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LTC2641/LTC2642

26412f

APPLICATIONS INFORMATION

Unbuffered Operation and VOUT Loading

The DAC output is available directly at the VOUT pin, which

swings from GND to VREF. Unbuffered operation provides

the lowest possible offset, full-scale and linearity errors, the

fastest settling time and minimum power consumption.

However, unbuffered operation requires that appropriate

loading be maintained on the VOUT pin. The LTC2641/

LTC2642 VOUT can be modeled as an ideal voltage source

in series with a source resistance of ROUT, typically 6.2k

(Figure 4). The DAC’s linear output impedance allows

it to drive medium loads (RL > 60k) without degrading

INL or DNL; only the gain error is increased. The gain

error (GE) caused by a load resistance, RL, (relative to

full scale) is:

OUT

⎛

⎝⎜

⎞

⎠⎟

–1

In 16-bit LSBs:

LSB

OUT

⎛

⎝⎜

⎞

⎠⎟

[]

–65536

ROUT has a low tempco (typically < ±50ppm/°C), and is

independent of DAC code. The variation of ROUT, part-to-

part, is typically less than ±20%.

Note on LSB units:

For the following error descriptions, “LSB” means 16-bit

LSB and 65,536 is rounded to 66k.

To convert to 14-bit LSBs (LTC2641-14/LTC2642-14)

divide by 4.

To convert to 12-bit LSBs (LTC2641-12/LTC2642-12)

divide by 16.

A constant current, IL, loading VOUT will produce an offset

of:

VOFFSET = –IL • ROUT

For VREF = 2.5V, a 16-bit LSB equals 2.5V/65,536, or 38μV.

Since ROUT is 6.2k, an IL of 6nA produces an offset of

1LSB. Therefore, to avoid degrading DAC performance,

it is critical to protect the VOUT pin from any sources of

leakage current.

Unbuffered VOUT Settling Time

The settling time at the VOUT pin can be closely approxi-

mated by a single-pole response where:

τ = ROUT • (COUT + CL)

(Figure 4). Settling to 1/2LSB at 16-bits requires about

12 time constants (ln(2 • 65,536)). The typical settling

time of 1μs corresponds to a time constant of 83ns, and

a total (COUT + CL) of about 83ns/6.2k = 13pF. The internal

capacitance, COUT is typically 10pF, so an external CL of

3pF corresponds to 1μs settling to 1/2LSB.

VOUT

0V TO VREF

ROUT

VOUT

COUT

LTC2641

LTC2642

VREF

REF

GND

CODE

VREF

()

26412 F04

–

Figure 4. VOUT Pin Equivalent Circuit

Op Amp Selection

The optimal choice for an external buffer op amp depends

on whether the DAC is used in the unipolar or bipolar

mode of operation, and also depends on the accuracy,

speed, power dissipation and board area requirements of

the application. The LTC2641/LTC2642’s combination of

tiny package size, rail-to-rail single supply operation, low

power dissipation, fast settling and nearly ideal accuracy

speciﬁcations makes it impractical for one op amp type

to ﬁt every application.

In bipolar mode (LTC2642 only), the ampliﬁer operates

with the internal resistors to provide bipolar offset and

scaling. In this case, a precision ampliﬁer operating from

dual power supplies, such as the the LT1678 provides the

±VREF output range (Figure 3).

In unipolar mode, the output ampliﬁer operates as a unity

gain voltage follower. For unipolar, single supply applica-

tions a precision, rail-to-rail input, single supply op amp

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