Datasheet

ADS7815
6
SBAS065B
www.ti.com
TIMING
The timing shown in Figure 2 and Table II is the recom-
mended method of operating the ADS7815. The falling edge
of CS initiates the conversion. During the conversion, the
digital outputs are tri-stated and BUSY is LOW. Near the
end of the conversion, the digital outputs become active with
the most recent conversion result. After a brief delay (see
time t
11
in Figure 2 and Table II), BUSY rises. The rising
edge of BUSY is used to latch the digital result in Figure 1.
R/C AND CS
The R/C (read/convert) and CS signals control the start of
conversion and, when a conversion is not in progress, the
status of the digital outputs D15 through D0. It is possible to
start a conversion by taking CS LOW and then taking R/C
LOW. However, this is not recommended and will result in
a significant decrease in signal-to-noise ratio. This is due to
FIGURE 3. Bus Timing.
SYMBOL DESCRIPTION MIN TYP MAX UNITS
t
1
CS to R/C Delay 200 ns
t
2
CS to BUSY Delay 40 ns
t
3
Aperture Delay 40 ns
t
4
BUSY LOW 3.3 µs
t
5
R/C LOW to CS LOW 100 ns
t
6
BUSY HIGH to CS HIGH 250 ns
t
7
Bus Access Time 10 83 ns
t
8
Bus Relinquish Time 83 ns
t
9
Throughput Time 4 µs
t
10
Conversion Time 3.3 µs
t
11
Data Valid to BUSY HIGH 25 35 ns
t
12
CS to R/C Setup Time 40 ns
TABLE II. Conversion Timing.
FIGURE 2. ADS7815 Timing.
t
12
CS
R/C
t
7
t
8
Hi-Z State
D15 - D0
Hi-Z State
MODE
Acquire
DataValid
CS
t
9
t
6
t
5
BUSY
t
4
R/C
t
1
t
2
Hi-Z State
D15 - D0
t
11
t
8
Hi-Z State
AcquireMODE
Convert
t
10
t
3
Acquire
DataValid
the digital outputs tri-stating while the sample and hold
transitions to the hold mode. The change in digital outputs
results in noise being coupled onto the hold capacitor.
If a conversion is not in progress or is just about to finish, the
digital outputs will be active when R/C is HIGH and CS is
LOW. This is shown in Figure 2 and Figure 3. It is possible
to return CS HIGH during the initial part of the conversion
(as is done with R/C) and prevent the digital outputs from
becoming active. At a later time, the digital results could be
read by taking CS LOW. It is also possible to leave R/C
LOW, take CS HIGH during the conversion, and read the
results at a later time by taking R/C HIGH and CS LOW.
Following a conversion, if R/C and CS are both LOW 250ns
after BUSY rises, then a new conversion will be initiated
without allowing the proper acquisition period for the sample
and hold. R/C must remain HIGH or CS must be taken
HIGH within 250ns of BUSY rising.