LTC1666CG datasheet(13/24 Pages) LINER

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LTC1666/LTC1667/LTC1668

APPLICATIO S I FOR ATIO

LADCOM

The LADCOM pin is the common connection for the

internal DAC attenuator ladder. It usually is tied to analog

ground, but more generally it should connect to the same

potential as the load resistors on IOUT A and IOUT B. The

LADCOM pin carries a constant current to VSS of approxi-

mately 0.32 • (IOUTFS), plus any current that flows from

IOUT A and IOUT B through the RIOUT A and RIOUT B resistors.

Output Compliance

The specified output compliance voltage range is

±1V. The

DC linearity specifications, INL and DNL, are trimmed and

guaranteed on IOUT A into the virtual ground of an

I-to-V converter, but are typically very good over the full

output compliance range. Above 1V the output current will

start to increase as the DAC current steering switch

impedance decreases, degrading both DC and AC linear-

ity. Below –1V, the DAC switches will start to approach the

transition from saturation to linear region. This will de-

grade AC performance first, due to nonlinear capacitance

and increased glitch impulse. AC distortion performance

is optimal at amplitudes less than

±0.5VP-P on IOUT A and

IOUT B due to nonlinear capacitance and other large-signal

effects. At first glance, it may seem counter-intuitive to

decrease the signal amplitude when trying to optimize

SFDR. However, the error sources that affect AC perfor-

mance generally behave as additive currents, so decreas-

ing the load impedance to reduce signal voltage amplitude

will reduce most spurious signals by the same amount.

Figure 4. AC Characterization Setup (LTC1668)

–

16-BIT

HIGH SPEED

DAC

HP1663EA

LOGIC ANALYZER WITH

PATTERN GENERATOR

VSS

VDD

–5V

LADCOM

AGND DGND

CLK

DB15

DB0

DIGITAL

DATA

CLK

1666/7/8 F05

IOUT A

0.1

µF

LTC1668

IREFIN

REFOUT

COMP1

COMP2

0.1

µF

0.1

µF

OUT 1 OUT 2

0.1

µF

RSET

IOUT B

HP8110A DUAL

PULSE GENERATOR

LOW JITTER

CLOCK SOURCE

CLK

Ω

0.1

µF

Ω

TO HP3589A

SPECTRUM

ANALYZER

Ω INPUT

110

Ω

MINI-CIRCUITS

T1–1T

2.5V

REFERENCE

Figure 3. Equivalent Analog Output Circuit

RIOUT B

1.1k

5pF

–5V

1666/7/8 F04

RIOUT A

1.1k

LADCOM

IOUT A

IOUT B

VSS

52.3

Ω

52.3

Ω

LTC1666/LTC1667/LTC1668

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