LMP8276 Datasheet, PDF (8/16 Pages) Texas Instruments – Bidirectional Precision Voltage Difference Amplifier

LMP8276
OBSOLETE
SNOSAK2K – DECEMBER 2004 – REVISED APRIL 2013
APPLICATION NOTE
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LMP8276
The LMP8276 is a single supply amplifier with a fixed gain of 20 and a common mode voltage range of −2V to
16V. The fixed gain is achieved in two separate stages, a preamplifier with gain of +10 and a second stage
amplifier with gain of +2. A block diagram of the LMP8276 is shown in Figure 20.
LMP8276
V_S = 4.75V to 5.5V
OFFSET
+IN
+
PROPRIETARY
A1
-IN
LEVEL-SHIFT
CIRCUITRY
GAIN = 10
-
100 k:
A2
GAIN = 2
OUT
GND
A1
A2
Figure 20. LMP8276
The overall offset of the LMP8276 is minimized by trimming amplifier A1. This is done so that the output referred
offset of A1 cancels the input referred offset of A2 or 10V_OS1 = −V_OS2
Because of this offset voltage relationship, the offset of each individual amplifier stage may be more than the limit
specified for the overall system in the datasheet tables. Care must be given when pin 3 and 4, A1 and A2, are
connected to each other. If the signal going from A1 to A2 is amplified or attenuated (by use of amplifiers and
resistors), the overall LMP8276 offset will be affected as a result. Filtering the signal between A1 and A2 or
simply connecting the two pins will not change the offset of the LMP8276.
Referencing the input referred offset voltages of each stage, the following relationship holds:
(10V_OS1) + (V_OS2) _{= VOS (LMP8276)}
10
(1)
If the signal on pin 3 is scaled, attenuated or amplified, by a factor X then the offset of the overall system will
become:
(10V_OS1) x (X) + (V_OS2)
10 (X)
= V_OS (LMP8276)
(2)
MID-RAIL OFFSET ADJUSTMENT PIN
The external mid-rail offset adjustment pin enables the user to utilize the LMP8276 for bidirectional current
sensing. If the offset pin, pin 7, is connected to ground then the LMP8276 is capable of sensing positive signals.
When the offset pin is connected to V_S then 1/2V_S is added to the output and the amplifier can sense both
positive and negative signals. If this offset pin is connected to any voltage between ground and V_S, then the
output is increased by a voltage equal to half of that offset. Namely, if the offset is connected to V_X, then the
output increases by 1/2V_X. This relationship is shown in Figure 21:
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