LTC2704IGW-16#PBF【PDF 9/22 页】IC DAC 16BIT QUAD VOUT 44-SSOP

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LTC2704

2704fd

APPLICATIONS INFORMATION

Overview

The LTC2704 is a highly integrated device, greatly sim-

plifying design and layout as compared to a design using

multiple current output DACs and separate ampliers. A

similar design using four separate current output DACs

would require six precision op amps, compensation capaci-

tors, bypass capacitors for each amplier, several times as

much PCB area and a more complicated serial interface.

Still, it is important to avoid some common mistakes in

order to achieve full performance. DC752A is the evalu-

ation board for the LTC2704. It is designed to meet all

data sheet specications, and to allow the LTC2704 to be

integrated into other prototype circuitry. All force/sense

lines are available to allow the addition of current booster

stages or other output circuits.

The DC752A design is presented as a tutorial on properly

applying the LTC2704. This board shows how to properly

return digital and analog ground currents, and how to

compensate for small differences in ground potential

between the two banks of two DACs. There are other ways

to ground the LTC2704, but the one requirement is that

analog and digital grounds be connected at the LTC2704

by a very low impedance path. It is NOT advisable to split

the ground planes and connect them with a jumper or

inductor. When in doubt, use a single solid ground plane

rather than separate planes.

The LTC2704 does allow the ground potential of the DACs

to vary by ±300mV with respect to analog ground, allowing

compensation for ground return resistance.

Power Supply Grounding and Noise

LTC2704 V+ and V– pins are the supplies to all of the

output ampliers, ground sense ampliers and reference

inversion ampliers. These ampliers have good power

supply rejection, but the V+ and V– supplies must be free

from wideband noise. The best scheme is to prelter low

noise regulators such as the LT1761 (positive) and LT1964

(negative). Refer to Linear Technology Application Note

101, Minimizing Switching Regulator Residue in Linear

Regulator Outputs.

The LTC2704 VDD pin is the supply for the digital logic

and analog DAC switches and is very sensitive to noise. It

must be treated as an analog supply. The evaluation board

uses an LT1790 precision reference as the VDD supply to

minimize noise.

The GND pin is the return for digital currents and the

AGND pin is a bias point for internal analog circuitry. Both

of these pins must be tied to the same point on a quiet

ground plane.

Each DAC has a separate ground sense pin that can be used

to compensate for small differences in ground potential

within a system. Since DACs A and B are associated with

REF1 and DACs C and D are associated with REF2, the

grounds must be grouped together as follows:

AGNDA, AGNDB and REFG1 tied together (“GND1” on

DC752A)

AGNDC, AGNDD and REFG2 tied together (“GND2” on

DC752A)

This scheme allows compensation for ground return IR

drops, as long as the resistance is shared by both DACs

in a group. This implies that the ground return for DACs

A and B must be as close as possible, and GND1 must

be connected to this point through a low current, low

resistance trace. (Similar for DACs C and D.)

Figure 3 shows the top layer of the evaluation board. The

GND1 trace connects REFG1, AGNDA, AGNDB and the

ground pin of the LT1236 precision reference (U4.) This

point is the ground reference for DACs A and B. The GND2

trace connects REFG2, AGNDC, AGNDD and the ground pin

of the other LT1236 precision reference (U5). This point

is the ground reference for DACs C and D.

Voltage Reference

A high quality, low noise reference such as the LT1236

or LT1027 must be used to achieve full performance.

The ground terminal of this reference must be connected

directly to the common ground point. If GND1 and GND2

are separate, then two references must be used.

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