AD1672

REV. 0

–9–

APPLYING THE AD1672

ANALOG INPUTS

Figure 9 shows the equivalent analog input of the AD1672. The

input SHA and associated resistor network topology can be eas-

ily configured for either unipolar (0 V to 2.5 V, 0 V to 5.0 V) or

bipolar (–2.5 V to 2.5 V) input signals as shown in Figure 10.

Ω for a

2.5 V span and 4 k

Ω for a 5 V span. The circuit topology both

level shifts and inverts the analog input for the various input

spans.

AIN 1

4k

Ω

2k

Ω

1.25V

4k

Ω

AIN 2

2k

Ω

Figure 9. Equivalent Analog Input Circuit

21

22

20

23

1µF

AIN1

AIN2

REFOUT

REFIN

AD1672

21

22

20

23

1µF

AIN1

AIN2

REFOUT

REFIN

AD1672

21

22

20

23

1µF

AIN1

AIN2

REFOUT

REFIN

AD1672

a. 0 to +2.5 V Input Range

b. 0 to +5.0 V Input Range

c. –2.5 to +2.5 V Input Range

Figure 10. Input Range Connections

In applications where ac coupling of the analog input signal is

appropriate such as in a single supply system, the user can

capacitively couple the input signal for a 2.5 V or 5 V span thus

removing any preceding system dc offsets. Figure 11 shows the

proper configurations of the AD1672 for ac coupling. Main-

taining the specifications outlined in the data sheet requires care-

ful selection of the component values. The most important

by the equation

/f

C1 is typically a large electrolytic or tantalum capacitor that be-

comes inductive at high frequencies. Adding a small ceramic

capacitor on the order of 0.1

µF that does not become inductive

until negligibly higher frequencies maintains a low impedance

over a wide frequency range.

21

22

25

20

23

C1

10µF

C2

1.0µF

10µF

1.0µF

ANALOG

INPUT

AIN1

AIN2

NCOMP2

REF

OUT

REFIN

AD1672

21

22

25

20

23

C1

10µF

C2

1.0µF

10µF

1.0µF

ANALOG

INPUT

AIN1

AIN2

NCOMP2

REF

OUT

REFIN

AD1672

2.5V Span

5.0V Span

Figure 11. AC Coupled Inputs

In applications requiring dc coupling, a buffer amplifier is rec-

ommended for driving the AD1672 input. Any source resis-

tance will contribute to both gain and offset error due to its

interaction with the AD1672’s input resistance. The particular

application and signal input range will determine how the buffer

amplifier is configured. For example, in dc precision applica-

tions, the buffer amplifier can be configured for convenient gain

and offset adjustment as shown in Figure 12. In spectral

analysis/signal processing applications, the buffer amplifier can

be configured as a 2nd order antialiasing filter in a Sallen-Key

or Multiple-Feedback topology as shown in Figure 13.

500

Ω

50

Ω

500

Ω

5k

Ω

50

Ω

5k

Ω

0.1µF

ANALOG

INPUT

OFFSET

ADJUSTMENTS

GAIN

ADJUSTMENT

AD1672

AIN

Figure 12. Offset and Gain Adjustment

C2

R4

C1

R2

R1

R3

C2

C1

R2

R1

R3

Figure 13. Sallen-Key and Multiple-Feedback

Antialiasing Filter Topologies

In imaging and multiplexed data acquisition applications, the

AD1672’s wide input bandwidth facilitates rapid acquisition of

transient input signals: the input SHA can typically settle to 12-

bit accuracy from a full scale input step in less than 150 ns. Fig-

ure 14 illustrates the typical acquisition of a full scale input step.

For amplifiers that are powered by supplies greater than

6.5 V, it is recommended that a clamping circuit be included at

the input of AD1672. This circuit limits the input voltage to

6.5 V under a fault condition.