AD9243ASZ Datasheet, PDF (11/24 Pages) Analog Devices – Complete 14-Bit, 3.0 MSPS Monolithic A/D Converter

AD9243
REFERENCE OPERATION
The AD9243 contains an onboard bandgap reference that pro-
vides a pin-strappable option to generate either a 1 V or 2.5 V
output. With the addition of two external resistors, the user can
generate reference voltages other than 1 V and 2.5 V. Another
alternative is to use an external reference for designs requiring
enhanced accuracy and/or drift performance. See Table II for a
summary of the pin-strapping options for the AD9243 reference
configurations.
Figure 26 shows a simplified model of the internal voltage
reference of the AD9243. A pin-strappable reference amplifier
buffers a 1 V fixed reference. The output from the reference
amplifier, A1, appears on the VREF pin. The voltage on the
VREF pin determines the full-scale input span of the A/D. This
input span equals,
Full-Scale Input Span = 2 × VREF
TO
A/D
5k⍀
AD9243
5k⍀
A2
5k⍀
DISABLE
A2
5k⍀
LOGIC
A1
1V
DISABLE
A1
LOGIC
CAPT
CAPB
VREF
7.5k⍀
5k⍀
SENSE
REFCOM
Figure 26. Equivalent Reference Circuit
The voltage appearing at the VREF pin as well as the state of
the internal reference amplifier, A1, are determined by the volt-
age appearing at the SENSE pin. The logic circuitry contains
two comparators which monitor the voltage at the SENSE pin.
The comparator with the lowest set point (approximately 0.3 V)
controls the position of the switch within the feedback path of
A1. If the SENSE pin is tied to REFCOM, the switch is con-
nected to the internal resistor network thus providing a VREF of
2.5 V. If the SENSE pin is tied to the VREF pin via a short or
resistor, the switch is connected to the SENSE pin. A short will
provide a VREF of 1.0 V while an external resistor network will
provide an alternative VREF between 1.0 V and 2.5 V. The
other comparator controls internal circuitry which will disable
the reference amplifier if the SENSE pin is tied AVDD. Dis-
abling 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 AD9243 appear on the CAPT and CAPB pins. For proper
operation when using the internal or an external reference, it is
necessary to add a capacitor network to decouple these pins.
Figure 27 shows the recommended decoupling network. This
capacitive network performs the following three functions: (1)
along with the reference amplifier, A2, it provides a low source
impedance over a large frequency range to drive the A/D inter-
nal circuitry, (2) it provides the necessary compensation for A2,
and (3) it bandlimits the noise contribution from the reference.
The turn-on time of the reference voltage appearing between
CAPT and CAPB is approximately 15 ms and should be evalu-
ated in any power-down mode of operation.
CAPT
AD9243
CAPB
0.1␮F
0.1␮F
10␮F
0.1␮F
Figure 27. Recommended CAPT/CAPB Decoupling Network
The A/D’s input span may be varied dynamically by changing
the differential reference voltage appearing across CAPT and
CAPB symmetrically around 2.5 V (i.e., midsupply). To change
the reference at speeds beyond the capabilities of A2, it will be
necessary to drive CAPT and CAPB with two high speed, low
noise amplifiers. In this case, both internal amplifiers (i.e., A1
and A2) must be disabled by connecting SENSE to AVDD and
VREF to REFCOM and the capacitive decoupling network
removed. The external voltages applied to CAPT and CAPB
must be 2.5 V + Input Span/4 and 2.5 V – Input Span/4 respec-
tively in which the input span can be varied between 2 V and 5 V.
Note that those samples within the pipeline A/D during any
reference transition will be corrupted and should be discarded.
Reference
Operating Mode
INTERNAL
INTERNAL
INTERNAL
EXTERNAL
(NONDYNAMIC)
EXTERNAL
(DYNAMIC)
Table II. Reference Configuration Summary
Input Span (VINA–VINB)
(V p-p)
2
5
2 ≤ SPAN ≤ 5 AND
SPAN = 2 × VREF
2 ≤ SPAN ≤ 5
2 ≤ SPAN ≤ 5
Required VREF (V)
1
2.5
1 ≤ VREF ≤ 2.5 AND
VREF = (1 + R1/R2)
1 ≤ VREF ≤ 2.5
CAPT and CAPB
Externally Driven
Connect
SENSE
SENSE
R1
R2
SENSE
VREF
SENSE
VREF
EXT. REF.
EXT. REF.
To
VREF
REFCOM
VREF AND SENSE
SENSE AND REFCOM
AVDD
EXT. REF.
AVDD
REFCOM
CAPT
CAPB
REV. A
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