Using an AC Reference

In applications where the reference has AC signal com-

ponents, the MAX5251 has multiplying capability within

the reference input range specifications. Figure 12

shows a technique for applying a sine-wave signal to

the reference input where the AC signal is offset before

being applied to REFAB/REFCD. The reference voltage

must never be more negative than DGND.

The MAX5251’s total harmonic distortion plus noise

(THD + N) is typically less than -72dB (full-scale code),

given a 1Vp-p signal swing and input frequencies up to

25kHz. The typical -3dB frequency is 650kHz, as

shown in the

Typical Operating Characteristics graphs.

Digitally Programmable Current Source

The circuit of Figure 13 places an NPN transistor

(2N3904 or similar) within the op-amp feedback loop to

implement a digitally programmable, unidirectional cur-

rent source. This circuit can be used to drive 4–20mA

current loops, which are commonly used in industrial-

control applications. The output current is calculated

with the following equation:

where NB is the numeric value of the DAC’s binary

input code and R is the sense resistor shown in

Figure 13.

+3V, Quad, 10-Bit Voltage-Output DAC

with Serial Interface

14

______________________________________________________________________________________

MAX5251

DAC A

DAC B

DAC C

DAC D

OUTA

10k

10k

10k

10k

16k

16k

16k

16k

OUTB

OUTC

OUTD

DGND

AGND

REFAB

REFCD

REFERENCE INPUTS

+3.3V

FBA

FBB

FBC

FBD

= 2.6

16k

10k

Figure 10. Unipolar Rail-to-Rail Output Circuit

DAC

+3.3V

-3.3V

R1 = R2 = 10k

Ω ± 0.1%

MAX5251

REF_

R1

R2

FB_

OUT_

Figure 11. Bipolar Output Circuit

DAC_

OUT_

MAX5251

10k

26k

REF_

AGND

DGND

+3.3V

AC

REFERENCE

INPUT

500mVp-p

1/2 MAX492

Figure 12. AC Reference Input Circuit