LTC1040_09 Datasheet, PDF (6/12 Pages) Linear Technology – Dual Micropower Comparator

LTC1040
APPLICATIO S I FOR ATIO
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.
S1
+
C_IN1 VIRTUAL
GROUND
V1
S2
–
C_IN2
+
V2
–
LTC1040 DUAL DIFFERENTIAL INPUT
LTC1040 • AI02
Figure 2. Dual Differential Equivalent Input Circuit
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.
6
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,
V_IN = V_REF
C_IN1
C_IN2
= 1V ± 1mV
because C_IN1 is guaranteed to equal C_IN2 to within 0.1%.
V_REF
+
–
+
V_IN
–
V_REF
+
V_IN
–
+
–
(a) OK
(b) Optimum
Figure 3. Two Ways to Do It
LTC1040 • TA03
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.
I_L
0.1Ω
–
R_L
+
+ 1/2
– LTC1040
OUT
+
V_S
100mV
OUT = HI IF I_L > 1A
OUT = LO IF I_L < 1A
LTC1040 • AI04
Figure 4. Load Current Detector
1040fa