AD9225

–11–

Table I. Analog Input Configuration Summary

Input

Input

Input Range (V)

Figure

Connection

Coupling

Span (V)

VINA

*

VINB

*

No.

Comments

Single-Ended

DC

2

0 to 2

1

8, 9

Best for stepped input response applications;

requires

±5 V op amp.

2

¥ VREF 0 to

VREF

8, 9

Same as above but with improved noise

2

¥ VREF

performance due to increase in dynamic

range. Headroom/settling time requirements

of

±5 V op amp should be evaluated.

40 to 4

2.0

8, 9

Optimum noise performance, excellent THD

performance, often requires low distortion op

amp with VCC > +5 V due to its headroom

issues.

2

¥ VREF 2.0 – VREF

2.0

21

Optimum THD performance with VREF = 1.

to

Single-supply operation (i.e., +5 V) for many

2.0 + VREF

op amps.

Single-Ended

AC

2 or

0 to 1 or

1 or VREF

10, 11

2

¥ VREF 0 to 2 ¥ VREF

4

0.5 to 4.5

2.0

11

Optimum noise performance, excellent THD

performance; ability to use

±5 V op amp.

2

¥ VREF 2.0 – VREF

2.0

10

Flexible input range, optimum THD perfor-

to

mance with VREF = 1. Ability to use either

2.0 + VREF

+5 V or

±5 V op amp.

Differential

AC/DC

2

2 to 3

3 to 2

12, 13

Optimum full-scale THD and SFDR perfor-

(via Transformer)

mance well beyond the ADC’s Nyquist

or Amplifier

frequency. Preferred mode for under-

sampling applications.

2

¥ VREF 2.0 – VREF/2 2.0 + VREF/2 12, 13 Same as above with the exception that full-

to

to

scale THD and SFDR performance can be

2.0 + VREF/2

2.0 – VREF/2

traded off for better noise performance.

4.0

1.5 to 3.5

3.5 to 1.5

12, 13

Optimum noise performance.

*VINA and VINB can be interchanged if signal inversion is required.

Table II. Reference Configuration Summary

Reference

Input Span (VINA–VINB)

Operating Mode

(V p-p)

Required VREF (V)

Connect

To

Internal

2

1

SENSE

VREF

Internal

4

2

SENSE

REFCOM

Internal

2

£ SPAN £ 4 and

1

£ VREF £ 2.0 and

R1

VREF and SENSE

SPAN = 2

VREF

VREF = (1 + R1/R2)

R2

SENSE and REFCOM

External

2

£ SPAN £ 41 £ VREF £ 2.0

SENSE

AVDD

(Nondynamic)

VREF

External Reference

External

2

£ SPAN £ 4

CAPT and CAPB

SENSE

AVDD

(Dynamic)

Externally Driven

VREF

AVSS

External Reference

CAPT

External Reference

CAPB

Rev. C