Typical Application Circuits

The AD863xA 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 AD863xA 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.

Figure 8. Current Shunt Monitor Circuit

480

μA, 6.5MHz, RRIO CMOS Operational Amplifiers

UMW

AD8631,AD8632,AD8634

R

www.umw-ic.com

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UTD Semiconductor Co.,Limited