LTC2757

15

2757f

Op Amp Selection

Because of the extremely high accuracy of the 18-bit

LTC2757, 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 3 and 4 contain equations for evaluating the ef-

fects of op amp parameters on the LTC2757’s accuracy.

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 3. Coefficients for the Equations of Table 4

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 4. 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.

OP

AMP

INL (LSB)

DNL (LSB)

UNIPOLAR

OFFSET (LSB)

BIPOLAR ZERO ERROR

(LSB)

UNIPOLAR GAIN

ERROR (LSB)

BIPOLAR GAIN

ERROR (LSB)

(mV)

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

(nA)

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

(V/mV)

A1•

66

⎛

⎝⎜

⎞

⎠⎟

A2•

6

⎛

⎝⎜

⎞

⎠⎟

00

A5•

524

⎛

⎝⎜

⎞

⎠⎟

A5•

524

⎛

⎝⎜

⎞

⎠⎟

(mV)

00

0

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

(nA)

00

0

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

5V

⎛

⎝⎜

⎞

⎠⎟

(V/mV)

00

0

A4•

262

⎛

⎝⎜

⎞

⎠⎟

524

⎛

⎝⎜

⎞

⎠⎟

524

⎛

⎝⎜

⎞

⎠⎟