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DAC16FS 数据表(PDF) 8 Page - Analog Devices

部件名 DAC16FS
功能描述  16-Bit High Speed Current-Output DAC
Download  12 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

DAC16FS 数据表(HTML) 8 Page - Analog Devices

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DAC16
REV. B
–8–
reference amplifier. Completely removing the compensation
network would introduce large linearity errors, reference amplifier
instability, wideband reference amplifier noise, and poor settling
time.
Because the DAC exhibits an internal current scaling factor of
eight times (8
×), the reference amplifier requires only 500 µA
input current from the user-supplied precision reference for a
4 mA full-scale output current. In applications that do not re-
quire such high output currents, good accuracy can be achieved
with input reference currents in the range of 350
µA ≤ I
REF
≤ 625 µA. The best signal-to-noise ratios, of course, will be
achieved with a 625
µA reference current which yields a maxi-
mum 5 mA output current. Figure 22 illustrates how to form
the reference input current with a REF02 and a 10 k
Ω precision
resistor.
REF GND
RREF
10k
IREF
+15V
0.1 F
DAC16
IOUT
REF02
IREF =
VREF
RREF
Figure 22. Generating the DAC16’s Reference Input Current
Reducing Voltage Reference Noise
In data converters of 16-bit and greater resolution, noise is of
critical importance. Surprisingly, the integrated voltage refer-
ence circuit used may contribute the dominant share of a
system’s noise floor, thereby degrading system dynamic range
and signal-to-noise ratio. To maximize system dynamic range
and SNR, all external noise contributions should be effectively
much less than 1/2 LSB. For example, in a 5 V DAC16 applica-
tion, one LSB is equivalent to 76
µV. This means that the total
wideband noise contribution due to a voltage reference and all
other sources should be less than 38
µV rms. These noise levels
are not easy targets to hit with standard off-the-shelf reference
devices. For example, commercially available references might
exhibit 5
µV rms noise from 0.1 Hz to 10 Hz: but, over a 100 kHz
bandwidth, its 300
µV rms of noise can easily swamp out a
16-bit system. Such noisy behavior can degrade a DAC’s effec-
tive resolution by increasing its differential nonlinearity which,
in turn, can lead to nonmonotonic behavior or analog errors.
The easiest way to reduce noise in the reference circuit is to
band-limit its noise before feeding it to the converter. In the
case of the DAC16, the reference is not a voltage, but a current.
Illustrated in Figure 23 is a simple way of hand-limiting
REF GND
R1
5k
IREF
+15V
0.1 F
DAC16
REF02
C1
22 F
R2
5k
AGND
Figure 23. Filtering a Reference’s Wideband Noise
voltage reference noise by splitting RREF into two equal resistors
and bypassing the common node with a capacitor. To minimize
thermally induced errors, R1 and R2 must be electrically and
thermally well-matched. Thin-film resistor networks work well
here. In this circuit, the parallel combination of R1 and R2
forms a 3 Hz low-pass filter with C1. The only noise source that
remains is the thermal noise of R2 which can be a significantly
lower noise generator than the voltage reference.
Input Coding
The unipolar digital input coding of the DAC16 employs nega-
tive logic to control the output current; that is, an all zero input
code (0000H) yields an output current 1 LSB below full scale.
Conversely, an all 1s input code (FFFFH) yields a zero analog
current output. An expression for the DAC16’s transfer equa-
tion can be expressed by:
I
OUT = 8 × IREF ×
65,535 –
Digital Code
65,536
Table II provides the relationship between the digital input
codes and the output current of the DAC16.
Table II. Unipolar Code Table
Digital Input
DAC16 Output
Word (Hex)
Current IOUT
Comment
0000
8
× (216 – 1)/216 × I
REF
Full Scale
7FFE
8
× (215 + 1)/216 × I
REF
Midscale + 1 LSB
7FFF
8
× (215/216) × I
REF
Midscale
8000
8
× (215– 1)/216 × I
REF
Midscale – 1 LSB
FFFF
0
Zero Scale
Since the DAC16 exhibits a small output voltage compliance on
the order of a few millivolts, a high accuracy operational ampli-
fier must be used to convert the DAC’s output current to a volt-
age. Refer to the section on selecting operation amplifiers for the
DAC16. The circuit shown in Figure 24 illustrates a unipolar
output configuration. In symbolic form, the transfer equation
for this circuit can be expressed by:
V
O = R3 × 8 × IREF
65,535 –
Digital Code
65,536


In this example, the reference input current was set to 500
µA
which produces a full-scale output current of 4 mA – 1 LSB.
The DAC’s output current was scaled by R3, a 1.25 k
Ω resistor,
to produce a 5 V full-scale output voltage. Bear in mind that to
ensure the highest possible accuracy, matched thin-film resistor
networks are almost a necessity, not an option. The resistors
used in the circuit must have close tolerance and tight thermal
tracking. Table III illustrates the relationship between the input
digital code and the circuit’s output voltage for the component
values shown.
Table III. Unipolar Output Voltage vs. Digital Input Code
Digital Input Word
Decimal Number in
Analog Output
(Hex)
in DAC Decoder
Voltage (V)
0000
65,535
4.999924
7FFE
32,769
2.500076
7FFF
32,768
2.500000
8000
32,767
2.499924
FFFF
0
0


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