Instrumentation Circuits

The input impedance of the previous difference amplifier is

set by the resistors R

, R

, and R

. To eliminate the prob-

lems of low input impedance, one way is to use a voltage

follower ahead of each input as shown in the following two

instrumentation amplifiers.

Three-Op-Amp Instrumentation Amplifier

The quad LMV324 can be used to build a three-op-amp in-

strumentation amplifier as shown in Figure 8.

10006085

FIGURE 8. Three-Op-Amp Instrumentation Amplifier

The first stage of this instrumentation amplifier is a differential-

input, differential-output amplifier, with two voltage followers.

These two voltage followers assure that the input impedance

is over 100 MΩ. The gain of this instrumentation amplifier is

set by the ratio of R

. R

should equal R

, and R

equal

. Matching of R

to R

and R

to R

affects the CMRR. For

good CMRR over temperature, low drift resistors should be

used. Making R

slightly smaller than R

and adding a trim

pot equal to twice the difference between R

and R

will allow

the CMRR to be adjusted for optimum performance.

Two-Op-Amp Instrumentation Amplifier

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

make a high-input-impedance DC differential amplifier (Fig-

ure 9). As in the three-op-amp circuit, this instrumentation

amplifier requires precise resistor matching for good CMRR.

should equal R

and, R

should equal R

10006011

10006035

FIGURE 9. Two-Op-Amp Instrumentation Amplifier

Single-Supply Inverting Amplifier

There may be cases where the input signal going into the

amplifier is negative. Because the amplifier is operating in

single supply voltage, a voltage divider using R

and R

implemented to bias the amplifier so the input signal is within

the input common-mode voltage range of the amplifier. The

capacitor C

is placed between the inverting input and resistor

to block the DC signal going into the AC signal source,

. The values of R

and C

affect the cutoff frequency, fc =

1/2πR

As a result, the output signal is centered around mid-supply

(if the voltage divider provides V

/2 at the non-inverting input).

The output can swing to both rails, maximizing the signal-to-

noise ratio in a low voltage system.

10006013

10006020

FIGURE 10. Single-Supply Inverting Amplifier

15 www.national.com

LMV321/LMV358/LMV324 Single/Dual/Quad

ACTIVE FILTER

Simple Low-Pass Active Filter

The simple low-pass filter is shown in Figure 11. Its low-fre-

quency gain (ω → 0) is defined by −R

. This allows low-

frequency gains other than unity to be obtained. The filter has

a −20 dB/decade roll-off after its corner frequency fc. R

should be chosen equal to the parallel combination of R

and

to minimize errors due to bias current. The frequency re-

sponse of the filter is shown in Figure 12.

10006014

10006037

FIGURE 11. Simple Low-Pass Active Filter

10006015

FIGURE 12. Frequency Response of Simple Low-Pass

Active Filter in Figure 11

Note that the single-op-amp active filters are used in the ap-

plications that require low quality factor, Q( ≤ 10), low fre-

quency (≤ 5 kHz), and low gain (≤ 10), or a small value for

the product of gain times Q (≤ 100). The op amp should have

an open loop voltage gain at the highest frequency of interest

at least 50 times larger than the gain of the filter at this fre-

quency. In addition, the selected op amp should have a slew

rate that meets the following requirement:

Slew Rate ≥ 0.5 × (ω

OPP

) × 10

−6

V/µsec

where ω

is the highest frequency of interest, and V

OPP

is the

output peak-to-peak voltage.

Sallen-Key 2nd-Order Active Low-Pass Filter

The Sallen-Key 2nd-order active low-pass filter is illustrated

in Figure 13. The DC gain of the filter is expressed as

(1)

Its transfer function is

(2)

10006016

FIGURE 13. Sallen-Key 2nd-Order Active Low-Pass Filter

The following paragraphs explain how to select values for

, R

, C

, and C

for given filter requirements, such

as A

, Q, and f

The standard form for a 2nd-order low pass filter is

(3)

where

Q: Pole Quality Factor

: Corner Frequency

A comparison between Equation 2 and Equation 3 yields

(4)

(5)

To reduce the required calculations in filter design, it is con-

venient to introduce normalization into the components and

design parameters. To normalize, let ω

= ω

= 1 rad/s, and

= C

= 1F, and substitute these values into Equation

4 and Equation 5. From Equation 4, we obtain

(6)

From Equation 5, we obtain

(7)

www.national.com 16

LMV321/LMV358/LMV324 Single/Dual/Quad

For minimum DC offset, V

= V

−

, the resistor values at both

inverting and non-inverting inputs should be equal, which

means

(8)

From Equation 1 and Equation 8, we obtain

(9)

(10)

The values of C

and C

are normally close to or equal to

As a design example:

Require: A

= 2, Q = 1, fc = 1 kHz

Start by selecting C

and C

. Choose a standard value that is

close to

From Equations 6, 7, 9, 10,

= 1Ω

= 4Ω

The above resistor values are normalized values with ω

= 1

rad/s and C

= C

= 1F. To scale the normalized cutoff

frequency and resistances to the real values, two scaling fac-

tors are introduced, frequency scaling factor (k

) and

impedance scaling factor (k

Scaled values:

= R

= 15.9 kΩ

= R

= 63.6 kΩ

= C

= 0.01 µF

An adjustment to the scaling may be made in order to have

realistic values for resistors and capacitors. The actual value

used for each component is shown in the circuit.

2nd-Order High Pass Filter

A 2nd-order high pass filter can be built by simply interchang-

ing those frequency selective components (R

, R

, C

) in

the Sallen-Key 2nd-order active low pass filter. As shown in

Figure 14, resistors become capacitors, and capacitors be-

come resistors. The resulted high pass filter has the same

corner frequency and the same maximum gain as the previ-

ous 2nd-order low pass filter if the same components are

chosen.

10006083

FIGURE 14. Sallen-Key 2nd-Order Active High-Pass Filter

State Variable Filter

A state variable filter requires three op amps. One convenient

way to build state variable filters is with a quad op amp, such

as the LMV324 (Figure 15).

This circuit can simultaneously represent a low-pass filter,

high-pass filter, and bandpass filter at three different outputs.

The equations for these functions are listed below. It is also

called "Bi-Quad" active filter as it can produce a transfer func-

tion which is quadratic in both numerator and denominator.

17 www.national.com

LMV321/LMV358/LMV324 Single/Dual/Quad

Part #	LMV321M5
Description	IC OPAMP GP 1MHZ RRO SOT23-5
Category	IC
Availability	Out of Stock
Qty	0

LMV321M5

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