AD7262BSTZ datasheet(25/32 Pages) AD

25 / 32 page

AD7262

Rev. 0 | Page 25 of 32

SERIAL INTERFACE

Figure 33 and Figure 34 show the detailed timing diagrams for

the serial interfacing of the AD7262/AD7262-5. The serial clock

provides the conversion clock and controls the transfer of

information from the AD7262/AD7262-5 after the conversion.

The AD7262/AD7262-5 has two output pins corresponding to

each ADC. Data can be read from the AD7262/AD7262-5 using

both DOUTA and DOUTB. Alternatively, a single output pin of the

user’s choice can be used. The SCLK input signal provides the

clock source for the serial interface.

The falling edge of CS puts the track-and-hold into hold mode,

at which point the analog input is sampled. The conversion is

also initiated at this point and requires a minimum of 19 SCLKs

to complete. The DOUTx lines remain in three-state while the

conversion is taking place. On the 19th SCLK falling edge, the

AD7262/AD7262-5 return to track mode and the DOUTA and

DOUTB lines are enabled. The data stream consists of 12 bits of

data, MSB first.

The MSB of the conversion result is clocked out on the 19th

SCLK falling edge to be read by the microcontroller or DSP on

either the subsequent SCLK falling edge (20th falling edge) or

the 20th SCLK rising edge. The choice of whether to read on the

rising or falling SCLK edge depends on the SCLK frequency

being used. When the maximum SCLK frequency of 40 MHz is

used with a VDRIVE voltage of 5 V, the maximum specified access

time (t4) is 23 ns, resulting in 2 ns of setup time, which may not

be sufficient for most DSPs or microcontrollers. Under these

conditions, it is recommended to use the rising SCLK edge to

read the data. In this case, the MSB of the conversion result is

clocked out on the 19th SCLK falling edge to be read on the 20th

SCLK rising edge, as shown in Figure 33. The remaining data is

then clocked out by subsequent SCLK falling edges. When using

a 40 MHz SCLK frequency, the 20th falling clock edge on the

serial clock clocks out the second data bit, which is provided for

reading on the 21st SCLK rising edge. The remainder of the 12-bit

result follows, with the final bit in the data transfer being valid

on the 31st rising edge. The LSB is provided on the 30th falling

clock edge.

An alternative to reading on the rising SCLK edge is to use a

slower SCLK frequency. If a slower SCLK frequency is used, for

example 32 MHz with the AD7262, this will enable reading on

the subsequent falling SCLK edge after the data has been

clocked out, as illustrated in Figure 35. A throughput rate of

1 MSPS can still be achieved for the AD7262 when a 32 MHz

SCLK frequency is used. The remaining data is then clocked out

by subsequent SCLK falling edges. When using a 32 MHz or

less SCLK frequency with the AD7262 or when using the

AD7262-5, the 20th falling clock edge on the serial clock has the

MSB provided for reading and also clocks out the second data bit.

The remainder of the 12-bit result follows, with the final bit in

the data transfer being valid on the 31st falling edge. The LSB is

provided on the 30th falling clock edge.

On the rising edge of CS, DOUTA and DOUTB go back into three-

state. If CS is not brought high after 31 SCLKs but is instead

held low for an additional 12 SCLK cycles, the data from

ADC B is output on DOUTA after the ADC A result. Likewise,

the data from ADC A is output on DOUTB after the ADC B

result. This is illustrated in

, which shows the DOUTA

example. In this case, the DOUT line in use goes back into three-

state on the 45th SCLK falling edge or the rising edge of

Figure 34

CS,

whichever occurs first.

If the falling edge of SCLK coincides with the falling edge of CS,

the falling edge of SCLK is not acknowledged by the AD7262

and the next falling edge of SCLK is the first one registered after

the falling edge of CS.

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