DAC729JH BURR-BROWN [Burr-Brown Corporation], DAC729JH Datasheet - Page 8

• Manufacturer Part Number: DAC729JH
• Description: Ultra-High Resolution 18-BIT DIGITAL-TO-ANALOG CONVERTER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

FIGURE 4. Equivalent Resistances.
FIGURE 6. Relationship of Offset and Gain Adjustments for
REFERENCE ADJUSTMENT
The internal reference may be fine adjusted using pin 35 as
shown in Figure 7. Adjusting the reference has a similar
effect on the DAC as gain adjust, except the transfer charac-
teristic rotates around bipolar zero for a bipolar connection
as shown in Figure 8.
LAYOUT/APPLICATIONS SUGGESTIONS
Obviously, the management of IR drops, power supply
noise, thermal stability, and environmental noise becomes
FIGURE 5. Relationship of Offset and Gain Adjustments for
Offset Adj.
Translates
Offset Adj.
Offset Adj.
Translates
Range of
the Line
the Line
+ Full
Scale
Full Scale
1LSB
Range
3.9M
3FFFF
Input =
a Bipolar D/A Converter.
a Unipolar D/A Converter.
H
DAC729
Bipolar V
Offset
3FFFF
Input =
Full Scale
Range
H
1LSB
Digital Input
180k
Digital Input
+ Full Scale
–Full Scale
MSB on,
All Others Off
Gain Adjustment
Rotates the Line
10k
Gain Adjustment
Rotates the Line
180k
Input =
00000
Range of
Gain Adj.
Range of
Gain Adj.
Input =
00000
H
H
8
much more critical as the accuracy of the system increases.
The DAC729 has been designed to minimize these applica-
tions problems to a large degree. The basics of “Kelvin
sensing” and “holy point” grounding will be the most impor-
tant considerations in optimizing the absolute accuracy of
the system. Figure 9 shows the proper connection of the
DAC with the holy-point ground and the Kelvin-sensed-
output connection at the load.
The DAC729 has three separate supply common (ground)
pins. Reference common (pin 33) carries the return current
from the internal reference and the output I/V converter
common. The current in pin 33 is stable and independent of
code or load. Digital common (pin 20) carries the variable
currents of the biasing circuits. Analog common (pin 30) is
the termination of the R-2R ladder and also carries the
“waste current” from the off side of the current switches.
These three ground pins must be star connected to system
ground for the DAC to bias properly and accurately. Good
ground connections are essential, because an IR drop of just
39 V completely swamps out a 10V FSR 18-bit LSB.
When the application is such that the DAC must control
loads of greater than 5mA with rated accuracy, it is recom-
mended that an external op amp or op amp buffer combina-
tion be used to dissipate the variable power external to the
DAC729. This minimizes the temperature variations on the
precision D/A converter. Figure 10 illustrates a method of
connecting the external amplifier for 10V operation, while
using an external reference.
When driving loads to greater than 10V, care must be taken
that the internal resistors are never exposed to greater than
that the voltage never exceeds 5V. Clamping the summing
junction with diodes (parallel opposing connection) to ground
will give the best transient response and settling times.
TRUE 18-BIT PERFORMANCE
(Differential Linearity Adjustment)
To take full advantage of the DAC729’s accuracy, the four
MSBs have adjustment capabilities. A simplified schematic
(Figure 11) shows the internal structure of the DAC current
source and the adjustment input terminal. The suggested
network for adjusting the linearity is shown in Figure 12.
This circuit has nearly twice the range that is required for the
DAC729JH. The range is intentionally narrow so as to
minimize the effect of temperature drift or stability problems
in the potentiometers. The potentiometers are biased in an
identical fashion to the internal DAC current sources to
minimize power supply sensitivity and drift over tempera-
ture. Low leakage capacitors such as Mylar or Teflon film
are essential.
The linearity adjustment requires a digital voltmeter with 7
digits of resolution on the 10V range (1 V resolution) and
excellent linearity. For the DAC, 1LSB of the 0V to 10V
scale (10 FSR) is 38 V. To be 1/2LSB linear, the measure-
ment must resolve 19 V. The meter must be properly
calibrated and linear to 1ppm of range.
10V, and that the summing junction is clamped to insure