AVDD

GND

DVDD

AVDD

DVDD

1 PF

1 PF

ADS7044

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SBAS682D – NOVEMBER 2014 – REVISED DECEMBER 2015

10 Power-Supply Recommendations

10.1 AVDD and DVDD Supply Recommendations

The device has two separate power supplies: AVDD and DVDD. The device operates on AVDD; DVDD is used

for the interface circuits. AVDD and DVDD can be independently set to any value within the permissible ranges.

The AVDD supply also defines the full-scale input range of the device. Decouple the AVDD and DVDD pins

individually with 1-µF ceramic decoupling capacitors, as shown in Figure 45. The minimum capacitor value

required for AVDD and DVDD is 200 nF and 20 nF, respectively. If both supplies are powered from the same

source, a minimum capacitor value of 220 nF is required for decoupling.

Figure 45. Power-Supply Decoupling

10.2 Estimating Digital Power Consumption

The current consumption from the DVDD supply depends on the DVDD voltage, load capacitance on the SDO

line, and the output code. The load capacitance on the SDO line is charged by the current from the SDO pin on

every rising edge of the data output and is discharged on every falling edge of the data output. The current

consumed by the device from the DVDD supply can be calculated by Equation 4:

IDVDD = C × V × f

where:

•

C = Load capacitance on the SDO line,

•

V = DVDD supply voltage, and

•

f = Number of transitions on the SDO output.

(4)

The number of transitions on the SDO output depends on the output code, and thus changes with the analog

input. The maximum value of f occurs when data output on the SDO change on every SCLK. SDO changing on

every SCLK results in an output code of AAAh or 555h. For an output code of AAAh or 555h at a 1-MSPS

throughput, the frequency of transitions on the SDO output is 6 MHz.

To keep the current consumption at the lowest possible value, the DVDD supply must be kept at the lowest

permissible value and the capacitance on the SDO line must be kept as low as possible.

10.3 Optimizing Power Consumed by the Device

•

Keep the analog supply voltage (AVDD) as per the analog input full-scale range (FSR) requirement.

•

Keep the digital supply voltage (DVDD) at the lowest permissible value.

•

Reduce the load capacitance on the SDO output.

•

Run the device at optimum throughput. Power consumption reduces with throughput.

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