AD9225ARS Analog Devices Inc, AD9225ARS Datasheet - Page 16

• Manufacturer Part Number: AD9225ARS
• Description: IC ADC 12BIT 25MSPS 28-SSOP
• Manufacturer: Analog Devices Inc

Specifications of AD9225ARS

• Mounting Type: Surface Mount
• Package / Case: 28-SSOP (0.200", 5.30mm Width)
• Rohs Status: RoHS non-compliant
• Number Of Bits: 12
• Sampling Rate (per Second): 25M
• Data Interface: Parallel
• Number Of Converters: 7
• Power Dissipation (max): 373mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Peak Reflow Compatible (260 C): No
• No. Of Bits: 12 Bit
• Leaded Process Compatible: No
• No. Of Channels: 2
• Interface Type: Parallel
• For Use With: AD9225-EB - BOARD EVAL FOR AD9225
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD9225
REFERENCE CONFIGURATIONS
The figures associated with this section on internal and external
reference operation do not show recommended matching series
resistors for VINA and VINB for the purpose of simplicity. Refer
to the Driving the Analog Inputs and Introduction sections for a
discussion of this topic. The figures do not show the decoupling
network associated with the CAPT and CAPB pins. Refer to the
Reference Operation section for a discussion of the internal refer-
ence circuitry and the recommended decoupling network shown
in Figure 16.
USING THE INTERNAL REFERENCE
Single-Ended Input with 0 to 2 3 VREF Range
Figure 16 shows how to connect the AD9225 for a 0 V to 2 V or
0 V to 4 V input range via pin strapping the SENSE pin. An inter-
mediate input range of 0 to 2 ¥ VREF can be established using the
resistor programmable configuration in Figure 19.
In either case, both the midscale voltage and input span are directly
dependent on the value of VREF. More specifically, the midscale
voltage is equal to VREF while the input span is equal to 2 ¥ VREF.
Thus, the valid input range extends from 0 to 2 ¥ VREF. When
VINA is £ 0 V, the digital output will be 0x000; when VINA is
≥ 2 ¥ VREF, the digital output will be 0xFFF.
Shorting the VREF pin directly to the SENSE pin places the inter-
nal reference amplifier in unity-gain mode and the resulting VREF
output is 1 V. Therefore, the valid input range is 0 V to 2 V.
However, shorting the SENSE pin directly to the REFCOM pin
configures the internal reference amplifier for a gain of 2.0 and the
resulting VREF output is 2.0 V. Therefore, the valid input range
becomes 0 V to 4 V. The VREF pin should be bypassed to the
REFCOM pin with a 10 mF tantalum capacitor in parallel with a
low inductance 0.1 mF ceramic capacitor.
Figure 18 shows the single-ended configuration that gives good
dynamic performance (SINAD, SFDR). To optimize dynamic
specifications, center the common-mode voltage of the analog
input at approximately by 2.5 V by connecting VINB to a low
impedance 2.5 V source. As described above, shorting the VREF
pin directly to the SENSE pin results in a 1 V reference voltage
and a 2 V p-p input span. The valid range for input signals is 1.5 V
to 3.5 V. The VREF pin should be bypassed to the REFCOM pin
with a 10 mF tantalum capacitor in parallel with a low-inductance
0.1 mF ceramic capacitor.
2
Figure 17. Internal Reference—2 V p-p Input Span,
V
VREF
CM
0V
= 1 V, or 4 V p-p Input Span
SHORT FOR 0V TO 2V
SHORT FOR 0V TO 4V
10 F
INPUT SPAN
INPUT SPAN
0.1 F
VINA
VINB
VREF
SENSE
REFCOM
AD9225
–16–
This reference configuration could also be used for a differential
input in which VINA and VINB are driven via a transformer as
shown in Figure 13. In this case, the common-mode voltage,
VCM , is set at midsupply by connecting the transformer’s center
tap to CML of the AD9225. VREF can be configured for 1.0 V
or 2.0 V by connecting SENSE to either VREF or REFCOM,
respectively. Note that the valid input range for each of the differ-
ential inputs is one half of the single-ended input and thus becomes
VCM – VREF/2 to VCM + VREF/2.
Resistor Programmable Reference
Figure 19 shows an example of how to generate a reference voltage
other than 1.0 V or 2.0 V with the addition of two external resistors
and a bypass capacitor. Use the equation
to determine appropriate values for R1 and R2. These resistors
should be in the 2 kW to 100 kW range. For the example shown,
R1 equals 2.5 kW and R2 equals 5 kW. From the equation above,
the resultant reference voltage on the VREF pin is 1.5 V. This
sets the input span to be 3 V p-p. To assure stability, place a 0.1 mF
ceramic capacitor in parallel with R1.
The midscale voltage can be set to VREF by connecting VINB to
VREF to provide an input span of 0 to 2 ¥ VREF. Alternatively,
the midscale voltage can be set to 2.5 V by connecting VINB to a
low impedance 2.5 V source. For the example shown, the valid
input single-ended range for VINA is 1 V to 4 V since VINB is set
to an external, low impedance 2.5 V source. The VREF pin should
be bypassed to the REFCOM pin with a 10 mF tantalum capacitor
in parallel with a low inductance 0.1 mF ceramic capacitor.
Figure 18. Internal Reference—2 V p-p Input Span,
V
CM
Figure 19. Resistor Programmable Reference
3 V p-p Input Span, V
10 F
4V
1V
3.5V
1.5V
= 2.5 V
2.5V
10 F
VREF = 1 V ¥ (1 + R1/R2)
0.1 F
R1
2.5k
R2
5k
0.1 F
CM
C1
0.1 F
= 2.5 V
1V
1.5V
VINA
VCM
VINB
VREF
SENSE
REFCOM
VINB
VINA
VREF
SENSE
REFCOM
AD9225
AD9225
Rev. C