LTC2757

16

2757f

APPLICATIONS INFORMATION

Table 5 contains a partial list of LTC precision op amps

recommended for use with the LTC2757. The easy-to-use

design equations simplify the selection of op amps to meet

the system’s specified error budget. Select the amplifier

from Table 5 and insert the specified op amp parameters

in Table 4. Add up all the errors for each category to de-

termine the effect the op amp has on the accuracy of the

part. Arithmetic summation gives an (unlikely) worst-case

effect. A root-sum-square (RMS) summation produces a

more realistic estimate.

Op amp offset contributes mostly to DAC output offset

and gain error, and has minimal effect on INL and DNL.

For example, consider the LTC2757 in unipolar 5V output

range. (Note that for this example, the LSB size is 19μV.)

An op amp offset of 35μV will cause 1.8LSB of output

offset, and 1.8LSB of gain error; but 0.4LSB of INL, and

just 0.1LSB of DNL.

While not directly addressed by the simple equations in

Tables 3 and 4, temperature effects can be handled just as

easily for unipolar and bipolar applications. First, consult

ture-induced effects.

For applications where fast settling time is important, Ap-

plication Note 120,

1ppm Settling Time Measurement for

a Monolithic 18-Bit DAC, offers a thorough discussion of

18-bit DAC settling time and op amp selection.

Recommendations

To achieve the full specified static and dynamic performance

of the LTC2757, the LT1468 amplifier is recommended;

it offers a unique combination of fast settling and excel-

lent DC precision. When using the LT1468 as an output

amp, however, the offset voltage (75μV max) must be

nulled to avoid degrading the linearity of the LTC2757.

The LT1468 datasheet shows how to do this with a digital

potentiometer.

For DC or low-frequency applications, the LTC1150 is the

simplest 18-bit accurate output amplifier. An auto-zero

amp, its exceptionally low offset (10μV max) and offset

drift (0.01μV/°C) make nulling unnecessary. Note: for

swings above 8V, use an LT1010 buffer to boost the load

current capability of the LTC1150. The settling of auto-zero

amps is a special case; see Application Note 120,

1ppm

Settling Time Measurement for a Monolithic 18-Bit DAC,

Appendix E, for details.

The LT1012 and LT1001 are good intermediate output-amp

solutions that achieve moderate speed and good accuracy.

They are also excellent choices for the reference inverting

amplifier in fixed-reference applications.

Figure 3 shows a composite output amplifier that achieves

fast settling (8μs) and very low offset (3μV max) without

offset nulling. This circuit offers high open-loop gain

(1000V/mV min), low input bias current (0.15nA max),

fast slew rate (25V/μs min), and a high gain-bandwidth

product (30MHz typ). The high speed path consists of

an LTC6240, which is an 18MHz ultra-low bias current

amplifier, followed by an LT1360, a 50MHz fast-slewing

amplifier which provides additional gain and the ability to

Table 5. Partial List of LTC Precision Amplifiers Recommended for Use with the LTC2757 with Relevant Specifications

AMPLIFIER

AMPLIFIER SPECIFICATIONS

μV

nA

V/mV

VOLTAGE

NOISE

nV/√Hz

CURRENT

NOISE

pA/√Hz

SLEW

RATE

V/μs

GAIN BANDWIDTH

PRODUCT

MHz

with LTC2757

μs

POWER

DISSIPATION

mW

LTC1150

10

0.05

5600

90

0.0018

3

2.5

10ms

24

LT1001

25

2

800

10

0.12

0.25

0.8

120

46

LT1012

25

0.1

2000

14

0.02

0.2

1

120

11.4

LT1097

50

0.35

2500

14

0.008

0.2

0.7

120

11

LT1468

75

10

5000

5

0.6

22

90

2.1

117