Datasheet

6
LTC1040
1040fa
Tracking Error
Tracking error is caused by the ratio error between C
IN1
and C
IN2
and is expressed as a percentage. For example,
consider Figure 3a with V
REF
= 1V. Then at null,
Minimizing Comparison Errors
The two differential input voltages, V1 and V2, are con-
verted to charge by the input capacitors C
IN1
and C
IN2
(see
Figure 2). The charge is summed at the virtual ground
point; if the net charge is positive, the comparator output
is high and if negative, it is low. There is an optimum way
to connect these inputs, in a specific application, to
minimize error.
APPLICATIO S I FOR ATIO
WUUU
Ignoring internal offset, the LTC1040 will be at its switch-
ing point when:
V1 • C
IN1
+ V2 • C
IN2
= 0.
Optimum error will be achieved when the differential
voltages, V1 and V2, are individually minimized. Figure 3
shows two ways to connect the LTC1040 to compare an
input voltage, V
IN
, to a reference voltage, V
REF
. Using the
above equation, each method will be at null when:
(a) (V
REF
– 0V) C
IN1
– (0V – V
IN
) C
IN2
= 0
or V
IN
= V
REF
(C
IN1
/C
IN2
)
(b) (V
REF
– V
IN
) C
IN1
– (0V – 0V) C
IN2
= 0
or V
IN
= V
REF
.
Notice that in method (a) the null point depends on the
ratio of C
IN1
/C
IN2
, but method (b) is independent of this
ratio. Also, because method (b) has zero differential input
voltage, the errors due to finite input resistance are
negligible. The LTC1040 has a high accuracy capacitor
array and even the non-optimum connection will only
result in ± 0.1% more error, worst-case compared to the
optimum connection.
LTC1040 • AI02
C
IN1
VIRTUAL
GROUND
LTC1040 DUAL DIFFERENTIAL INPUT
S1
S2
+
–
+
–
V1
V2
C
IN2
Figure 2. Dual Differential Equivalent Input Circuit
Common Mode Range
The input switches of the LTC1040 are capable of
switching to either the V
+
or V
–
supply. This means that the
input common mode range includes both supply rails.
Many applications, not feasible with conventional com-
parators, are possible with the LTC1040. In the load
current detector shown in Figure 4, a 0.1Ω resistor is used
to sense the current in the V
+
supply. This application
requires the dual differential input and common mode
capabilities of the LTC1040.
Figure 4. Load Current Detector
V
IN
= V
REF
C
IN1
C
IN2
= 1V ± 1mV
Figure 3. Two Ways to Do It
V
REF
V
IN
V
REF
V
IN
LTC1040 • TA03
–
–
+
+
–
–
+
+
(a) OK (b) Optimum
because C
IN1
is guaranteed to equal C
IN2
to within 0.1%.
V
S
R
L
LTC1040 • AI04
I
L
100mV
–
–
+
+
+
0.1Ω
OUT
1/2
LTC1040
OUT = HI IF I
L
> 1A
OUT = LO IF I
L
< 1A