Comparators – a brief overview
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Similar to an Op Amp…including the symbol
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AV = ∞
IBIAS = 0
VOS = 0
BW = ∞
Except…
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Output
p stage
g is likely
y different ((open
p collector/drain, open
p
emitter/source)
Designed to saturate and recover rapidly
Not compensated,
p
, i.e. no provision
p
for unity
yg
gain stability
y
Not intended to be used as an amplifier
Used open loop or with POSITIVE feedback
More other difference can be subtle
More…other
Comparators – function and application
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Decides if an input voltage is above or below a reference voltage
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Often have unique output structures for speed or interfacing
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ii.e. Si
Single-bit
l bit analog-to-digital
l t di it l converter
t
Zero-crossing detector
Oscillators
Drivers (switch, relay, LED, etc.)
Compatibility
p
y with standard logic
g levels
Latching outputs
Open-collector/drain, open-emitter/source
Suffer from offset and bias current problems just like Op Amps
Input Overdrive – magnitude of ΔVIN
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Affects output response time
Op Amps as Comparators
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This is often done with the extra amp in a dual or quad package
Op Amp input protection diodes often prevent using an Op Amp as a
Comparator
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Large input differential causes large currents and damage
+
-
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Op Amps have better accuracy and lower drift (DC and dynamic)
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Op Amp is really the only option for microvolt comparisons
True Comparators are usually faster (no internal compensation)
Output voltage may not be compatible with logic levels
Very poor saturation recovery
Comparator as an Op Amp – don’t even try
Comparators make simple “digital” decisions
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Simple view of LM311 Comparator operation:
+
+
+
+
No Current Flow
Current Flows
VOUT
VIN < 0
-
GND
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GND
-
Output acts as Transistor “switch”
switch – only allows current to flow one direction
External connections (i.e. pull-up, pull-down) define output operation
“Open” Emitter not available on most comparators
High-speed comparators have a defined output circuit to preserve speed
“O
“Open”
” Collector
C ll t allows
ll
fl
flexibility
ibilit iin iinterfacing
t f i
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VOUT
VIN > 0
Voltage level shifting
LED or Relay drivers
If VIN = 0 V, output is INDETERMINATE!
LM311 – Output configurations are limitless
VDC
VDC
+V
B/S
OUT
3
G
4
-
V-
VREF
8
5
R1
6
VIN
2
V+
B
+
+
B/S
VOUT
7
OUT
VREF
1
U1
LM311
3
-
0
-V
„
G
4
2
V-
VIN
V+
B
8
5
+V
7
VOUT1
1
VOUT2
U3
LM311
R4
-V
VIN < VREF, VOUT = 0; VIN > VREF, VOUT = VDC
0
„
+V
VIN
2
8 5
+
V+B
B/S
OUT
VREF
3
-
6
VIN < VREF, VOUT1 = VOUT2 = VDCR4/(R3 + R4);
VIN > VREF, VOUT1 = VDC, VOUT2 = 0
VDC
VDC
+V
7
1
G
V-U2
LM311
4
-V
VOUT
VREF
2
8 5
+
VIN
3
-
„
6
7
R5
VOUT
1
G
V-U4
LM311
4
-V
VIN < VREF, VOUT = VDC; VIN > VREF, VOUT = 0
V+B
B/S
OUT
R2
0
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R3
6
-VDC
VIN < VREF, VOUT = VDC; VIN > VREF, VOUT = VDC
Problems with Comparators (LM311 specifically)
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Input Offset Voltage
Input Bias Current
Common-mode input range
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VCM
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+
IN
−
IN
V −V
=
2
VIN+ < VSS + 30V
VIN+ < VDD − 1.25V
VIN− > VDD − 30V > VSS
VIN− > VSS + 0.4V
With passive pull-up, output rise asymmetrical
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Exceeding the CM input range or absolute input range can cause very
strange responses (Definition,
(Definition Survival,
Survival Operation)
t(VOL → VOH) ≠ t(VOH → VOL)
Noise – causes jitter is switching
Overdrive – small overdrive = long
g switching
g time