DAC8806 Datasheet, PDF (14/20 Pages) Texas Instruments – 14-Bit, Parallel Input Multiplying Digital-to-Analog Converter

DAC8806
SBAS385A – APRIL 2006 – REVISED JUNE 2006
APPLICATION INFORMATION
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Multiplying Mode THD versus Frequency
Figure 35 and Figure 36 show the DAC8806 bipolar 4-quadrant multiplying mode total harmonic distortion (THD)
versus frequency. Figure 35 shows the bipolar mode THD with the DAC8806 set to a full-scale code of 3FFFh.
Figure 36 shows the bipolar multiplying mode THD with the DAC8806 set to a minus full-scale code of 0000h. In
both graphs, two OPA627s are used for both the DAC output op amp and the reference inverting amplifier. A
6V_RMS sine wave is used for the reference input V_REF and is swept in frequency from 10Hz to 30kHz. The THD
levels versus frequency are illustrated at various DAC output filtering levels using an external ac-coupled
low-pass filter.
Figure 37 illustrates the DAC8806 unipolar 2-quadrant multiplying mode THD versus frequency. The DAC8806
is set to a full-scale code of 3FFFh. A single OPA627 is used for the DAC output op amp.
Stability Circuit
For a current-to-voltage (I/V) design, as shown in Figure 41, the DAC8806 current output (I_OUT) and the
connection with the inverting node of the op amp should be as short as possible and laid out according to
correct printed circuit board (PCB) layout design. For each code change there is a step function. If the gain
bandwidth product (GBP) of the op amp is limited and parasitic capacitance is excessive at the inverting node,
then gain peaking is possible. Therefore, a compensation capacitor C1 (4pF to 20pF, typ) can be added to the
design for circuit stability, as shown in Figure 41.
V_DD
U1
V_DD R_OFS R_FB
C1
V_REF
V_{REF DAC8806}
I_OUT
OPA277
V_OUT
GND
U2
Figure 41. Gain Peaking Prevention Circuit with Compensation Capacitor
Bipolar Output Circuit
The DAC8806, as a 4-quadrant multiplying DAC, can be used to generate a bipolar output. The polarity of the
full-scale output (I_OUT) is the inverse of the input reference voltage at V_REF.
Using a dual op amp, such as the OPA2277, full 4-quadrant operation can be achieved with minimal
components. Figure 42 demonstrates a ±10V_OUT circuit with a fixed +10V reference.
ǒ Ǔ V_OUT +
D
8192
*
1
V_REF
(2)
14
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