Typical Application Circuits

BUFFERED CHEMICAL SENSORS

Figure 7 Buffered pH Probe

The AD854xA 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 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 AD854xA 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). For a 3-shunt solution

in motor phase current sensing, if the smaller duty cycle of the PWM is defined at 45% (In

fact, the phase with minimum PWM duty cycle, such as 5%, is not detected current directly,

and it can be calculated from the other two phase currents), 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

1.1MHz, 85µA, CMOS, RRIO Operational Amplifiers

UMW

AD8541,AD8542,AD8544

R

www.umw-ic.com

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UTD Semiconductor Co.,Limited