Datasheet

R
i
+IN
-IN
+
-
Preamplifier
Output Buffer
Gain = K2
A1
A2
Gain = 10
OFFSET
Internal
Resistor
100 k:
OUT
Level
shift
1
5
3 4
7
8
R
i
= 900 k:x
G
i
- 100
G
i
LMP8602, LMP8602Q, LMP8603, LMP8603Q
www.ti.com SNOSB36D –JULY 2009–REVISED MARCH 2013
and for the LMP8603 with:
(24)
It should be noted from the equation for the gain G
i
that for large gains R
i
approaches 100 kΩ x (K
2
- 1). In this
case, the denominator in the equation becomes close to zero. In practice, for large gains the denominator will be
determined by tolerances in the values of the external resistor R
i
and the internal 100 kΩ resistor, and the K
2
gain error. In this case, the gain becomes very inaccurate. If the denominator becomes equal to zero, the system
will even become unstable. It is recommended to limit the application of this technique to gain increases of a
factor 2.5 or smaller.
Figure 46. Increase Gain
BIDIRECTIONAL CURRENT SENSING
The signal on the A1 and OUT pins is ground-referenced when the OFFSET pin is connected to ground. This
means that the output signal can only represent positive values of the current through the shunt resistor, so only
currents flowing in one direction can be measured. When the offset pin is tied to the positive supply rail, the
signal on the A1 and OUT pins is referenced to a mid-rail voltage which allows bidirectional current sensing.
When the offset pin is connected to a voltage source, the output signal will be level shifted to that voltage divided
by two. In principle, the output signal can be shifted to any voltage between 0 and V
S
/2 by applying twice that
voltage from a low impedance source to the OFFSET pin.
With the offset pin connected to the supply pin (V
S
) the operation of the amplifier will be fully bidirectional and
symmetrical around 0V differential at the input pins. The signal at the output will follow this voltage difference
multiplied by the gain and at an offset voltage at the output of half V
S
.
Example:
With 5V supply and a gain of 50x for the LMP8602, a differential input signal of +10 mV will result in 3.0V at the
output pin. similarly -10 mV at the input will result in 2.0V at the output pin.
With 5V supply and a gain of 100x for the LMP8603, a differential input signal of +10 mV will result in 3.5V at the
output pin. similarly -10 mV at the input will result in 1.5V at the output pin.
(1)
POWER SUPPLY DECOUPLING
In order to decouple the LMP8602/LMP8602Q/LMP8603/LMP8603Q from AC noise on the power supply, it is
recommended to use a 0.1 µF bypass capacitor between the V
S
and GND pins. This capacitor should be placed
as close as possible to the supply pins. In some cases an additional 10 µF bypass capacitor may further reduce
the supply noise.
(1) The OFFSET pin has to be driven from a very low-impedance source (<10Ω). This is because the OFFSET pin internally connects
directly to the resistive feedback networks of the two gain stages. When the OFFSET pin is driven from a relatively large impedance
(e.g. a resistive divider between the supply rails) accuracy will decrease.
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