LT1011 Datasheet, PDF (6/16 Pages) Linear Technology – Voltage Comparator

LT1011/LT1011A
TYPICAL PERFOR A CE CHARACTERISTICS
Input Offset Voltage
vs Common Mode Voltage
2.5
T_J = 25°C
2.0
1.5
1.0
UPPER
COMMON MODE
0.5
LIMIT = V ⁺ – (1.5V)
0
– 0.5
–1.0
–1.5
V ^– (OR GND WITH
SINGLE SUPPLY)
– 2.0
–
2.5
V
–
0.1
0.2
0.3
0.4
0.5
0.6
0.7
V⁺
COMMON MODE VOLTAGE (V)
1011 G19
Offset Pin Characteristics
0.8
0.6
0.4
0.2
CHANGE IN V_OS FOR CURRENT
INTO PINS 5 OR 6
0
–150mV
VOLTAGE ON PINS 5 AND 6
WITH RESPECT TO V ⁺
–100mV
– 50mV
0
– 50 – 25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1011 G20
APPLICATIONS INFORMATION
Preventing Oscillation Problems
Oscillation problems in comparators are nearly always
caused by stray capacitance between the output and
inputs or between the output and other sensitive pins on
the comparator. This is especially true with high gain
bandwidth comparators like the LT1011, which are
designed for fast switching with millivolt input signals.
The gain bandwidth product of the LT1011 is over 10GHz.
Oscillation problems tend to occur at frequencies around
5MHz, where the LT1011 has a gain of ≈ 2000. This
implies that attenuation of output signals must be at
least 2000:1 at 5MHz as measured at the inputs. If the
source impedance is 1kΩ, the effective stray capaci-
tance between output and input must have a reactance
of more than (2000)(1kΩ) = 2MΩ, or less than 0.02pF.
The actual interlead capacitance between input and out-
put pins on the LT1011 is less than 0.002pF when cut to
printed circuit mount length. Additional stray capaci-
tance due to printed circuit traces must be minimized by
routing the output trace directly away from input lines
and, if possible, running ground traces next to input
traces to provide shielding. Additional steps to ensure
oscillation-free operation are:
1. Bypass the STROBE/BALANCE pins with a 0.01µF
capacitor connected from Pin 5 to Pin 6. This elimi-
nates stray capacitive feedback from the output to the
BALANCE pins, which are nearly as sensitive as the
inputs.
2. Bypass the negative supply (Pin 4) with a 0.1µF
ceramic capacitor close to the comparator. 0.1µF can
also be used for the positive supply (Pin 8) if the pull-
up load is tied to a separate supply. When the pull-up
load is tied directly to Pin 8, use a 2µF solid tantalum
bypass capacitor.
3. Bypass any slow moving or DC input with a capacitor
(≥ 0.01µF) close to the comparator to reduce high
frequency source impedance.
4. Keep resistive source impedance as low as possible. If
a resistor is added in series with one input to balance
source impedances for DC accuracy, bypass it with a
capacitor. The low input bias current of the LT1011
usually eliminates any need for source resistance bal-
ancing. A 5kΩ imbalance, for instance, will create only
0.25mV DC offset.
5. Use hysteresis. This consists of shifting the input
offset voltage of the comparator when the output
changes state. Hysteresis forces the comparator to
move quickly through its linear region, eliminating
oscillations by “overdriving” the comparator under all
input conditions. Hysteresis may be either AC or DC.
AC techniques do not shift the apparent offset voltage
6