LTC2758

18

2758fb

Op Amp Selection

Because of the extremely high accuracy of the 18-bit

LTC2758, 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 ef-

fects of op amp parameters on the LTC2758’s accuracy

when 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.

applicaTions inForMaTion

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.

Table 4. Coefficients for the Equations of Table 5

OUTPUT RANGE

A1

A2

A3

A4

A5

5V

1.1

2

1

1

10V

2.2

3

0.5

1.5

±5V

2

2

1

1

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 contains a partial list of LTC precision op amps

recommended for use with the LTC2758. The easy-to-use

designequationssimplifytheselectionofopampstomeet

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

( )

66

OP AMP

A1•

0

0

0

INL (LSB)

( )

5V

( )

5V

( )

6

( )

262

( )

524

( )

524

( )

524

( )

524

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 6. Partial List of LTC Precision Amplifiers Recommended for Use with the LTC2758 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 LTC2758

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