BASIC OPERATION
Figure 1 shows a basic circuit to operate the ADS7805 with
a full parallel data output. Taking R/C (pin 24) LOW for a
minimum of 40ns (7µs max) will initiate a conversion.
BUSY (pin 26) will go LOW and stay LOW until the
conversion is completed and the output registers are up-
dated. Data will be output in Binary Two’s Complement
with the MSB on pin 6. BUSY going HIGH can be used to
latch the data. All convert commands will be ignored while
BUSY is LOW.
The ADS7805 will begin tracking the input signal at the end
of the conversion. Allowing 10µs between convert com-
mands assures accurate acquisition of a new signal.
The offset and gain are adjusted internally to allow external
trimming with a single supply. The external resistors com-
pensate for this adjustment and can be left out if the offset
and gain will be corrected in software (refer to the
Calibra-
tion
section).
Table II for a summary of CS, R/C, and BUSY states and
Figures 3 through 5 for timing diagrams.
CS and R/C are internally OR’d and level triggered. There
is not a requirement which input goes LOW first when
initiating a conversion. If, however, it is critical that CS or
R/C initiates conversion ‘n’, be sure the less critical input is
LOW at least 10ns prior to the initiating input.
To reduce the number of control pins, CS can be tied LOW
using R/C to control the read and convert modes. This will
have no effect when using the internal data clock in the serial
output mode. However, the parallel output will become
active whenever R/C goes HIGH. Refer to the
Reading
Data
section.
CS
1
↓
0
0
↓
↓
0
0
R/C
X
0
↓
1
1
1
↑
0
BUSY
X
1
1
↑
1
0
0
↑
OPERATION
None. Databus is in Hi-Z state.
Initiates conversion “n”. Databus remains
in Hi-Z state.
Initiates conversion “n”. Databus enters Hi-Z
state.
Conversion “n” completed. Valid data from
conversion “n” on the databus.
Enables databus with valid data from
conversion “n”.
Enables databus with valid data from
conversion “n-1”
(1)
. Conversion n in progress.
Enables databus with valid data from
conversion “n-1”
(1)
. Conversion “n” in progress.
New conversion initiated without acquisition
of a new signal. Data will be invalid. CS and/or
R/C must be HIGH when BUSY goes HIGH.
New convert commands ignored. Conversion
“n” in progress.
STARTING A CONVERSION
The combination of CS (pin 25) and R/C (pin 24) LOW for
a minimum of 40ns immediately puts the sample/hold of the
ADS7805 in the hold state and starts conversion ‘n’. BUSY
(pin 26) will go LOW and stay LOW until conversion ‘n’ is
completed and the internal output register has been updated.
All new convert commands during BUSY LOW will be
ignored. CS and/or R/C must go HIGH before BUSY goes
HIGH or a new conversion will be initiated without suffi-
cient time to acquire a new signal.
The ADS7805 will begin tracking the input signal at the end
of the conversion. Allowing 10µs between convert com-
mands assures accurate acquisition of a new signal. Refer to
X
X
0
NOTE: (1) See Figures 3 and 4 for constraints on data valid from
conversion “n-1”.
Table II. Control Line Functions for “Read” and “Convert”.
200Ω
1
33.2kΩ
+
2
2.2µF
3
2.2µF
28
27
26
25
24
23
22
ADS7805
21
20
19
18
17
16
15
B0 (LSB)
B1
B2
B3
B4
B5
B6
B7
40ns min
6µs max
Convert Pulse
+
0.1µF
+
+5V
10µF
+
4
5
B15 (MSB)
B14
B13
B12
B11
B10
B9
B8
6
7
8
9
10
11
12
13
14
FIGURE 1. Basic Operation.
®
7
ADS7805
BURR-BROWN [ BURR-BROWN CORPORATION ]
