LMP8602MME datasheet(19/33 Pages) TI

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+IN

-IN

Preamplifier

Output Buffer

Gain = K2

Gain = 10

OFFSET

Internal

Resistor

100 k:

OUT

Level

shift

Ri = 900 k: x

Gi - 100

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 Gi that for large gains Ri approaches 100 kΩ x (K2 - 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 Ri and the internal 100 kΩ resistor, and the K2

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 VS/2 by applying twice that

voltage from a low impedance source to the OFFSET pin.

With the offset pin connected to the supply pin (VS) 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 VS.

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 VS 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 (<10

Ω). 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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Product Folder Links: LMP8602 LMP8602Q LMP8603 LMP8603Q

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