LTC2755

18

2755f

Op Amp Selection

Because of the extremely high accuracy of the 16-bit

LTC2755-16, careful thought should be given to op amp

selection in order to achieve the exceptional performance

of which the part is capable. Fortunately, the sensitivity of

INL and DNL to op amp offset has been greatly reduced

compared to previous generations of multiplying DACs.

Tables 4 and 5 contain equations for evaluating the effects

of op amp parameters on the LTC2755’s accuracy when

APPLICATIONS INFORMATION

programmed in a unipolar or bipolar output range. These

are the changes the op amp can cause to the INL, DNL,

unipolar offset, unipolar gain error, bipolar zero and bipolar

gain error. Tables 4 and 5 can also be used to determine

the effects of op amp parameters on the LTC2755-14

and the LTC2755-12. However, the results obtained from

Tables 4 and 5 are in 16-bit LSBs. Divide these results

by 4 (LTC2755-14) and 16 (LTC2755-12) to obtain the

correct LSB sizing.

Table 6 contains a partial list of LTC precision op amps

recommended for use with the LTC2755. 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 6 and insert the specified op amp parameters

in Table 5. 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.

0

A4 •

()

5V

()

5V

()

16.5k

OP AMP

A1 •

0

0

0

INL (LSB)

()

5V

()

5V

()

1.5k

()

66k

()

131k

()

131k

()

131k

()

131k

A2 •

0

0

0

DNL (LSB)

()

5V

()

5V

0

0

0

0

UNIPOLAR

OFFSET (LSB)

()

5V

()

5V

()

5V

A5 •

BIPOLAR GAIN

ERROR (LSB)

()

5V

()

5V

()

()

()

5V

()

5V

BIPOLAR ZERO

ERROR (LSB)

UNIPOLAR GAIN

ERROR (LSB)

()

5V

()

5V

()

5V

()

5V

()

5V

A5 •

Table 4. Variables for Each Output Range that Adjust the

Equations in Table 5

OUTPUT RANGE

A1

A2

A3

A4

A5

5V

1.1

2

1

1

10V

2.2

3

0.5

1.5

±5V

2211

1.5

±10V

4

4

0.83

1

2.5

±2.5V

1

1

1.4

1

1

–2.5V to 7.5V

1.9

3

0.7

0.5

1.5

Table 6. Partial List of LTC Precision Amplifiers Recommended for Use with the LTC2755 with Relevant Specifications

AMPLIFIER

AMPLIFIER SPECIFICATIONS

μV

nA

V/mV

VOLTAGE

NOISE

nV/√⎯H⎯z

CURRENT

NOISE

pA/√⎯H⎯z

SLEW

RATE

V/μs

GAIN BANDWIDTH

PRODUCT

MHz

with LTC2755

μs

POWER

DISSIPATION

mW

LT1001

25

2

800

10

0.12

0.25

0.8

120

46

LT1097

50

0.35

1000

14

0.008

0.2

0.7

120

11

LT1112 (Dual)

60

0.25

1500

14

0.008

0.16

0.75

115

10.5/Op Amp

LT1124 (Dual)

70

20

4000

2.7

0.3

4.5

12.5

19

69/Op Amp

LT1468

75

10

5000

5

0.6

22

90

2

117

LT1469 (Dual)

125

10

2000

5

0.6

22

90

2

123/Op Amp

Table 5. Easy-to-Use Equations Determine Op Amp Effects on DAC Accuracy in All Output Ranges (Circuit of Page 1). Subscript 1

Refers to Output Amp, Subscript 2 Refers to Reference Inverting Amp.