Datasheet

AD7887
Rev. D | Page 14 of 24
second conversion can put the part back into Mode 1, and the
part goes into power-down mode when
CS
returns high.
Mode 2 (PM1 = 0, PM0 = 1)
In this mode of operation, the AD7887 remains fully powered
up regardless of the status of the
CS
line. It is intended for fastest
throughput rate performance because the user does not have to
worry about the 5 μs power-up time previously mentioned.
shows the general diagram of the operation of the
AD7887 in this mode.
Figure 17
The data presented to the AD7887 on the DIN line during the
first eight clock cycles of the data transfer are loaded to the
control register. To continue to operate in this mode, the user
must ensure that PM1 is loaded with 0 and PM0 is loaded with
1 on every data transfer.
The falling edge of
CS
initiates the sequence, and the input
signal is sampled on the second rising edge of the SCLK input.
Sixteen serial clock cycles are required to complete the conversion
and access the conversion result. Once a data transfer is complete
(that is, once
CS
returns high), another conversion can be initiated
immediately by bringing
CS
low again.
Mode 3 (PM1 = 1, PM0 = 0)
In this mode, the AD7887 automatically enters its full shutdown
mode at the end of every conversion. It is similar to Mode 1
except that the status of
CS
does not have any effect on the
power-down status of the AD7887.
Figure 18 shows the general diagram of the operation of the
AD7887 in this mode. On the first falling SCLK edge after
CS
goes low, all on-chip circuitry starts to power up. It takes
approximately 5 μs for the AD7887 internal circuitry to be fully
powered up. As a result, a conversion (or sample-and-hold
acquisition) should not be initiated during this 5 μs. The input
signal is sampled on the second rising edge of SCLK following
the
CS
falling edge. The user should ensure that 5 μs elapses
between the first falling edge of
SCLK
and the second rising
edge of SCLK after the
CS
falling edge, as shown in .
In microcontroller applications (or with a slow serial clock), this
is readily achievable by driving the
Figure 18
CS
input from one of the
port lines and ensuring that the serial data read (from the
microcontroller’s serial port) is not initiated for 5 μs. However,
for higher speed serial clocks, it will not be possible to have a
5 μs delay between powering up and the first rising edge of the
SCLK. Therefore, the user must write to the control register to
exit this mode and (by writing PM1 = 0 and PM0 = 1) put the
part into Mode 2. A second conversion needs to be initiated
when the part is powered up to get a conversion result, as
shown in . The write operation that takes place in
conjunction with this second conversion can put the part back
into Mode 3, and the part goes into power-down mode when
the conversion sequence ends.
Figure 19
Mode 4 (PM1 = 1, PM0 = 1)
In this mode, the AD7887 automatically enters a standby (or
sleep) mode at the end of every conversion. In this standby
mode, all on-chip circuitry, apart from the on-chip reference, is
powered down. This mode is similar to Mode 3, but, in this
case, the power-up time is much shorter because the on-chip
reference remains powered up at all times.
Figure 20 shows the general diagram of the operation of the
AD7887 in this mode. On the first falling SCLK edge after
CS
goes low, the AD7887 comes out of standby. The AD7887 wake-
up time is very short in this mode, so it is possible to wake up
the part and carry out a valid conversion in the same read/write
operation. The input signal is sampled on the second rising
edge of SCLK following the
CS
falling edge. At the end of
conversion (last rising edge of SCLK), the part automatically
enters its standby mode.