5 Power Supplies

The default ADC power-supply voltages (DVDD and AVDD) are generated by the PHI controller using power from the USB. The PHI provides 3.3-V and 5.5-V power rails. The 3.3-V power rail directly supplies the ADC DVDD voltage. The 5.5-V power rail is supplied to the input of the TPS7A4700, a low-noise, configurable-output LDO, to supply AVDD. The LDO output voltage depends on the internal reference voltage (V_{LDO_REF}) of 1.4 V and the pin connections. Grounding any LDO pin between pins 4 through pin 12 adds that pin voltage to V_{LDO_REF} and increases the LDO output voltage, LDO_VOUT. Figure 5-1 shows that the ADS1258EVM grounds pins 6 and 10 (3P2V and 0P4V, respectively) to set LDO_VOUT equal to 5 V.

Figure 5-1 ADS1258EVM LDO Circuit and External Power Source Connections

Figure 5-1 shows that the 5V_LDO output is used for the AVDD connections across the EVM. Additionally, the user can measure the AVDD current by connecting an ammeter between pins 1 and 2 of JP3 or the DVDD current by connecting an ammeter between pins 1 and 2 of JP4.

External power sources can also be provided for each supply voltage:

• LDO: Remove the shunt on JP1 and connect external power to JP1:2
• AVDD: Remove the shunt on JP3 and connect external power to JP3:1
• DVDD: Remove the shunt on JP4 and connect external power to JP4:1

When using either internal or external power supplies, LEDs D1 and D2 indicate if power is connected to AVDD and DVDD, respectively. These LEDs do not necessarily indicate if AVDD is 5 V or if DVDD s 3.3 V. Check the AVDD voltage using test point TP3 and check the DVDD voltage using test point TP5. Figure 5-2 shows the portion of the ADS1258EVM schematic that includes the LEDs and test points.

Figure 5-2 ADS1258EVM AVDD and DVDD Indicator LEDs