Datasheet

AD9204
Rev. 0 | Page 22 of 36
VOLTAGE REFERENCE
A stable and accurate 1.0 V voltage reference is built into the
AD9204. The VREF can be configured using either the internal
1.0 V reference or an externally applied 1.0 V reference voltage.
The various reference modes are summarized in the sections that
follow. The Reference Decoupling section describes the best
practices PCB layout of the reference.
Internal Reference Connection
A comparator within the AD9204 detects the potential at the
SENSE pin and configures the reference into two possible
modes, which are summarized in Table 10. If SENSE is grounded,
the reference amplifier switch is connected to the internal resistor
divider (see Figure 46), setting VREF to 1.0 V.
VREF
SENSE
0.5V
ADC
SELECT
LOGIC
0.1µF1.0µF
VIN–A/VIN–B
VIN+A/VIN+B
ADC
CORE
08122-012
Figure 46. Internal Reference Configuration
In either internal or external reference mode, the maximum
input range of the ADC can be varied by configuring SPI
Address 0x18 as shown in Tabl e 11 , resulting in a selectable
differential span from 1 V p-p to 2 V p-p.
If the internal reference of the AD9204 is used to drive multiple
converters to improve gain matching, the loading of the reference
by the other converters must be considered. Figure 47 shows
how the internal reference voltage is affected by loading.
0
–3.0
02
LOAD CURRENT (mA)
REFERENCE VOLTAGE ERROR (%)
.0
–0.5
–1.0
–1.5
–2.0
–2.5
0.2 0.4 0.6 0.8 1.0 1.4 1.6 1.81.2
08122-014
INTERNAL V
REF
= 0.993V
Figure 47. VREF Accuracy vs. Load Current
Table 10. Reference Configuration Summary
Selected Mode SENSE Voltage (V) Resulting VREF (V) Resulting Differential Span (V p-p)
Fixed Internal Reference AGND to 0.2 1.0 internal 2.0
Fixed External Reference AVDD 1.0 applied to external VREF pin 2.0
Table 11. Scaled Differential Span Summary
Selected Mode Resulting VREF (V) SPI Register 0x18 (Hex) Resulting Differential Span (V p-p)
Fixed Internal or External Reference 1.0 (internal or external) 0xC0 1.0
Fixed Internal or External Reference 1.0 (internal or external) 0xC8 1.14
Fixed Internal or External Reference 1.0 (internal or external) 0xD0 1.33
Fixed Internal or External Reference 1.0 (internal or external) 0xD8 1.6
Fixed Internal or External Reference 1.0 (internal or external) 0xE0 2.0