9

LTC1661

Voltage Outputs

Each of the rail-to-rail output amplifiers contained in the

LTC1661 can typically source or sink up to 5mA

of either supply when unloaded and have an equivalent

output resistance of 85

Ω (typical) when driving a load to

the rails. The output amplifiers are stable driving capaci-

tive loads up to 1000pF.

A small resistor placed in series with the output can be

used to achieve stability for any load capacitance. A 1

µF

load can be successfully driven by inserting a 20

Ω resistor

resistor, and a 10

µF electrolytic capacitor can be used

without any resistor (the equivalent series resistance of

the capacitor itself provides the required small resis-

tance). In any of these cases, larger values of resistance,

capacitance or both may be substituted for the values

given.

OPERATIO

Rail-to-Rail Output Considerations

In any rail-to-rail DAC, the output swing is limited to

voltages within the supply range.

If the DAC offset is negative, the output for the lowest

codes limits at 0V as shown in Figure 2b.

Similarly, limiting can occur near full scale when the REF

(FSE) is positive, the output for the highest codes limits at

Offset and linearity are defined and tested over the region

of the DAC transfer function where no output limiting can

occur.

Figure 2. Effects of Rail-to-Rail Operation On a DAC Transfer Curve. (a) Overall Transfer Function (b) Effect of Negative

1661 F02

INPUT CODE

(b)

OUTPUT

VOLTAGE

NEGATIVE

OFFSET

0V

512

0

1023

INPUT CODE

OUTPUT

VOLTAGE

(a)

(c)

INPUT CODE

OUTPUT

VOLTAGE

POSITIVE

FSE