Datasheet

8
LTC1040
1040fa
Output Logic
In addition to the normal outputs (A
OUT
and B
OUT
), two
additional outputs, A + B and ON/0FF, are provided (see
Figure 8 and Table 1). All logic is powered from V
+
and
ground, thus input and output logic levels are independent
of the V
–
supply. The LTC1040 is directly compatible with
CMOS logic and is TTL compatible for 4.75V ≤ V
+
≤ 5.25V.
No external pull-up resistors are required.
ΣA INPUTS ΣB INPUTS A
OUT
B
OUT
A + B ON/OFF
++HHLL
+–HLLL
–+LHLH
––LLHI*
*I = indeterminate. When both A and B outputs are low, the ON/OFF output
remains in the state it was in prior to entering A
OUT
= B
OUT
= L.
Using External Strobe
A positive pulse on the strobe input, with the 0SC input tied
to ground, will initiate a comparison cycle. The STROBE
input is edge-sensitive and pulse widths of 50ns will
typically trigger the device.
APPLICATIO S I FOR ATIO
WUUU
Because of the sampling nature of the LTC1040, some
sensitivity exists between the offset voltage and the falling
edge of the input strobe. When the falling edge of the
strobe signal falls within the comparator’s active time
(80µs after rising edge), offset changes of as much as 2mV
can occur. To eliminate this problem, make sure the strobe
pulse width is greater than the response time, t
D
.
Using Internal Strobe
An internal oscillator allows the LTC1040 to strobe itself.
The frequency of oscillation, and hence sampling rate, is
set by an external RC network (see typical curve of
Sampling Rate vs R
EXT
, C
EXT
).
For self-oscillation, the STROBE pin must be tied to
ground. The external RC network is connected as shown
in Figure 9.
To assure oscillation, R
EXT
must be between 100k and
10M. There is no limit to the size of C
EXT
.
R
EXT
is very important in determining the power
consumption. The average voltage at the oscillator pin is
approximately V
+
/2. The power consumed by R
EXT
is then:
P
REXT
= (V
+
/2)
2
/R
EXT
.
LTC1040 • AI08
B
OUT
A
OUT
A + B
ON/OFF
80µs
COMPARATOR A
OUTPUT
COMPARATOR B
OUTPUT
STROBE
D
D
C
CQ
Q
2
4
3
15
R
EXT
C
EXT
LTC1040 • AI09
1
9
18
17
16
10
LTC1040
V
+
Figure 9. External RC Connection
Figure 8. LTC1040 Logic Diagram
Table 1. Output Logic Truth Table
Example: R
EXT
= 1M, V
+
5V, P
REXT
= (2.5)
2
/10
6
=
6.25 • 10
–6
W.
This is about four times the power consumed by the
LTC1040 at V
+
= 5V and f
S
= 1 sample/second. Where
power is a premium R
EXT
should be made as large as
possible. Note that the power consumed by R
EXT
is not a
function of f
S
or C
EXT
.