LMP2232 Datasheet, PDF(16/20 Page) National Semiconductor (TI) – Micropower, 1.8V, Precision, Operational Amplifier with CMOS Input

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

LMP2232 Datasheet, PDF (16/20 Pages) National Semiconductor (TI) – Micropower, 1.8V, Precision, Operational Amplifier with CMOS Input

◁

Figure 2 shows a schematic of this input voltage noise reduc-

tion circuit. Typical resistor values are: RG = 10â¦, RF = 1 kâ¦,

and RO = 1 kâ¦.

FIGURE 2. Noise Reduction Circuit

30033946

PRECISION INSTRUMENTATION AMPLIFIER

Measurement of very small signals with an amplifier requires

close attention to the input impedance of the amplifier, gain

of the signal on the inputs, and the gain on each input of the

amplifier. This is because the difference of the input signal on

the two inputs is of the interest and the common signal is

considered noise. A classic circuit implementation is an in-

strumentation amplifier. Instrumentation amplifiers have a

finite, accurate, and stable gain. They also have extremely

high input impedances and very low output impedances. Fi-

nally they have an extremely high CMRR so that the amplifier

can only respond to the differential signal. A typical instru-

mentation amplifier is shown in Figure 3.

FIGURE 3. Instrumentation Amplifier

30033936

There are two stages in this amplifier. The last stage, output

stage, is a differential amplifier. In an ideal case the two am-

plifiers of the first stage, the input stage, would be set up as

buffers to isolate the inputs. However they cannot be con-

nected as followers because of mismatch of amplifiers. That

is why there is a balancing resistor between the two. The

product of the two stages of gain will give the gain of the in-

strumentation amplifier. Ideally, the CMRR should be infinite.

However the output stage has a small non-zero common

mode gain which results from resistor mismatch.

In the input stage of the circuit, current is the same across all

resistors. This is due to the high input impedance and low

input bias current of the LMP2232.

(1)

By Ohmâs Law:

(2)

However:

(3)

So we have:

VO1âVO2 = (2a+1)(V1âV2)

(4)

Now looking at the output of the instrumentation amplifier:

www.national.com

(5)

Substituting from Equation 4:

(6)

This shows the gain of the instrumentation amplifier to be:

âK(2a+1)

Typical values for this circuit can be obtained by setting:

a = 12 and K= 4. This results in an overall gain of â100.

▷