AD7849
Rev. C | Page 14 of 20
Unipolar Configuration
configuration. The DAC is driven by the
AD586, 5 V reference.
Because ROFS is tied to 0 V, the output amplifier has a gain of ×2,
and the output range is 0 V to 10 V. If a 0 V to 5 V range is
required, ROFS should be tied to VOUT, configuring the output
stage for a gain of ×1.
Table 7 gives the code table for the circuit
ROFS
VDD
VCC
VREF+
VOUT
(0V TO 10V)
AGND
VREF–
VSS
–15V
AD7849*
R1
10k
AD586
C1
1nF
SIGNAL GND
6
8
4
5
*ADDITIONAL PINS OMITTED FOR CLARITY.
2
DGND
+15V
+5V
01
00
8-
02
1
Figure 20. Unipolar Binary Operation
Binary Number in DAC Latch
MSB
LSB
Analog Output (VOUT)
1111 1111 1111 1111
10 (65,535/65,536) V
1000 0000 0000 0000
10 (32,768/65,536) V
0000 0000 0000 0001
10 (1/65,536) V
0000 0000 0000 0000
0 V
Table 7 assumes a 16-bit resolution; 1 LSB = 10 V/216 = 10 V/65,536 = 152 μV.
Offset and gain can be adjusted in
Figure 20 as follows:
To adjust offset, disconnect the VREF input from 0 V, load the
DAC with all 0s, and adjust the VREF voltage until VOUT = 0 V.
To adjust gain, load the
AD7849 with all 1s and adjust R1
until VOUT = 10 (65,535/65,536) = 9.9998474 V for the 16-bit,
B and C versions. For the 14-bit A version, VOUT should be
10 (16,383/16,384) = 9.9993896 V.
If a simple resistor divider is used to vary the VREF voltage, it is
important that the temperature coefficients of these resistors
match that of the DAC input resistance (300 ppm/°C). Otherwise,
extra offset errors will be introduced over temperature. Many
circuits do not require these offset and gain adjustments. In
these circuits, R1 can be omitted. Pin 5 of the
AD586 may be
left open circuit, and Pin 2 (VREF) of the AD7849 tied to 0 V. Bipolar Configuration
The
AD588 provides precision ±5 V tracking outputs that are
fed to the VREF+ and VREF inputs of the AD7849.The code table Full-scale and bipolar-zero adjustment are provided by varying
the gain and balance on the
AD588. R2 varies the gain on the
AD588, while R3 adjusts the +5 V and 5 V outputs together
with respect to ground.
VOUT
(–10V TO +10V)
+15V
+5V
VDD
VCC
VREF+
VOUT
ROFS
AGND
DGND
VREF–
VSS
–15V
AD7849*
SIGNAL
GND
*ADDITIONAL PINS OMITTED FOR CLARITY
AD588
C1
1F
R2
100k
R3
100k
R1
39k
6
15
2
8
5
14
7
9
3
1
10
12
11
4
13
16
01
008-
022
Figure 21. Bipolar ±10 V Operation
Binary Number in DAC Latch
MSB
LSB
Analog Output (VOUT)
1111 1111 1111 1111
+10 (32,767/32,768) V
1000 0000 0000 0001
+10 (1/32,768) V
1000 0000 0000 0001
0 V
0111 1111 1111 1111
10 (1/32,768) V
0000 0000 0000 0000
10 (32,768/32,768) V
Table 8 assumes a 16-bit resolution; 1 LSB = 20 V/216 = 305 μV. For bipolar-zero adjustment on the
AD7849, load the DAC with
100 … 000 and adjust R3 until VOUT = 0 V. Full scale is adjusted
by loading the DAC with all 1s and adjusting R2 until VOUT =
9.999694 V.
When bipolar-zero and full-scale adjustment are not needed,
omit R2 and R3, connect Pin 11 to Pin 12 on the
AD588 and
leave Pin 5 on the
AD588 floating.
If a ±5 V output range is desired with the circuit shown in
Figure 21, tie Pin 20 (ROFS) to Pin 19 (VOUT), thus reducing the output gain stage to unity and giving an output range of ±5 V.