AD9244-40PCB Analog Devices Inc, AD9244-40PCB Datasheet - Page 20

• Manufacturer Part Number: AD9244-40PCB
• Description: BOARD EVAL FOR AD9244-40
• Manufacturer: Analog Devices Inc

Specifications of AD9244-40PCB

• Rohs Status: RoHS non-compliant
• Number Of Adc's: 1
• Number Of Bits: 14
• Sampling Rate (per Second): 40M
• Data Interface: Parallel
• Inputs Per Adc: 1 Differential
• Input Range: 2 Vpp
• Power (typ) @ Conditions: 345mW @ 40MSPS
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9244-40

AD9244
The circuit in Figure 47 shows a method for applying a differential,
direct-coupled signal to the AD9244. An AD8138 amplifier is used
to derive a differential signal from a single-ended signal.
REFERENCE OPERATION
The AD9244 contains a band gap reference that provides a pin-
strappable option to generate either a 1 V or 2 V output. With
the addition of two external resistors, the user can generate
reference voltages between 1 V and 2 V. Another alternative is
to use an external reference for designs requiring enhanced
accuracy and/or drift performance, as described later in this
section. Figure 48 shows a simplified model of the internal
voltage reference of the AD9244. A reference amplifier buffers a
1 V fixed reference. The output from the reference amplifier,
A1, appears on the VREF pin. As stated earlier, the voltage on
the VREF pin determines the full-scale differential input span
of the ADC.
The voltage appearing at the VREF pin and the state of the
internal reference amplifier, A1, are determined by the voltage
present at the SENSE pin. The logic circuitry contains compara-
tors that monitor the voltage at the SENSE pin. The various
reference modes are summarized in Table 9 and are described
in the next few sections.
0V
Figure 47. Direct-Coupled Drive Circuit with AD8138 Differential Op Amp
1V p-p
0.1 μ F
1kΩ
50Ω
DISABLE
ADC
Figure 48. Equivalent Reference Circuit
TO
A1
1V
475Ω
499Ω
499Ω
1kΩ
2.5V
499Ω
499Ω
0.1 μ F
10 μ F
AD9244
AD8138
+
A2
A1
LOGIC
33Ω
33Ω
20pF
5V
R
R
VIN+
VIN–
AD9244
AVDD
REFB
REFT
REFT
REFB
VREF
SENSE
REFGND
10 μ F
10 μ F
+
0.1μF
0.1μF
0.1μF
+
10μF
Rev. C | Page 20 of 36
The actual reference voltages used by the internal circuitry of
the AD9244 appear on the REFT and REFB pins. The voltages
on these pins are symmetrical about midsupply or CML. For
proper operation, it is necessary to add a capacitor network to
decouple these pins. Figure 49 shows the recommended
decoupling network. The turn-on time of the reference voltage
appearing between REFT and REFB is approximately 10 ms and
should be taken into consideration in any power-down mode of
operation. The VREF pin should be bypassed to the REFGND
pin with a 10 μF tantalum capacitor in parallel with a low
inductance 0.1 μF ceramic capacitor.
Pin-Programmable Reference
By shorting the VREF pin directly to the SENSE pin, the inter-
nal reference amplifier is placed in a unity gain mode, and the
resulting VREF output is 1 V. By shorting the SENSE pin directly to
the REFGND pin, the internal reference amplifier is configured
for a gain of 2, and the resulting VREF output is 2 V.
Resistor-Programmable Reference
Figure 50 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. Use the equation
to determine the appropriate values for R1 and R2. These resistors
should be in the 2 kΩ to 10 kΩ range. For the example shown, R1
equals 2.5 kΩ and R2 equals 5 kΩ. From the previous equation, the
resulting reference voltage on the VREF pin is 1.5 V. This sets the
differential input span to 1.5 V p-p. The midscale voltage can also
be set to VREF by connecting VIN− to VREF.
3.25V
1.75V
VREF = 1 V × (1 + R1/R2)
+
1
+
LOCATE AS CLOSE AS POSSIBLE TO REFT/REFB PINS.
10μF
10μF
(1.5 V p-p Input Span, Differential Input with V
2.5V
Figure 50. Resistor-Programmable Reference
0.1μF
0.1μF
33Ω
33Ω
Figure 49. Reference Decoupling
20pF
VREF
REFGND
R1
2.5kΩ
R2
5kΩ
AD9244
1.5V
REFB
REFT
VIN+
VIN–
VREF
SENSE
REFGND
AD9244
REFB
REFT
0.1μF
1
+
10μF
0.1μF
0.1μF
0.1μF
CM
0.1μF
0.1μF
= 2.5 V)
+
10μF
(5)