ADDS-21535-EZLITE View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
ADDS-21535-EZLITE Datasheet PDF : 44 Pages
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AD15700
The AD15700’s ADC has five different ground pins: INGND,
REFGND, AGND, DGND, and OGND. INGND is used to
sense the analog input signal. REFGND senses the reference
voltage and should be a low impedance return to the reference
because it carries pulsed currents. AGND is the ground to which
most internal ADC analog signals are referenced. This ground
must be connected with the least resistance to the analog ground
plane. DGND must be tied to the analog or digital ground plane
depending on the configuration. OGND is connected to the
digital system ground.
The layout of the decoupling of the reference voltage is important.
The decoupling capacitor should be close to the ADC and connected
with short and large traces to minimize parasitic inductances.
100 CLOCK (5V/DIV)
90
VREF = 2.5V
VDD = 5V
TA = 25؇C
VOUT = (50mV/DIV)
10
0%
2s/DIV
Figure 27. Digital Feedthrough
100
90
CS (5V/DIV)
VREF = 2.5V
VDD = 5V
TA = 25؇C
VREF = 2.5V
100
VDD = 5V
TA = 25؇C
VOUT (1V/DIV)
90
VOUT (50mV/DIV)
GAIN = –216
10
0%
0.5s/DIV
Figure 30. Small Signal Settling Time
DAC Circuit Information
The DAC is a single 14-bit, serial input voltage output. It
operates from a single supply ranging from 2.7 V to 5 V and
consumes typically 300 mA with a supply of 5 V. Data is written
to the devices in a 14-bit word format, via a 3- or 4-wire serial
interface. To ensure a known power-up state, the parts were
designed with a power-on reset function. In unipolar mode, the
output is reset to 0 V.
Digital-to-Analog Section
The DAC architecture consists of two matched DAC sections.
A simplified circuit diagram is shown in Figure 31. The four
MSBs of the 14-bit data-word are decoded to drive 15 switches,
E1 to E15. Each of these switches connects one of 15 matched
resistors to either AGND or VREF. The remaining 10 bits of
the data-word drive switches S0 to S9 of a 10-bit voltage mode
R-2R ladder network.
R
R
2R
2R
2R
2R
2R 2R
VOUT
2R
S0
S1
S9
E1 E2
E15
VOUT (0.1V/DIV)
10
0%
2s/DIV
Figure 28. Digital-to-Analog Glitch Impulse
2s/DIV
100
90
10pF
50pF
100pF
10
0%
200pF
VREF = 2.5V
VDD = 5V
TA = 25؇C
CS
(5V/DIV)
VOUT
(0.5V/DIV)
Figure 29. Large Signal Settling Time
10-BIT R-2R LADDER
FOUR MSBS DECODED INTO
15 EQUAL SEGMENTS
Figure 31. DAC Architecture
With this type of DAC configuration, the output impedance is
independent of code, while the input impedance seen by the
reference is heavily code dependent. The output voltage is
dependent on the reference voltage as shown in the following
equation.
VOUT
=
VREF ¥
2N
D
where D is the decimal data-word loaded to the DAC register
and N is the resolution of the DAC. For a reference of 2.5 V,
the equation simplifies to the following.
VOUT
=
2.5 ¥ D
16, 384
giving a VOUT of 1.25 V with midscale loaded, and 2.5 V with
full scale loaded to the DAC.
The LSB size is VREF/16,384.
–36–
REV. A
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