11

LM321/LM358/ LM324

1.2MHz, Low-Power 36V Op Amps

www.3peakic.com.cn

Rev. B.01

and types of components, where possible to match the number and type of thermocouple junctions. For example,

dummy components such as zero value resistors can be used to match real resistors in the opposite input path.

Matching components should be located in close proximity and should be oriented in the same manner. Ensure leads

are of equal length so that thermal conduction is in equilibrium. Keep heat sources on the PCB as far away from

amplifier input circuitry as is practical.

The use of a ground plane is highly recommended. A ground plane reduces EMI noise and also helps to maintain a

constant temperature across the circuit board.

Instrumentation Amplifier

The LM321/358/324 OPA is well suited for conditioning sensor signals in battery-powered applications. Figure 4

shows a two op-amp instrumentation amplifier, using the LM321/358/324 OPA.

The circuit works well for applications requiring rejection of Common Mode noise at higher gains. The reference

11

12

2

2

=(

)(1

)

OUT

REF

G

RR

V

V

V

V

RR









Figure 4

Two-Pole Micro-power Sallen-Key Low-Pass Filter

Figure 5 shows a micro-power two-pole Sallen-Key Low-Pass Filter with 400Hz cut-off frequency.

For best results,

the filter’s cut-off frequency should be 8 to 10 times lower than the OPA’s crossover frequency. Additional OPA’s

phase margin shift can be avoided if the

OPA’s bandwidth-to-signal ratio is greater than 8. The design equations for

the 2-pole Sallen-Key low-pass filter are given below with component values selected to set a 400Hz low-pass filter

cutoff frequency:

R1

1M

Ω

R2

1M

Ω

C1

400pF

C2

400pF

R3

2M

Ω

R4

2M

Ω

12

12

-3dB

3

4

3

4

R = R = R = 1M

C = C = C = 400pF

Q = Filter Peaking Factor = 1

f

= 1/(2

) = 400

R = R /(2-1/Q) ; with Q = 1, R =R

RC

Hz





Figure 5