Datasheet

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4 Power Supplies
4.1 Reference Voltage control via W4
Power Supplies
The ADS78/8505EVM board requires +5V DC for both the analog and digital sections of the ADC. Power
to the ADC is sourced from J3 pin 3 and pin 10 (+5VA and +5VD - see table below). The digital I/O
voltage can be set for +3.3V or +5.0V depending on the needs of the host processor driving the ADC.
Note:
VDIG must be less than or equal to VANA.
Table 3 shows the pin out of J3:
Table 3. Pinout of the Power Supply Connector, J3
Signal Pin Number Signal
+VA 1 2 -VA
+5VA 3 4 Unused
DGND 5 6 AGND
Unused 7 8 Unused
+3.3VD 9 10 +5VD
The EVM is configured with two small LC filters that take the voltage applied to J3 pin 3 and splits it into
+5VA and +5VD nodes. For stand alone operation, power sources can be applied via various test points
located on the EVM (VANA to TP2 and VDIG to TP4) provided that inductor L2 is removed. Refer to the
schematic at the end of this document for details.
The optional amplifier located at position U8 (user supplied) can be powered through J3 pins 1 and 2. See
the earlier discussion about the power supply restrictions in section 1.2 of this manual.
Note:
While filters are provided for all power supply inputs, optimal performance of the EVM
requires a clean, well-regulated power source.
The ADS78/8505 is normally configured to use its internal reference. Jumper W4 provides various options
to allow the EVM user to send the reference voltage off board to external amplifier circuits (W4 pins 7-8,
default state). An external reference source applied to J1 pin 20 can be sent to the ADC by moving the
shunt at W4 to pins 1-2. An on board trim pot is provided at R8, and can be used with a shunt jumper
placed on W6 pins 3-4. Finally, an on-board fixed 2.5V reference from U2 (REF3125) can be applied to
the ADC by moving the shunt at W4 to cover pins 5-6.
ADS78/8505EVM User's Guide4 SLAU145 – December 2004