Datasheet
+IN
-IN
V
CC
4.5V < V
CC
< 76V
4.0V < V
IN
< 76V
R
SENSE
GND
V
OUT
LMP8481
I
L
-+
+
-
L
o
a
d
+
-
V
REFA
V
REFB
V
5()¶
100 k:
100 k:
1.95 M:
2 M:
Internal
14V LDO
Regulator
R
GP
V
CM
SENSE
R
GN
400 k:
400 k:
V to I
Converter
Difference
Amplifier
(x5)
V
SENSE
LMP8480, LMP8481
www.ti.com
SNVS829A –JUNE 2012–REVISED AUGUST 2012
Figure 7. LMP8481 Functional Diagram
Internally, the voltage on each input pin is converted to a current by the internal precision thin-film input resistors
R
GP
and R
GN
. A second set of much higher value V
CM
sense resistors between the inputs provide a sample of
the input common mode voltage for internal use by the differential amplifier.
V
SENSE
is applied to the differential amplifier through R
GP
and R
GN
. These resistors change the input voltage to a
differential current. The differential amplifier then servos the resistor currents through the MOSFETs to maintain
a zero balance across the differential amplifier inputs.
With no input signal present, the currents in R
GP
and R
GN
are equal. When a signal is applied to V
SENSE
, the
current through R
GP
and R
GN
are imbalanced and are no longer equal. The amplifier then servos the MOSFETS
to correct this current imbalance, and the extra current required to balance the input currents is then reflected
down into the two lower 400kΩ “tail” resistors. The difference in the currents into the tail resistors is therefore
proportional to the amplitude and polarity of V
SENSE
. The tail resistors, being larger than the input resistors for the
same current, then provide voltage gain by changing the current into a proportionally larger voltage. The gain of
the first stage is then set by the tail resistor value divided by R
G
value.
The differential amplifier stage then samples the voltage difference across the two 400K tail resistors and also
applies a further gain-of-five and output level-shifting according to the applied reference voltage (V
REF
).
The resulting output of the amplifier will be equal to the differential input voltage times the gain of the device, plus
any voltage value applied to the two VREF pins.
The resistor values in the schematic are ideal values for clarity and understanding. The table below shows the
actual values used that account for parallel combinations and loading. This table can be used for calculating the
effects of any additional external resistance.
Table 2. Actual Internal Resistor Values
R
GP and
R
GN
R
VCMSENSE
R
TAIL
Differential Amp FB V
REFx
Resistors
Gain Option (each) (each) (each) (each) (each)
20x 98.38k 491.9k 393.52k 1967.6k 98.38k
50x 39.352k 196.76k 393.52k 1967.6k 98.38k
60x 32.793k 172.165k 393.52k 1967.6k 98.38k
100x 19.676k 98.38k 393.52k 1967.6k 98.38k
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