LTC2656

21

2656f

load conditions. The measured change in output voltage

per milliampere of forced load current change is expressed

in LSB/mA.

DC output impedance is equivalent to load regulation, and

may be derived from it by simply calculating a change in

units from LSB/mA to Ohms. The amplifiers’ DC output

impedance is 0.04Ω when driving a load well away from

the rails.

When drawing a load current from either rail, the output

voltage headroom with respect to that rail is limited by

the 30Ω typical channel resistance of the output devices;

e.g., when sinking 1mA, the minimum output voltage =

30Ω • 1mA = 30mV. See the graph Headroom at Rails vs

Output Current in the Typical Performance Characteristics

section.

The amplifiers are stable driving capacitive loads of up

to 1000pF.

Board Layout

The excellent load regulation and DC crosstalk performance

of these devices is achieved in part by keeping “signal”

and “power” grounds separate.

The PC board should have separate areas for the analog

and digital sections of the circuit. This keeps digital signals

away from sensitive analog signals and facilitates the use of

separate digital and analog ground planes which have mini-

mal capacitive and resistive interaction with each other.

Digital and analog ground planes should be joined at only

one point, establishing a system star ground as close to

the device’s ground pin as possible. Ideally, the analog

ground plane should be located on the component side of

the board, and should be allowed to run under the part to

shield it from noise. Analog ground should be a continuous

and uninterrupted plane, except for necessary lead pads

and vias, with signal traces on another layer.

The GND pin functions as a return path for power supply

currents in the device and should be connected to analog

ground. The REFLO pin should be connected to the system

star ground. Resistance from the REFLO pin to the system

star ground should be as low as possible.

Rail-to-Rail Output Considerations

In any rail-to-rail voltage output device, the output is limited

to voltages within the supply range.

Since the analog outputs of the device cannot go below

ground, they may limit the lowest codes as shown in Fig-

ure 3b. Similarly, limiting can occur in external reference

are shown in Figure 3c. No full-scale limiting can occur if

Offset and linearity are defined and tested over the region

of the DAC transfer function where no output limiting can

occur.

OPERATION

2656 F03

INPUT CODE

(3b)

OUTPUT

VOLTAGE

NEGATIVE

OFFSET

0V

32,768

0

65,535

INPUT CODE

OUTPUT

VOLTAGE

(3a)

(3c)

INPUT CODE

OUTPUT

VOLTAGE

POSITIVE

FSE

Figure 3. Effects of Rail-to-Rail Operation on a DAC Transfer Curve. (3a) Overall Transfer Function (3b) Effect of

Negative Offset for Codes Near Zero-Scale (3c) Effect of Positive Full-Scale Error for Codes Near Full-Scale