AD7226BRSZ Datasheet, PDF (9/16 Pages) Analog Devices – LC2MOS Quad 8-Bit D/A Converter

AD7226
V_REF
MSB
DB7
DAC A
DB0
LSB
DAC B
WR
DAC C
A1
A0
DAC D
V_DD
V_OUTA
V_OUTB
V_OUTC
V_OUTD
V_SS ^AGND
DGND
Figure 8. AD7226 Unipolar Output Circuit
Table II. Unipolar Code Table
DAC Latch Contents
MSB
LSB
Analog Output
1111
1111
+V_REF ^ÊËÁ ₂₂₅₅₆₅^ˆ¯˜
1000
0001
+V
REF
ÊËÁ
129
256
ˆ¯˜
1000
0000
+V _REF
ÊËÁ 122586ˆ¯˜
=
+
V
REF
2
0111
1111
+V
REF
ÊËÁ
127
256
ˆ¯˜
0000
0001
+V
REF
ÊËÁ
1
256
ˆ¯˜
0000
0000
0V
( )( ) ^{Note: LSB = V}REF
2^–8
= V_REF
Ê 1ˆ
ËÁ ²⁵⁶¯˜
(2)
Bipolar Output Operation
Each of the DACs of the AD7226 can be individually config-
ured to provide bipolar output operation. This is possible using
one external amplifier and two resistors per channel. Figure 9
shows a circuit used to implement offset binary coding (bipolar
operation) with DAC A of the AD7226. In this case
( ) ( ) ^VOUT
=
ÊËÁ_{1 +}
R2ˆ
R1_¯˜
Â¥
D_AV_REF
–
Ê
ËÁ
R2ˆ
R1_¯˜
Â¥
V_REF
(3)
With R1 = R2
( ) ^VOUT ^{= 2D}A ^{– 1 ¥V}REF
(4)
where D_A is a fractional representation of the digital word in latch A.
Mismatch between R1 and R2 causes gain and offset errors and
therefore these resistors must match and track over tempera-
ture. Once again the AD7226 can be operated in single supply
or from positive/negative supplies. Table III shows the digital
code versus output voltage relationship for the circuit of Figure 9
with R1 = R2.
V_REF
V_REF
AD7226^*
DAC A
V_{SS AGND}
R1
V_DD
V_OUTA
R2
+15V
V_OUT
–15V
DGND
R1, R2 = 10k⍀ ؎0.1%
*_{DIGITAL INPUTS OMITTED}
FOR CLARITY
Figure 9. AD7226 Bipolar Output Circuit
Table III. Bipolar (Offset Binary) Code Table
DAC Latch Contents
MSB
LSB
Analog Output
1111
1000
1111
0001
+V_REF ^ÊËÁ₁₁₂₂₈₇^ˆ¯˜
+V
REF
ÊËÁ
1
128
ˆ¯˜
1000
0111
0000
0000
0000
1111
0001
0000
0V
–V
REF
ÊËÁ
1
128
ˆ¯˜
–V_REF ^ÊËÁ₁₁₂₂₈₇^ˆ¯˜
–V_REF ^{ÊËÁ112288ˆ¯˜} = –V_REF
AGND BIAS
The AD7226 AGND pin can be biased above system GND
(AD7226 DGND) to provide an offset “zero” analog output
voltage level. Figure 10 shows a circuit configuration to achieve
this for channel A of the AD7226. The output voltage, V_OUTA,
can be expressed as:
( ) ^VOUT ^{A = V}BIAS ^{+ D}A ^VIN
(5)
where D_A is a fractional representation of the digital input word
(0 £ D £ 255/256).
REV. D
–9–