12

REV A.02

www.3peakic.com.cn

LMV321TP/LMV358TP/LMV324TP

40μA, 1.27MHz, Micro-Power Rail-to-Rail I/O Op Amps

Figure 6: Frequency response of 100kHz, 74uA Gain-of-100 Amplifier

Buffered Chemical Sensor (pH) Probe

The LMV321TP/358TP/324TP OPA has input bias current in the pA range. This is ideal in buffering high

impedance chemical sensors such as pH probe.

As an example, the circuit in Figure 7 eliminates expansive

low-leakage cables that that is required to connect pH probe to metering ICs such as ADC, AFE and/or MCU.

A

LMV321TP/358TP/324TP OPA and a lithium battery are housed in the probe assembly.

A conventional low-cost

coaxial cable can be used to carry OPA’s output signal to subsequent ICs for pH reading.

R1

10M

R2

10M

COAX

BATTERY

3V

(DURACELL

DL1620)

ALL COMPONENTS CONTAJNED WITHIN THE pH PROBE

GENERAL PURPOSE

COMBINATION

pH PROBE

(CORNING 476540)

pH

PROBE

To

ADC/AFE/MCU

Figure 7: Buffer pH Probe

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

Figure 8 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: