LTC1666CG datasheet(12/24 Pages) LINER

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LTC1666/LTC1667/LTC1668

Adjusting the Full-Scale Output

In Figure 2, a serial interfaced DAC is used to set IOUTFS.

The LTC1661 is a dual 10-bit VOUT DAC with a buffered

voltage output that swings from 0V to VREF.

DAC Transfer Function

The LTC1666/LTC1667/LTC1668 use straight binary digital

coding. The complementary current outputs, IOUTA andIOUT

B, sink current from 0 to IOUTFS. For IOUTFS = 10mA (nomi-

nal), IOUT 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). IOUT B is comple-

mentary to IOUTA. IOUTA and IOUTB are given by the following

formulas:

LTC1666:

IOUT A = IOUTFS • (DAC Code/4096)

(2)

IOUT B = IOUTFS • (4095 – DAC Code)/4096

(3)

LTC1667:

IOUT A = IOUTFS • (DAC Code/16384)

(4)

IOUT B = IOUTFS • (16383 – DAC Code)/16384

(5)

LTC1668:

IOUT A = IOUTFS • (DAC Code/65536)

(6)

IOUT B = IOUTFS • (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 IOUT A and

IOUT 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

VOUT A = IOUT A • RLOAD

(8)

VOUT B = IOUT B • RLOAD

(9)

The differential voltage is:

VDIFF = VOUT A – VOUT B

(10)

= (IOUT A – IOUT B) • (RLOAD)

Substituting the values found earlier for IOUT A, IOUT B and

IOUTFS (LTC1668):

VDIFF = {2 • DAC Code – 65535)/65536} • 8 •

(RLOAD/RSET) • (VREF)

(11)

From these equations some of the advantages of differen-

tial mode operation can be seen. First, any common mode

noise or error on IOUTA and IOUTB is cancelled. Second, the

signal power is twice as large as in the single-ended case.

Third, any errors and noise that multiply times IOUT A and

IOUT B, such as reference or IOUTFS 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 (RLOAD/RSET) appears in both the

differential and single-ended transfer functions. This means

that the Gain Error of the DAC depends on the ratio of

RLOAD to RSET, and the Gain Error tempco is affected by the

temperature tracking of RLOAD with RSET. Note also that

the absolute tempco of RLOAD is very critical for DC

nonlinearity. As the DAC output changes from 0mA to

10mA the RLOAD 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, IOUT A and IOUT B (see DAC Transfer

Function). The output impedance of IOUT A and IOUT B

(RIOUT A and RIOUT B) is typically 1.1kΩ to LADCOM. (See

Figure 3.)

–

IREFIN

2.5V

REFERENCE

RSET

1.9k

REF

0.1

µF

1/2 LTC1661

1666/7/8 F03

LTC1666/

LTC1667/

LTC1668

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