Datasheet

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SLAS060F − JANUARY 1995 − REVISED APRIL 2009
7
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
PRINCIPLES OF OPERATION
AGND bias for direct bipolar output operation
The TLC7226 can be used in bipolar operation without adding more external operational amplifiers as shown
in Figure 4 by biasing AGND to V
SS
. This configuration provides an excellent method for providing a direct
bipolar output with no additional components. The transfer values are shown in Table 1.
_
+
DAC A
AGND
DGND
TLC7226
‡
REF (V
ref
= 5 V)
OUT
4 18
2
63
V
DD
5
V
SS
−5 V
Output range
(5 V to −5 V)
‡
Digital inputs omitted for clarity.
Figure 4. AGND Bias for Direct Bipolar Operation
LSBMSB
DAC LATCH CONTENTS
ANALOG OUTPUT
) V
ref
ǒ
127
128
Ǔ
1111 1111
) V
ref
ǒ
1
128
Ǔ
1000 0001
1000 0000
–V
ref
ǒ
128
128
Ǔ
+*V
ref
0111 1111
0000 0001
* V
ref
ǒ
127
128
Ǔ
0000 0000
0 V
Table 1. Bipolar (Offset Binary) Code
* V
ref
ǒ
1
128
Ǔ
AGND bias for positive output offset
The TLC7226 AGND terminal can be biased above or below the system ground terminal, DGND, to provide an
offset analog output voltage level. Figure 5 shows a circuit configuration to achieve this for channel A of the
TLC7226. The output voltage, V
O
, at OUTA can be expressed as:
V
O
+ V
BIAS
) D
A
ǒ
V
I
Ǔ
where D
A
is a fractional representation of the digital input word (0 ≤ D ≤ 255/256).
(1)
Increasing AGND above system GND reduces the output range. V
DD
− V
ref
must be at least 4 V to ensure
specified operation. Since the AGND terminal is common to all four DACs, this method biases up the output
voltages of all the DACs in the TLC7226. Supply voltages V
DD
and V
SS
for the TLC7226 should be referenced
to DGND.