Datasheet

12
LTC1666/LTC1667/LTC1668
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
.
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:
Figure 2. Adjusting the Full-Scale Current of
the LTC1666/LTC1667/LTC1668 with a DAC
APPLICATIO S I FOR ATIO
WUUU
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.)
+
–
I
REFIN
2.5V
REFERENCE
R
SET
1.9k
REF
0.1µF
1/2 LTC1661
5V
1666/7/8 F03
LTC1666/
LTC1667/
LTC1668