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LTC2000

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OPERATION

Table 2. Full-Scale Gain Adjustment

ADDRESS

BIT

NAME

DESCRIPTION

0x09

[5:0]

GAIN_ADJ

GAIN_ADJ (HEX)

GAIN_ADJ (DECIMAL)

GAIN ADJUSTMENT

FULL-SCALE CURRENT

(RFSADJ = 500Ω, VREFIO = 1.25V)

0x1F

+31

89.2%

35.68mA

0x1E

+30

89.5%

35.80mA

—

0x01

99.6%

39.84mA

0x00

100.0%

40.00mA

0x3F

–1

100.4%

40.16mA

—

0x21

–31

113.8%

45.51mA

0x20

–32

114.3%

45.71mA

Note: Register 0x09 resets to 0x00 (default).

DAC Transfer Function

The LTC2000 contains an array of current sources that

are steered through differential switches to either IOUTP

or IOUTN, depending on the DAC code programmed

through the LVDS parallel interface. The LTC2000 uses a

16-/14-/11-bit two’s complement DAC code. The comple-

mentary current outputs, IOUTP and IOUTN, source current

from 0mA to IOUTFS. For IOUTFS = 40mA (nominal), IOUTP

swings from 0mA (for zero-scale DAC code) to 40mA (for

full-scale DAC code). IOUTN is complementary to IOUTP.

When the DAC code is set to mid-scale (all zeros), IOUTFS

is evenly divided between IOUTP and IOUTN. IOUTP and

IOUTN are given by the following formulas:

LTC2000-16:

IOUTP = IOUTFS • (CODE + 32768)/65536 + IOUTCM

IOUTN = IOUTFS • (32768 – CODE – 1)/65536 + IOUTCM

LTC2000-14:

IOUTP = IOUTFS • (CODE + 8192)/16384 + IOUTCM

IOUTN = IOUTFS • (8192 – CODE – 1/4)/16384 + IOUTCM

LTC2000-11:

IOUTP = IOUTFS • (CODE + 1024)/2048 + IOUTCM

IOUTN = IOUTFS • (1024 – CODE – 1/32)/2048 + IOUTCM

The DAC code ranges from –2N-1 to 2N-1 – 1, with N being

the DAC resolution (16/14/11). IOUTCM is a small, constant

common-mode output current that is equal to approxi-

mately 0.2% full-scale, or 80µA for IOUTFS = 40mA.

The LTC2000 differential output currents typically drive a

resistive load either directly or drive an equivalent resistive

load through a transformer (see the Output Configurations

section). The voltage outputs generated by the IOUTP and

IOUTN outputs currents are then:

VOUTP = IOUTP • RLOAD

VOUTN = IOUTN • RLOAD

VDIFF = VOUTP – VOUTN = (IOUTP-IOUTN) • RLOAD

Substituting the values above gives:

LTC2000-16:

VDIFF = VREFIO • (RLOAD/RFSADJ) • (2 • CODE + 1)/4096

LTC2000-14:

VDIFF = VREFIO • (RLOAD/RFSADJ) • (2 • CODE + 1/4)/1024

LTC2000-11:

VDIFF = VREFIO • (RLOAD/RFSADJ) • (2 • CODE + 1/32)/128

Note that the gain of the DAC depends on the ratio of

RLOAD to RFSADJ, and the gain error tempco is affected by

the temperature tracking of RLOAD with RFSADJ.

Analog Outputs (IOUTP/N)

The two complementary analog outputs (IOUTP/N) have

low output capacitance that, with appropriate RLOAD val-

ues, can achieve high output bandwidths of 2.1GHz. The

analog outputs also have an internal impedance of 50Ω

to GND that will affect the calculation of RLOAD and the

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