Data Sheet

12
Rev. A.02 www.3peakic.com.cn
LMV321A / LMV358A / LMV324A
80μA, 1.0MHz, 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 LMV321A/358A/324A 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 LMV321A/358A/324A
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: