Datasheet

AD9237 Data Sheet
Rev. B | Page 18 of 24
Table 7. Reference Configuration Summary
Selected Mode SENSE Voltage Resulting VREF (V) Span Factor Resulting Differential Span (V p-p)
External Reference AVDD N/A 2
FactorSpan
ReferenceExternal
_
4

1
Internal Fixed Reference VREF 0.5 2 1.0 V
1 4.0 V
Programmable Reference 0.2 V to VREF
0.5 × (1 + R2/R1)
(See Figure 40)
2
FactorSpan
VREF
_
4

1
Internal Fixed Reference AGND to 0.2 V 1.0 2 2.0 V
1 1.0 V
VOLTAGE REFERENCE
A stable and accurate 0.5 V voltage reference is built into
the AD9237. The input range can be adjusted by varying
the reference voltage applied to the AD9237, using either the
internal reference or an externally applied reference voltage.
The input span of the ADC tracks reference voltage changes
linearly.
In all reference configurations, REFT and REFB drive the
A/D conversion core and, in conjunction with the span factor,
establish its input span. The input range of the ADC always
equals four times the voltage at the reference pin divided by
the span factor for either an internal or an external reference.
It is required to decouple REFT to REFB with 0.1 μF and 10 μF
decoupling capacitors, as shown in Figure 39.
Internal Reference Connection
A comparator within the AD9237 detects the potential at
the SENSE pin and configures the reference into one of four
possible states, which are summarized in Table 7. If SENSE is
grounded, the reference amplifier switch is connected to the
internal resistor divider, setting VREF to 1 V (see Figure 39).
Connecting the SENSE pin to VREF switches the reference
amplifier output to the SENSE pin, completing the loop and
providing a 0.5 V reference output. If a resistor divider is
connected, as shown in Figure 40, then the switch is again set to
the SENSE pin. This puts the reference amplifier in a non-
inverting mode with the VREF output defined as







1
2
150
R
R
.VREF
05455-043
ADC
CORE
VIN–
SENSE
VREF
VIN+
REFT
REFB
0.1F
0.1F
0.1F
10F
+
0.5V
SELECT
LOGIC
0.1F
+
10F
AD9237
Figure 39. Internal Reference Configuration
05455-044
ADC
CORE
VIN–
SENSE
R2
R1
VREF
VIN+
REFT
REFB
0.1F
0.1F
0.1F
10F
+
0.5V
SELECT
LOGIC
0.1F
+
10F
AD9237
Figure 40. Programmable Reference Configuration