AD768_15 Datasheet, PDF (7/20 Pages) Analog Devices – 16-Bit, 30 MSPS D/A Converter

AD768
REFERENCE OUTPUT
The internal 2.5 V bandgap reference is provided for generation
of the I_REFIN current, and must be compensated externally with
a capacitor of 0.1 µF or greater from REFOUT to REFCOM. If
an external reference is used, REFOUT should be tied directly
to the positive supply voltage, V_DD. This effectively turns off the
internal reference, eliminating the need for the external capaci-
tor at REFOUT. The reference is specified to drive a nominal
load of 5 mA with a maximum of 15 mA. Operation with a
heavier load will result in degradation of supply rejection and
reference voltage accuracy. Therefore, the reference output
should be buffered with an amplifier when additional load cur-
rent is required. A properly sized pull-up resistor can also be
used to source additional current to the load. The resistors value
should be selected such that REFOUT will always source a
minimum of 5 mA to IREFIN and the additional load.
CREFCOMP
1µF
500Ω
AD768
6 IREFIN
3 REFOUT
5 REFCOM
Figure 6. Typical Reference Hookup
TEMPERATURE CONSIDERATIONS
Note that the reference plays a key role in the overall tempera-
ture performance of the AD768. Any drift of I_REFIN shows up
directly in I_OUT. When the output is taken as a current, the drift
of I_REFIN (which depends on both V_REF and R_REF) must be mini-
mized. This can be done by using the internal temperature com-
pensated reference for V_REF and a low temperature coefficient
resistor for R_REF. If the output is taken as a voltage, it is a func-
tion of a resistor ratio, not an absolute resistor value. By select-
ing resistors with matched temperature coefficients for R_REF
and R_LOAD, the drift in the resistor values will cancel, providing
optimal drift performance.
REFERENCE NOISE REDUCTION AND MULTIPLYING
BANDWIDTH
For application flexibility and multiplying capabilities, the refer-
ence amplifier is designed to offer adjustable bandwidth that can
be reduced by connecting an external capacitor from the NR
node to the negative supply pin, V_EE. This capacitor limits the
bandwidth and acts as a filter to reduce the noise contribution
from the reference amplifier.
The noise reduction capacitor, C_NR, is not required for stability
and does not affect the settling time of the DAC output. With-
out this capacitor, the I_REFIN bandwidth is 15 MHz allowing
high frequency modulation of the DAC full-scale range through
the reference input node. Figure 7 shows the relationship be-
tween the external noise reduction capacitor and the –3 dB
bandwidth of the reference amplifier.
100M
10M
1M
100k
10k
1k
10p
100p
1n
10n
100n
1µ
NOISE REDUCTION CAPACITOR – F
Figure 7. External Noise Reduction Capacitor vs. –3 dB
Bandwidth
The sensitivity of the NR node requires that care be taken in
capacitor placement. The capacitor should be located as physi-
cally close to the package pins as possible and lead lengths
should be minimized. For this purpose, the use of a chip
capacitor is recommended. For applications that do not require
high frequency modulation at IREFIN, it is recommended that
a capacitor on the order of 1 µF be connected from NR to V_EE.
If the reference input is purely dc, noise may be minimized with
multiple capacitors, such as 1 µF and 0.1 µF, to more effectively
filter both high and low frequency disturbances.
ANALOG OUTPUTS
The AD768 offers two analog outputs; IOUTA is trimmed for
optimal INL and DNL performance and has a full-scale output
when all bits are high. For applications that require the specified
dc accuracy, IOUTA should be used. IOUTB is the comple-
mentary output with full-scale output when all bits are low.
Both IOUTA and IOUTB provide similar dynamic perfor-
mance. Refer to Figures 8 and 9 for typical INL and DNL per-
formance curves. The outputs can also be used differentially.
Refer to the section “Applying the AD768” for examples of vari-
ous output configurations.
8
6
4
2
0
–2
–4
–6
–8
0 5 10 15 20 25 30 35 40 45 50 55 60 65
DIGITAL INPUT CODE – k
Figure 8. Typical INL Performance
REV. B
–7–