8408 Datasheet, PDF (11/20 Pages) Maxwell Technologies – Quad 8-Bit Multiplying CMOS D/A Converter with Memory

Quad 8-Bit Multiplying CMOS
D/A Converter with Memory
TABLE 4. MODE SELECTION TABLE
8408
BASIC APPLICATIONS
Some basic circuit configurations are shown in Figures 8 and 9. Figure 8 shows the 8408 connected in a unipolar con-
figuration (2-Quadrant Multiplication), and Table 5 shows the Code Table. Resistors R1, R2, R3, and R4 are used to
trim full scale output. Full-scale output voltage = VREF –1 LSB = VREF (1–2–8) or VREF x (255/256) with all digital
inputs high. Low temperature coefficient (approximately 50 ppm/°C) resistors or trimmers should be selected if used.
Full scale can also be adjusted using VREF voltage. This will eliminate resistors R1, R2, R3, and R4. In many applica-
tions, R1 through R4 are not required, and the maximum gain error will then be that of the DAC.
Each DAC exhibits a variable output resistance that is code dependent.This produces a code-dependent, differential
nonlinearity term at the amplifier’s output which can have a maximum value of 0.67 times the amplifier’s offset voltage.
This differential nonlinearity term adds to the R-2R resistor ladder differential-nonlinearity; the output may no longer be
monotonic. To maintain monotonicity and minimize gain and linearity errors, it is recommended that the op amp offset
voltage be adjusted to less than 10% of 1 LSB (1 LSB = 2–8 x VREF or 1/256 x VREF), or less than 3.9 mV over the
operating temperature range. Zeroscale output voltage (with all digital inputs low) may be adjusted using the op amp
offset adjustment. Capacitors C1, C2, C3, and C4 provide phase compensation and help prevent overshoot and ring-
ing when using high speed op amps.
Figure 9 shows the recommended circuit configuration for the bipolar operation (4-quadrant multiplication), and Table
6 shows the Code Table. Trimmer resistors R17, R18, R19, and R20 are used only if gain error adjustments are
required and range between 50 Ω and 1000 Ω. Resistors R21, R22, R23, and R24 will range between 50 Ω and 500
Ω. If these resistors are used, it is essential that resistor pairs R9–R13, R10–R14, R11–R15, R12–R16 are matched
both in value and tempco. They should be within 0.01%; wire wound or metal foil types are preferred for best temper-
ature coefficient matching. The circuits of Figure 8 and 9 can either be used as a fixed reference D/A converter, or as
an attenuator with an ac input voltage.
08.20.02 REV 1
All data sheets are subject to change without notice 11
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