AD9244-EVAL Analog Devices, AD9244-EVAL Datasheet - Page 16

• Manufacturer Part Number: AD9244-EVAL
• Description: 14-Bit/ 40/65 MSPS Monolithic A/D Converter
• Manufacturer: Analog Devices

REV. PrD 01/22/02
In addition to the limitations placed on the input voltages
VIN+ and VIN- by Equation 2, boundaries on the inputs
also exist based on the power supply voltages according to
the conditions
where AGND is nominally 0V and AVDD is nominally
+5 V. The range of valid inputs for VIN+ and VIN- is
any combination that satisfies both Equations 2 and 3.
For additional information showing the relationship be-
tween VIN+, VIN-, VREF and the analog input range of
the AD9244, see Tables I and II on page 15.
REFERENCE OPERATION
The AD9244 contains a bandgap reference which pro-
vides a pin-strappable option to generate either a 1V or
2V output. With the addition of two external resistors, the
user can generate reference voltages between 1V and 2V.
Another alternative is to use an external reference for de-
signs requiring enhanced accuracy and/or drift perfor-
mance as described later in this section. Figure 5a shows a
simplified model of the internal voltage reference of the
AD9244. A reference amplifier buffers a 1V fixed refer-
ence. The output from the reference amplifier, A1, ap-
pears 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 REFSENSE pin. The logic cir-
cuitry contains comparators that monitor the voltage at the
REFSENSE pin. If REFSENSE is tied to AGND, the
switch is connected to the internal resistor network thus
providing a VREF of 2.0V. If REFSENSE is tied to
VREF pin via a short or resistor, the switch will connect
to the REFSENSE pin. This connection will provide a
VREF of 1.0V. An external resistor network will provide
an alternative VREF between 1.0V and 2.0V (see Figure
6). Another comparator controls internal circuitry which
disables the reference amplifier if REFSENSE is tied to
AVDD. Disabling the reference amplifier allows the
VREF pin to be driven by an external voltage reference.
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 the analog
supply. For proper operation when using an internal or
external reference, it is necessary to add a capacitor net-
work to decouple these pins. Figure 5b shows the recom-
mended decoupling network. The turn-on time of the
reference voltage appearing between REFT and REFB is
approximately 10ms and should be evaluated in any power
down mode of operation.
USING THE INTERNAL REFERENCE
The AD9244 can be easily configured for either a 1V p-p
differential input span or 2V p-p input span by setting the
internal reference. Other input spans can be realized with
two external gain-setting resistors as shown in Figure 6 of
this data sheet, or using an external reference.
AD9244
AGND – 0.3V < VIN+ < AVDD + 0.3V
AGND – 0.3V < VIN- < AVDD + 0.3V
PRELIMINARY TECHNICAL DATA
(3)
–16–
Pin Programmable Reference
By shorting the VREF pin directly to the REFSENSE
pin, the internal reference amplifier is placed in a unity
gain mode and the resultant VREF output is 1V. By short-
ing the REFSENSE pin directly to the REFGND pin, the
internal reference amplifier is configured for a gain of 2.0
and the resultant VREF output is 2.0V. The VREF pin
should be bypassed to the REFGND pin with a 10µF
tantalum capacitor in parallel with a low-inductance
0.1µF ceramic capacitor as shown in Figure 6.
Resistor Programmable Reference
Figure 6 shows an example of how to generate a reference
voltage other than 1.0V or 2.0V with the addition of two
external resistors. Use the equation,
VREF = 1V ×(1 + R1/R2)
to determine appropriate values for R1 and R2. These
resistors should be in the 2K
example shown, R1 equals 2.5K
From the equation above, the resultant reference voltage
on the VREF pin is 1.5 V. This sets the differential input
span to be 1.5V p-p. The midscale voltage can also be set
to VREF by connecting VIN- to VREF.
Figure 6. Resistor Programmable Reference (1.5V p-p
Input Span, Differential Input with VCM = 2.5V
10 F
3.25V
1.75V
Figure 5a. AD9244 Equivalent Reference Circuit
10 F
Figure 5b. REFT and REFB Decoupling
2.5V
0.1 F
0.1 F
ADC
TO
DISABLE
1V
VREF
33
2.5V
R1
2.5K
A1
R2
5K
33
AD9244
15pF
AD9244
A2
A1
REFT
REFB
LOGIC
1.5V
VIN+
VIN-
VREF
REFSENSE
REFGND
to 10K
AD9244
0.1 F
and R2 equals 5K .
REFT
REFB
REFT
REFB
VREF
REFSENSE
REFGND
range. For the
10 F
0.1 F
0.1 F
0.1 F
0.1 F
0.1 F
10 F