6
LTC1040
1040fa
Tracking Error
Tracking error is caused by the ratio error between CIN1
and CIN2 and is expressed as a percentage. For example,
consider Figure 3a with VREF = 1V. Then at null,
Minimizing Comparison Errors
The two differential input voltages, V1 and V2, are con-
verted to charge by the input capacitors CIN1 and CIN2 (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
WU
UU
Ignoring internal offset, the LTC1040 will be at its switch-
ing point when:
V1 CIN1 + V2 CIN2 = 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, VIN, to a reference voltage, VREF. Using the
above equation, each method will be at null when:
(a)
(VREF – 0V) CIN1 – (0V – VIN) CIN2 = 0
or VIN = VREF (CIN1/CIN2)
(b) (VREF – VIN) CIN1 – (0V – 0V) CIN2 = 0
or VIN = VREF.
Notice that in method (a) the null point depends on the
ratio of CIN1/CIN2, 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
CIN1
VIRTUAL
GROUND
LTC1040 DUAL DIFFERENTIAL INPUT
S1
S2
+
–
+
–
V1
V2
CIN2
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 0.1
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
VIN = VREF
CIN1
CIN2
= 1V
± 1mV
Figure 3. Two Ways to Do It
VREF
VIN
VREF
VIN
LTC1040 TA03
–
+
–
+
(a) OK
(b) Optimum
because CIN1 is guaranteed to equal CIN2 to within 0.1%.
VS
RL
LTC1040 AI04
IL
100mV
–
+
0.1
OUT
1/2
LTC1040
OUT = HI IF IL > 1A
OUT = LO IF IL < 1A