External DATACLK Active After the Conversion
and During the Next Conversion
CHIP SELECT (CS)
The CS input allows the digital outputs of the ADS7812 to
be disabled and gates the external DATACLK signal when
EXT/INT is HIGH. See Figure 9 for the enable and disable
time associated with CS and Figure 3 for a block diagram of
the ADS7812’s logic. The digital outputs can be disabled at
any time.
Figure 8 shows a method that is a hybrid of the two previous
approaches. This method works very well for microcontrollers
that do serial transfers 8 bits at a time and for slower
microcontrollers. For example, if the fastest serial clock that
the microcontroller can produce is 1µs, and two 8-bit trans-
fers must be used to obtain the serial data, the approach
shown in Figure 6 would result in a diminished throughput
(26kHz maximum conversion rate). The method described
in Figure 7 could not be used because time t25 would be
violated. The approach in Figure 8 results in an improved
throughput rate (33kHz maximum with a 1µs clock) and
DATACLK is LOW during t25.
Note that a conversion is initiated on the falling edge of CONV
even if CS is HIGH. If the EXT/INT input is LOW (internal
DATACLK) and CS is HIGH during the entire conversion, the
previous conversion result will be lost (the serial transmission
occurs but DATA and DATACLK are disabled).
CS
COMPATIBILITY WITH THE ADS7813
The only difference between the ADS7812 and the ADS7813
is in the internal control logic and the digital interface. Since
the ADS7813 is a 16-bit converter, the internal shift register
is 16 bits wide. In addition, only 16-bit decisions are made
during the conversion. Thus, the ADS7813’s conversion
time is approximately 133% of the ADS7812’s.
t26
t27
BUSY, DATA,
DATACLK(1)
HI-Z
Active
HI-Z
NOTE: (1) DATACLK is an output only when EXT/INT is LOW.
FIGURE 9. Enable and Disable Timing for Digital Outputs.
The timing presented in this data sheet will allow as much
compatibility as possible with the ADS7813. The main
concern will be the different number of serial clocks. If a
design must be compatible with both the ADS7812 and
ADS7813, it is recommended to consider the ADS7813
first. If the design works with the ADS7813, it will certainly
work with the ADS7812. This is also true in regards to
layout (see the Layout section of this data sheet).
ANALOG INPUT
The ADS7812 offers a number of input ranges. This is
accomplished by connecting the three input resistors to
either the analog input (VIN), to ground (GND), or to the
2.5V reference buffer output (BUF). Table I shows the
input ranges that are typically used in data acquisition
applications. These ranges are all guaranteed to meet the
specifications given in the Specifications table. Table IV
contains a complete list of ideal input ranges, associated
input connections, and comments regarding the range.
ANALOG
INPUT
RANGE (V)
CONNECT
R1IN
CONNECT
R2IN
CONNECT
R3IN
TO
INPUT
IMPEDANCE
(kΩ)
TO
TO
COMMENT
0.3125 to 2.8125
–0.417 to 2.916
0.417 to 3.750
±3.333
VIN
VIN
VIN
VIN
VIN
BUF
GND
VIN
> 10,000
26.7
26.7
21.3
45.7
45.7
21.3
45.7
45.7
45.7
21.3
21.3
26.7
26.7
45.7
21.3
21.3
26.7
26.7
Guaranteed offset and gain
VIN cannot go below GND – 0.3V
Offset and gain not guaranteed
Guaranteed offset and gain
VIN
VIN
VIN
BUF
BUF
BUF
GND
GND
GND
VIN
–15 to 5
VIN
BUF
GND
VIN
Offset and gain not guaranteed
Guaranteed offset and gain
±10
VIN
0.833 to 7.5
–2.5 to 17.5
2.5 to 22.5
0 to 2.857
–1 to 3
VIN
Offset and gain not guaranteed
Exceeds absolute maximum VIN
Exceeds absolute maximum VIN
Offset and gain not guaranteed
VIN cannot go below GND – 0.3V
Guaranteed offset and gain
VIN
BUF
GND
VIN
VIN
BUF
BUF
BUF
BUF
BUF
GND
GND
GND
GND
GND
VIN
BUF
GND
VIN
0 to 4
VIN
–6.25 to 3.75
0 to 10
BUF
GND
VIN
Offset and gain not guaranteed
Guaranteed offset and gain
VIN
0.357 to 3.214
–0.5 to 3.5
0.5 to 4.5
±5
VIN
Offset and gain not guaranteed
VIN cannot go below GND – 0.3V
Guaranteed offset and gain
VIN
BUF
GND
VIN
VIN
BUF
GND
Guaranteed offset and gain
1.25 to 11.25
VIN
Offset and gain not guaranteed
TABLE IV. Complete List of Ideal Input Ranges.
®
ADS7812
12
BB [ BURR-BROWN CORPORATION ]
