Rainbow Electronics MAX5251 - Manual (Page 14)

MAX5251
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:
I OUT = (V REF / R) x (NB / 1024)
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
V
REFAB
= V
REFCD
= 1.25V
OUTA
10k
10k
10k
10k
16k
16k
16k
16k
OUTB
OUTC
OUTD
DGND AGND
REFAB REFCD
REFERENCE INPUTS +3.3V
V
DD FBA
FBB
FBC
FBD
A
V
= 1 + = 2.6
16k
10k
Figure 10. Unipolar Rail-to-Rail Output Circuit
DAC
V OUT
+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_ V DD
AGND DGND
+3.3V
AC
REFERENCE
INPUT
500mVp-p
1/2 MAX492
Figure 12. AC Reference Input Circuit