AD7457BRT-R2 datasheet(14/20 Pages) AD

14 / 20 page

AD7457

Rev. A | Page 14 of 20

In applications with a slow SCLK, it is possible to read in data

on each SCLK rising edge. In this case, the first falling edge of

SCLK after the CS rising edge clocks out the second leading

zero and can be read in on the following rising edge. If the first

SCLK edge after the CS rising edge is a falling edge, the first

leading zero that was clocked out when CS went high is missed,

unless it was not read on the first SCLK falling edge. The fif-

teenth falling edge of SCLK clocks out the last bit of data, which

can be read in by the following rising SCLK edge.

POWER CONSUMPTION

The AD7457 automatically enters power-down at the end of

each conversion. When in the power-down mode, all analog

circuitry is powered down and the current consumption is 1 µA.

To achieve the specified power consumption (which is the

lowest), there are a few things the user should keep in mind.

The conversion time of the device is determined by the serial

clock frequency. The faster the SCLK frequency, the shorter the

conversion time. Therefore, as the clock frequency used is

increased, the ADC is dissipating power for a shorter period of

time (during conversion) and it remains in power-down for a

longer percentage of the cycle time or throughput rate. This

can be seen in Figure 23, which shows typical IDD vs. SCLK

frequency for VDD of 3 V and 5 V, when operating the device at

the maximum throughput of 100 kSPS.

SCLK Frequency (MHz)

2.5

2.0

1.0

1.5

0.5

410

VDD = 3V

VDD = 5V

TA = 25°C

Figure 23. IDD vs. SCLK Frequency for VDD = 3 V and 5 V

when Operating at 100 kSPS

Figure 24 shows typical power consumption vs. throughput rate

for the maximum SCLK frequency of 10 MHz. In this case, the

conversion time is the same for all throughputs, because the

SCLK frequency is fixed. As the throughput rate decreases, the

average power consumption decreases, because the ADC spends

more time in power-down.

020

THROUGHPUT (kSPS)

VDD = 3V

VDD = 5V

100

2.5

2.0

1.5

1.0

0.5

TA = 25°C

Figure 24. Power vs. Throughput Rate for SCLK = 10 MHz for VDD = 3 V and 5 V

MICROPROCESSOR INTERFACING

The serial interface of the AD7457 allows the part to be con-

nected to a range of different microprocessors. This section

explains how to interface the AD7457 with the ADSP-218x

serial interface.

AD7457 to ADSP-218x

The ADSP-218x family of DSPs can be interfaced directly to the

AD7457 without any glue logic. The serial clock for the ADC is

provided by the DSP. SDATA from the ADC is connected to the

data receive (DR) input of the serial port and CS can be con-

trolled by a flag (FL0). The connection diagram is shown in

Figure 25.

SCLK

SDATA

SCLK

DR0

RFS

FL0

1ADDITIONAL PINS OMITTED FOR CLARITY.

SPORT0

SPORT1

AD74571

ADSP-21xx1

Figure 25. AD7457 to ADSP-218x

SPORT0 must be enabled to receive the conversion data and to

provide the SCLK, while SPORT1 must be configured for flags

and so on.

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