AD9869BCPZ datasheet(34/37 Pages) AD

34 / 37 page

AD9869

Rev. A | Page 33 of 36

The ADC is based on a pipeline architecture with each stage

consisting of a switched capacitor amplifier. Therefore, its

performance vs. bias level is mostly dependent on the sample

rate. Figure 38 shows how the typical current consumption seen

at AVDD varies as a function of Register 0x13, Bits[2:0] and

sample rate, while the remaining bits are maintained at the

default setting of 0. Setting Bit 4 or Register 0x07 corresponds

to the 011 setting, and the settings of 101 and 111 result in

higher current consumption. Figure 39 shows how the SNR and

THD performance are affected for a 10 MHz sine wave input

for the lower power settings as the ADC sample rate is swept

from 20 MHz to 80 MHz.

SAMPLE RATE (MSPS)

220

120

130

140

150

160

170

180

190

200

210

000

001

010

011

100

101

101 OR 111

Figure 38. AVDD Current vs. ADC Bias Setting and Sample Rate

SAMPLE RATE (MSPS)

–54

–74

–72

–70

–68

–66

–64

–62

–60

–58

–56

SNR-00

SNR-01

SNR-10

SNR-11

THD-00

THD-01

THD-10

THD-11

Figure 39. SNR and THD Performance vs. fADC and ADC Bias Setting with

RxPGA = 0 dB, fIN = 10 MHz, AIN = −1 dBFS

A sine wave input is a standard and convenient method of

analyzing the performance of a system. However, the amount of

power reduction that is possible is application dependent, based

on the nature of the input waveform (such as frequency content,

and peak-to-rms ratio), the minimum ADC sample, and the

minimum acceptable level of performance. Thus, it is advisable

that power-sensitive applications optimize the power bias setting

of the Rx path using an input waveform that is representative of

the application.

POWER DISSIPATION

The power dissipation of the AD9869 can become quite high in

full-duplex applications in which the Tx path and Rx path are

simultaneously operating with nominal power bias settings. In

fact, some applications that use the IAMP may need to either

reduce its peak power capabilities or reduce the power

consumption of the Rx path so that the device’s maximum

allowable power consumption, PMAX, is not exceeded.

PMAX is specified at 1.66 W to ensure that the die temperature

does not exceed 125°C at an ambient temperature of 85°C. This

specification is based on the 64-lead LFSCP having a thermal

resistance, θJA, of 24°C/W with its heat slug soldered. (The θJA is

30.8°C/W if the heat slug remains unsoldered.) If a particular

application’s maximum ambient temperature, TA, falls below

85°C, the maximum allowable power dissipation can be

determined by the following equation:

PMAX = 1.66 + (85 − TA)/24

(10)

Assuming the IAMP common-mode bias voltage is operating

off the same analog supply as the AD9869, the following equa-

tion can be used to calculate the maximum total current

consumption, IMAX, of the IC:

IMAX = (PMAX − PIAMP)/3.47

(11)

With an ambient temperature of up to 85°C, IMAX is 478 mA.

If the IAMP is operating off a different supply or in the voltage

mode configuration, first calculate the power dissipated in the

IAMP, PIAMP, using Equation 3, and then recalculate IMAX using

Equation 11.

Figure 33, Figure 34, Figure 36, and Figure 38 can be used to

calculate the current consumption of the Rx and Tx paths for a

given setting.

MODE SELECT UPON POWER-UP AND RESET

The AD9869 power-up state is determined by the logic levels

appearing at the MODE and CONFIG pins. The MODE pin is

used to select a half- or full-duplex interface by pin strapping it

low or high, respectively. The CONFIG pin is used in conjunc-

tion with the MODE pin to determine the default settings for

the SPI registers as outlined in Table 10.

The intent of these particular default settings is to allow some

applications to avoid using the SPI (disabled by pin strapping

SEN high), thereby reducing implementation costs. For

example, setting MODE low and CONFIG high configures the

AD9869 to be backward compatible with the AD9975, while

setting MODE high and CONFIG low makes it backward

compatible with the AD9875. Other applications must use the

SPI to configure the device.

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