DAC7642_15 Datasheet, PDF (14/25 Pages) Texas Instruments – 16-Bit, Dual Voltage Output DIGITAL-TO-ANALOG CONVERTER

ANALOG OUTPUTS
When V_SS = –5V (dual-supply operation), the output amplifier
can swing to within 2.25V of the supply rails over the –40°C
to +85°C temperature range. When V_SS = 0V (single-supply
operation), and with R_LOAD also connected to ground, the
output can swing to ground. Care must also be taken when
measuring the zero-scale error when V_SS = 0V. Since the
DAC output cannot swing below ground, the output voltage
may not change for the first few digital input codes (0000_H,
0001_H, 0002_H, etc.) if the output amplifier has a negative
offset. At the negative limit of –2mV, the first specified output
starts at code 0040_H.
Due to the high accuracy of these DACs, system design
problems such as grounding and contact resistance become
very important. A 16-bit converter with a 2.5V full-scale range
has a 1LSB value of 38µV. With a load current of 1mA, a series
wiring and connector resistance of only 40mΩ (R_W2) will cause
a voltage drop of 40µV, as shown in Figure 4. To understand
what this means in terms of a system layout, the resistivity of
a typical 1 ounce copper-clad printed circuit board is 1/2 mΩ
per square. For a 1mA load, a 10 milli-inch wide printed circuit
conductor 600 milli-inches long will result in a voltage drop of
30µV.
The DAC7642 and DAC7643 offer a force and sense output
configuration for the high open-loop gain output amplifiers.
This feature allows the loop around the output amplifier to be
closed at the load (shown in Figure 4), thus ensuring an
accurate output voltage.
REFERENCE INPUTS
The reference inputs, V_REFL and V_REFH, can be any voltage
between V_SS + 2.5V and V_CC – 2.5V provided that V_REFH is
at least 1.25V greater than V_REFL. The minimum output of
each DAC is equal to V_REFL plus a small offset voltage
(essentially, the offset of the output op amp). The maximum
output is equal to V_REFH plus a similar offset voltage. Note
V_OUTA 32
V_OUTA Sense 31
V_REFL Sense 30
DAC7642
DAC7643
V_REFL 29
V_REFH 28
V_REFH Sense 27
V_OUTB Sense 26
V_OUTB 25
R_W2
R_W1
R_W1
R_W2
V_OUT
+V
+2.5V
V_OUT
FIGURE 4. Analog Output Closed-Loop Configuration. R_W
represents wiring resistances.
that V_SS (the negative power supply) must either be con-
nected to ground or must be in the range of –4.75V to
–5.25V. The voltage on V_SS sets several bias points within
the converter. If V_SS is not in one of these two configurations,
the bias values may be in error and proper operation of the
device may be affected.
The current into the V_REFH input and out of V_REFL depends
on the DAC output voltages and can vary from a few
microamps to approximately 0.5mA. The reference input
appears as a varying load to the reference. If the references
applied can sink or source the required current, a reference
buffer is not required. The DAC7642 and DAC7643 feature
reference drive and sense connections such that the internal
errors caused by the changing reference current and the
circuit impedances can be minimized. Figures 5 through 13
show different reference configurations and the effect on the
linearity and differential linearity.
V_OUTA 32
V_OUTA Sense 31
V_REFL Sense 30
DAC7642
DAC7643
V_REFL 29
V_REFH 28
V_REFH Sense 27
V_OUTB Sense 26
V_OUTB 25
V_OUT
1000pF
100Ω ^2200pF
1000pF
V_OUT
100Ω
2200pF
+V
OPA2234
–V
–2.5V
–V
+V
+2.5V
FIGURE 5. Dual Supply Configuration-Buffered References, Used for Dual-Supply Characteristic Curves.
14
DAC7642, DAC7643
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