Typical Application Circuits

Buffered Chemical Sensors

Fig. 18 Buffered pH Probe

The LMV72xA family has input bias current in the pA range. This is ideal in

buffering high impedance chemical sensors, such as pH probes. As an example, the

circuit in Figure 7 eliminates expansive low-leakage cables that is required to

connect a pH probe (general purpose combination pH probes, e.g Corning 476540)

to metering ICs such as ADC, AFE and/or MCU. An LMV72xA op-amp and a lithium

battery are housed in the probe assembly. A conventional low-cost coaxial cable can

be used to carry the op-amp’s output signal to subsequent ICs for pH reading.

Shunt-Based Current Sensing Amplifier

The current sensing amplification shown in Figure 8 has a slew rate of 2πfVPP for the

output of sine wave signal, and has a slew rate of 2fVPP for the output of triangular wave

signal. In most of motor control systems, the PWM frequency is at 10kHz to 20kHz, and one

cycle time is 100μs for a 10kHz of PWM frequency. In current shunt monitoring for a motor

phase, the phase current is converted to a phase voltage signal for ADC sampling. This

sampling voltage signal must be settled before entering the ADC. As the Figure 8 shown, the

total settling time of a current shunt monitor circuit includes: the rising edge delay time (tSR)

due to the op-amp’s slew rate, and the measurement settling time (tSET). If the minimum duty

cycle of the PWM is defined at 5%, and the tSR is required at 20% of a total time window for a

phase current monitoring, in case of a 3.3V motor control system (3.3V MCU with 12-bit

ADC), the op-amp’s slew rate should be more than:

3.3V / (100μs× 5% × 20%) = 3.3 V/μs

At the same time, the op-amp’s bandwidth should be much greater than the PWM

frequency, like 10 time at least.

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LMV721, LMV722, LMV724

10MHz, RRIO CMOS Operational Amplifiers

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