AD7894 Analog Devices, AD7894 Datasheet - Page 8

• Manufacturer Part Number: AD7894
• Manufacturer: Analog Devices

Specifications of AD7894

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 200kSPS
• Interface: Ser
• Analog Input Type: SE-Bip,SE-Uni
• Ain Range: Bip (Vref),Bip (Vref) x 4,Bip 10V,Bip 2.5V,Uni (Vref),Uni 2.5V
• Adc Architecture: SAR
• Pkg Type: SOIC

AD7894
Mode 2 Operation (Auto Sleep After Conversion)
The timing diagram in Figure 4 is for optimum performance in
operating Mode 2, where the part automatically goes into sleep
mode once BUSY goes low, after conversion and “wakes up”
before the next conversion takes place. This is achieved by keep-
ing CONVST low at the end of conversion, whereas it was high
at the end of conversion for Mode 1 Operation. The rising edge
of CONVST “wakes up” the AD7894. This wake-up time is
typically 5 s and is controlled internally by a monostable cir-
cuit. While the AD7894 is waking up there is some digital activ-
ity internal to the part. If the falling edge of CONVST (putting
the track/hold amplifier into hold mode) should occur during
this digital activity, noise will be injected into the track/hold
amplifier resulting in a poor conversion. For optimum results
the CONVST pulse should be between 40 ns and 2 s or greater
than 6 s in width. The narrower pulse will allow a system to
instruct the AD7894 to begin waking up and perform a conver-
sion when ready, whereas the pulse greater than 6 s will give
control over when the sampling instant takes place. Note that
the 10 s wake-up time shown in Figure 4 is for a CONVST pulse
less than 2 s. If a CONVST pulse greater than 6 s is used, the
conversion will not complete for a further 5 s after the falling edge
of CONVST. Even though the part is in sleep mode, data can still
be read from it. The read operation consists of 16 clock cycles as in
Mode 1 Operation. For the fastest serial clock of 16 MHz, the read
operation will take 1.0 s and this must be complete at least 250 ns
before the falling edge of the next CONVST, to allow the track/
hold amplifier to have enough time to settle. This mode is very
useful when the part is converting at a slow rate, as the power
consumption will be significantly reduced from that of Mode 1
Operation.
Serial Interface
The serial interface to the AD7894 consists of just three wires, a
serial clock input (SCLK) and the serial data output (SDATA)
and a conversion status output (BUSY). This allows for an
CONVST
BUSY
SCLK
Figure 4. Mode 2 Timing Diagram Where Automatic Sleep Function is Initiated
DOUT (O/P)
SCLK (I/P)
WAKES
PART
UP
THREE-STATE
CONVERSION
TRACK/HOLD
IS INITIATED;
GOES INTO
t
2
HOLD
t
=
CONVERT
t
3
= 31.25ns MIN,
= 10 s
Figure 5. Data Read Operation
CONVERSION
10 s LATER
ENDS
t
4
= 60ns MAX,
t
1
2 LEADING
2
ZEROS
–8–
t
t
SERIAL READ
5
4
OPERATION
= 10ns MIN,
2
easy-to-use interface to most microcontrollers, DSP processors
and shift registers.
Figure 5 shows the timing diagram for the read operation to the
AD7894. The serial clock input (SCLK) provides the clock
source for the serial interface. Serial data is clocked out from
the SDATA line on the falling edge of this clock and is valid on
both the rising and falling edges of SCLK. The advantage of
having the data valid on both the rising and falling edges of the
SCLK is to give the user greater flexibility in interfacing to the
part and so a wider range of microprocessor and microcontrol-
ler interfaces can be accommodated. This also explains the two
timing figures, t
time t
next data bit becomes valid, whereas the time t
how long after the falling edge of the SCLK the current data bit
is valid. The first leading zero is clocked out on the first rising
edge of SCLK. Note that the first zero will be valid on the first
falling edge of SCLK even though the data access time is speci-
fied at 60 ns for the other bits. The reason for this is that the
first bit will be clocked out faster than the other bits is due to
the internal architecture of the part. Sixteen clock pulses must
be provided to the part to access to full conversion result. The
AD7894 provides two leading zeros followed by the 14-bit
conversion result starting with the MSB (DB13). The last data
bit to be clocked out on the penultimate falling clock edge is the
LSB (DB0). On the 16th falling edge of SCLK the LSB (DB0)
will be valid for a specified time to allow the bit to be read on
the falling edge of the SCLK and then the SDATA line is dis-
abled (three-stated). After this last bit has been clocked out,
the SCLK input should return low and remain low until the
next serial data read operation. If there are extra clock pulses
after the 16th clock, the AD7894 will start over again with
outputting data from its output register and the data bus will
no longer be three-stated even when the clock stops. Provided
the serial clock has stopped before the next falling edge of
DB13
3
4
t
6
specifies how long after the falling edge of the SCLK the
= 20ns MAX @ 5V, A, B, VERSIONS
DB12
SHOULD END 250ns
READ OPERATION
t
5
RISING EDGE OF
PRIOR TO NEXT
4
CONVST
4
and t
15
5
t
, that are quoted on the diagram. The
3
250ns MIN
DB0
16
THREE-
STATE
t
6
SERIAL SHIFT
REGISTER
IS RESET
OUTPUT
5
specifies for
REV. 0