LOAD is very critical for DC
nonlinearity. As the DAC output changes from 0mA to
10mA the R_LOAD resistor will heat up slightly, and even a
very low tempco can produce enough INL bowing to be
significant at the 16-bit level. This effect disappears with
medium to high frequency AC signals due to the slow
thermal time constant of the load resistor.
Analog Outputs
The LTC1666/LTC1667/LTC1668 have two complemen-
tary current outputs, I_{OUT A} and I_{OUT B} (see DAC Transfer
Function). The output impedance of I_{OUT A} and I_{OUT B}
(R_{IOUT A} and R_{IOUT B}) is typically 1.1kΩ to LADCOM. (See
Figure 3.)
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