AD1672

REV. 0

–11–

REFERENCE INPUT

The nominal reference input is 2.5 V, taken with respect to

REFCOM. The reference input pin (REFIN) can be connected

to the reference output pin (REFOUT) or a standard external

+2.5 Volt reference can be selected to meet specific system re-

quirements. Figure 17 shows the AD1672’s REFIN being

driven from an external precision voltage reference such as the

REF192. If an external reference is used, REFOUT can be ef-

fectively disabled by tying it directly to the positive supply volt-

all three conversions in the pipeline will be invalidated.

The nominal 2.5 V reference input is rescaled to 1.25 V via a

resistor divider network as shown in Figure 18. The nominal

resistance of this resistor network is 8 k

Ω. Table IV summa-

rizes various 2.5 V references for use with the AD1672 (see

REFERENCE OUTPUT section).

Table IV. Suitable 2.5 Volt References

Drift

Initial Accuracy

(ppm/ C)

%

REF43B

10 (max)

0.1

REF192E

5 (max)

0.08

AD780B

3 (max)

0.04

0.1µF

10µF

AD780

OUTPUT

GND

1µF

AD1672

REFIN

REFOUT

Figure 17. Circuit using AD780 Optional +2.5 V

Reference Input Circuit

REFERENCE NOISE REDUCTION

Figure 18 shows the proper connection of two external 1

µF

duction nodes NCOMP1 and NCOMP2 (Pins 25 and 26) of

the AD1672. Although these noise reduction capacitors, CNR, are

not required for stability, the most optimum noise and distortion per-

formance will be attained with a 1

µF ceramic capacitor connected

the output of the DAC amp and is required for optimum dis-

tortion performance and SHA settling time. It provides a low

source impedance for signal-dependent, transient currents asso-

the reference buffer amplifier.

CNR2

1µF

1µF

CNR1

1µF

NCOMP2

REFOUT

REFIN

AD1672

REFCOM

NCOMP1

DAC

AMP

REF

AMP

4k

Ω

4k

Ω

20

23

24

25

26

Figure 18. Reference Noise Reduction

REFERENCE OUTPUT

The AD1672 includes an onboard +2.5 V curvature compen-

sated bandgap reference that has been laser trimmed for both

absolute value and temperature drift. The reference must be

externally compensated with a capacitor of 1

µF or greater from

REFOUT to REFCOM.

The reference is specified to provide an additional load current

up to 500

µA. For large dynamic loads and/or external loads

greater than 500

µA, an external buffer amplifier or pull-up re-

sistor is required. If a pull-up resistor is used, its value should

be selected such that REFOUT will not be required to sink any

current under all operating conditions. Figure 19 illustrates the

load regulation of the reference. The power supply rejection of

the reference is better than 0.2 % of FSR at dc.

LOAD CURRENT – µA

2.500

2.493

2.490

0

100

200

300

400

500

600

700

800

900

1000

2.499

2.494

2.492

2.491

2.498

2.496

2.497

2.495

Figure 19. Reference Output Voltage vs. Load Current

DIGITAL OUTPUTS

The AD1672 output data is presented in positive true straight

binary for both the 0 V to 2.5 V and 0 V to 5 V unipolar input

ranges and positive true offset binary for the

±2.5 V bipolar

range. Table V indicates the output data formats for the three

analog input ranges. Users requiring twos complement encoding

for the

±2.5 V range may simply invert the MSB (Pin 13).

Table V. Output Data Format

Input Voltage Range

+FS (V)

–FS (V)

LSB (mV)

0 to +2.5

+2.5

0.0

0.610

0 to +5.0

+5.0

0.0

1.22

–2.5 to +2.5

+2.5

–2.5

1.22

–FS+1/2 LSB

–FS

–FS –1/2 LSB

+FS

+FS –1/2 LSB

OTR

+FS –1 1/2 LSB

OTR

DATA OUTPUTS

1

1111 111 1111

0

1111 1111 1111

0

1111 1111 1110

0

0000 0000 0001

0

0000 0000 0000

1

0000 0000 0000

Figure 20. Output Data Format