LTC1666CG datasheet(14/24 Pages) LINER

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LTC1666/LTC1667/LTC1668

APPLICATIO S I FOR ATIO

Operating with Reduced Output Currents

The LTC1666/LTC1667/LTC1668 are specified to operate

with full-scale output current, IOUTFS, from the nominal

10mA down to 1mA. This can be useful to reduce power

dissipation or to adjust full-scale value. However, the DC

and AC accuracy is specified only at IOUTFS = 10mA, and

DC and AC accuracy will fall off significantly at lower IOUTFS

values. At IOUTFS = 1mA, the LTC1668 INL and DNL

typically degrade to the 14-bit to 13-bit level, compared to

16-bit to 15-bit typical accuracy at 10mA IOUTFS. Increas-

ing IOUTFS from 1mA, the accuracy improves rapidly,

roughly in proportion to 1/IOUTFS. Note that the AC perfor-

mance (SFDR) is affected much more by reduced IOUTFS

than it is by reduced digital amplitude (see Typical Perfor-

mance Characteristics). Therefore it is usually better to

make large gain adjustments digitally, keeping IOUTFS

equal to 10mA.

Output Configurations

Based on the specific application requirements, the

LTC1666/LTC1667/LTC1668 allow a choice of the best of

several output configurations. Voltage outputs can be

generated by external load resistors, transformer coupling

or with an op amp I-to-V converter. Single-ended DAC

output configurations use only one of the outputs, prefer-

ably IOUT A, to produce a single-ended voltage output.

Differential mode configurations use the difference be-

tween IOUT A and IOUT B to generate an output voltage,

VDIFF, as shown in equation 11. Differential mode gives

much better accuracy in most AC applications. Because

the DAC chip is the point of interface between the digital

input signals and the analog output, some small amount

of noise coupling to IOUT A and IOUT B is unavoidable. Most

of that digital noise is common mode and is canceled by

the differential mode circuit. Other significant digital noise

components can be modeled as VREF or IOUTFS noise. In

single-ended mode, IOUTFS noise is gone at zero scale and

is fully present at full scale. In differential mode, IOUTFS

noise is cancelled at midscale input, corresponding to zero

analog output. Many AC signals, including broadband and

multitone communications signals with high peak to aver-

age ratios, stay mostly near midscale.

Differential Transformer-Coupled Outputs

Differential transformer-coupled output configurations

usually give the best AC performance. An example is

shown in Figure 5. The advantages of transformer cou-

pling include excellent rejection of common mode distor-

tion and noise over a broad frequency range and conve-

nient differential-to-single-ended conversion with isola-

tion or level shifting. Also, as much as twice the power can

be delivered to the load, and impedance matching can be

accomplished by selecting the appropriate transformer

turns ratio. The center tap on the primary side of the

transformer is tied to ground to provide the DC current

path for IOUT A and IOUT B. For low distortion, the DC

average of the IOUT A and IOUT B currents must be exactly

equal to avoid biasing the core. This is especially impor-

tant for compact RF transformers with small cores. The

circuit in Figure 5 uses a Mini-Circuits T1-1T RF trans-

former with a 1:1 turns ratio. The load resistance on

IOUT A and IOUT B is equivalent to a single differential

resistor of 50

Ω, and the 1:1 turns ratio means the output

impedance from the transformer is 50

Ω. Note that the

load resistors are optional, and they dissipate half of the

output power. However, in lab environments or when

driving long transmission lines it is very desirable to have

a 50

Ω output impedance. This could also be done with a

Ω resistor at the transformer secondary, but putting

the load resistors on IOUT A and IOUT B is preferred since

it reduces the current through the transformer. At signal

frequencies lower than about 1MHz, the transformer core

size required to maintain low distortion gets larger, and at

some lower frequencies this becomes impractical.

Figure 5. Differential Transformer-Coupled Outputs

IOUT B

IOUT A

Ω

110

Ω

MINI-CIRCUITS

T1-1T

RLOAD

1666/7/8 F06

LTC1666/

LTC1667/

LTC1668

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