LMV716 datasheet(11/15 Pages) NSC

11 / 15 page

Application Information (Continued)

Now looking at the output of the instrumentation amplifier:

(5)

Substituting from Equation (4):

(6)

This shows the gain of the instrumentation amplifier to be:

−K(2a+1)

Typical values for this circuit can be obtained by setting: a =

12 andK=4. This results in an overall gain of −100.

Three LMV716 amplifiers are used along with 1% resistors

to minimize resistor mismatch. Resistors used to build the

circuit are: R

1 = 21.6 k

Ω,R

11 = 1.8 k

Ω,R

2 = 2.5 k

Ω with K =

40 and a = 12. This results in an overall gain of −K(2a+1) =

−1000.

Two-Op-Amp Instrumentation Amplifier

A two-op-amp instrumentation amplifier can also be used to

make a high-input impedance DC differential amplifier Figure

6). As in the three op amp circuit, this instrumentation am-

plifier requires precise resistor matching for good CMRR. R

should be equal to R

1, and R3 should equal R2.

ACTIVE FILTERS

Active filters are circuits with amplifiers, resistors, and ca-

pacitors. The use of amplifiers instead of inductors, which

are used in passive filters, enhances the circuit performance

while reducing the size and complexity of the filter. The

simplest active filters are designed using an inverting op

amp configuration where at least one reactive element has

been added to the configuration. This means that the op amp

will provide "frequency-dependent" amplification, since reac-

tive elements are frequency dependent devices.

Low Pass Filter

The following shows a very simple low pass filter.

The transfer function can be expressed as follows:

By KCL:

(7)

Simplifying this further results in:

(8)

(9)

Now, substituting

ω=2πf, so that the calculations are in f(Hz)

rather than in

ω(rad/s), and setting the DC gain

and

(10)

set:

(11)

Low pass filters are known as lossy integrators because they

only behave as integrators at higher frequencies. The gen-

eral form of the bode plot can be predicted just by looking at

the transfer function. When the f/f

O ratio is small, the capaci-

tor is, in effect, an open circuit and the amplifier behaves at

a set DC gain. Starting at f

O, which is the −3 dB corner, the

capacitor will have the dominant impedance and hence the

circuit will behave as an integrator and the signal will be

attenuated and eventually cut. The bode plot for this filter is

shown in Figure 8.

20179513

FIGURE 6. Two-Op-Amp Instrumentation Amplifier

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FIGURE 7. Low Pass Filter

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