AD7811/AD7812
OPERATING MODES
should occur before the end of the conversion process if the part
is not to be powered down. A serial read can take place at any
stage after the rising edge of CONVST. If a serial read is initi-
ated before the end of the current conversion process (i.e., at
time “A”), the result of the previous conversion is shifted out on
the DOUT pin. It is possible to allow the serial read to extend
beyond the end of a conversion. In this case the new data will
not be latched into the output shift register until the read
has finished. The dynamic performance of the AD7811 and
AD7812 typically degrades by up to 3 dBs while reading during
a conversion. If the user waits until the end of the conversion
process, i.e., 2.3 µs after the falling edge of CONVST (Point
“B”) before initiating a read, the current conversion result is
shifted out. The serial read must finish at least 100 ns prior to
the next falling edge of CONVST to allow the part to accurately
acquire the input signal.
The mode of operation of the AD7811 and AD7812 is selected
when the (logic) state of the CONVST is checked at the end of
a conversion. If the CONVST signal is logic high at the end
of a conversion, the part does not power down and is operat-
ing in Mode 1. If, however, the CONVST signal is brought
logic low before the end of a conversion, the AD7811 and AD7812
will power down at the end of the conversion. This is Mode 2
operation.
Mode 1 Operation (High Speed Sampling)
When the AD7811 and AD7812 are operated in Mode 1 they
are not powered down between conversions. This mode of opera-
tion allows high throughput rates to be achieved. The timing
diagram in Figure 16 shows how this optimum throughput rate
is achieved by bringing the CONVST signal high before the end
of the conversion.
Mode 2 Operation (Automatic Power-Down)
The sampling circuitry leaves its tracking mode and goes into
hold on the falling edge of CONVST. A conversion is also initi-
ated at this time. The conversion takes 2.3 µs to complete. At
this point, the result of the current conversion is latched into the
serial shift register and the state of the CONVST signal checked.
The CONVST signal should be logic high at the end of the
conversion to prevent the part from powering down. The serial
port on the AD7811 and AD7812 is enabled on the rising edge
of the first SCLK after the rising edge of the RFS signal—see
Serial Interface section. As explained earlier, this rising edge
When used in this mode of operation the part automatically
powers down at the end of a conversion. This is achieved by
leaving the CONVST signal low until the end of the conversion.
Because it takes approximately 1.5 µs for the part to power-up
after it has been powered down, this mode of operation is intended
to be used in applications where slower throughput rates are
required, i.e., in the order of 250 kSPS and improved power
performance is required—see Power vs. Throughput section.
There are two power-down modes the AD7811/AD7812 can
V
DD
tCONVERT
2.3s
tPOWER-UP
1.5s
tCONVERT
2.3s
CONVST
6040 HEX
4040 HEX
4040 HEX
DIN
NOT VALID
VALID DATA
VALID DATA
DOUT
Figure 15. Read/Write Sequence for AD7812
t1
CONVST
A
B
t
2
t12
SCLK
DOUT
CURRENT CONVERSION
RESULT
Figure 16. Mode 1 Operation Timing Diagram
–14–
REV. B
ADI [ ADI ]
