ADI pH meter application solutions enable accurate and efficient water quality measurement

A typical pH probe electrode is made of glass, which creates an extremely high resistance, ranging from 1 MΩ to 1 GΩ, acting as a resistor in series with the pH voltage source. The buffer amplifier bias current flowing through this series resistor will give The system introduces offset errors. To isolate the circuit from this high source resistance, a high input impedance, ultra-low input bias current buffer amplifier is required in this application. The ADA4661-2 or LTC6078 provides a precision buffer for high-impedance sensors such as pH probes and drives the ADC.

The ADA4661-2 is a dual-channel, precision, rail-to-rail input/output amplifier optimized for low power, high bandwidth, and wide operating supply voltage range applications. To minimize offset errors caused by bias current flowing through a high output impedance (approximately 1 GΩ) pH sensor, a typical input bias current is 0.15 pA. The ADA4661-2 has an offset error of 150μV. This is equivalent to a pH error of 0.0025 pH at 25°C. Proper layout measurements are recommended in PCB layout through the use of guard rings, shielding, and other techniques that are not affected by low currents.

Predicting system noise performance for PH channels

At an output data rate of 25 SPS and a gain of 1, the AD7124-4 has an rms noise of 570 nV in full power mode (noise referred to input, from AD7124-4 data sheet). At this time, the peak-to-peak noise can be calculated by the following formula:

Peak-to-peak noise = 6.6×rms noise=6.6×570nV=3.76μV

Adding the noise contribution from the ADA4661-2 (3μVp-p) in root sum square (rss) terms, the total system noise is predicted to be 4.818μVp-p. If the sensitivity of the pH meter is 59mV/PH, the noise-free resolution PH level that the pH meter can measure is 4.818μV/(59mV/PH)=0.0000816PH

The full-scale ADC input range is 6.6 V, so the predicted peak-to-peak resolution is

temperature measurement

The following is a 2-wire RTD temperature measurement:

Figure 2. 2-wire RTD temperature measurement

Three analog pins of the AD7124-4/AD4130-8 are used to implement a 2-wire configuration: AIN0, AIN2, and AIN3. AIN2 and AIN3 are configured as fully differential input channels for sensing the voltage on the RTD. The reference inputs used are REFIN+ and REFIN1-. Since a low-side reference resistor is used, a reference margin resistor is required.

Two-wire configuration requires an excitation current source. The excitation current source used to excite the RTD resistor, reference resistor, and margin resistor is generated by AVDD and flows to AIN0 (IOUT0). The same current flows through the RTD and the precision reference resistor (which produces the reference voltage), ensuring a ratiometric measurement.

The ADI official website can retrieve the tested and verified laboratory circuit Circuit Note: "CN-0381, fully integrated 4-wire RTD measurement system using low-power, precision, 24-bit Σ-Δ ADC"; and "CN0383, using Fully integrated 3-wire RTD measurement system with low-power, precision, 24-bit Σ-Δ ADC.

power supply

MAX42402/MAX42403 is a new low-cost, cost-effective switching power supply chip launched by ADI based on the new P90 process. It is a small synchronous buck converter with integrated high-side and low-side switches that can deliver up to 2.5A/3.5A. Voltage quality can be monitored by observing the PGOOD signal. The IC can operate at low dropout at 99% duty cycle, making it ideal for industrial applications.

Main advantages

MAX42402/MAX42403 adopts ADI's unique Silent Switcher technology, uses symmetrical input and flip-chip packaging technology, has low noise and low EMI characteristics, and can be used for low electromagnetic interference solutions and applications requiring excellent electromagnetic interference performance.

The MAX42402/MAX42403 feature high efficiency and low quiescent current in forced-PWM and skip modes of operation.

The MAX42402/MAX42403 provide the flexibility to operate two converters in parallel to meet higher power requirements

Features:

• Versatile, small size

• Operating input voltage range: 4.5V to 36V

• Synchronous DC-DC converter with integrated up to 2.5A/3.5A FETs

• Quiescent current in skip mode is 27µA

• Switching frequency: 1.5MHz/400kHz spread spectrum option 3.5Ms (1.5Mhz)/2.5ms (400kHz) internal soft start

• Programmable output voltage range: 0.8V to 12V

• 99% duty cycle operation mode for low dropout

• High accuracy meets safety critical application requirements

• Precision enable threshold implements fully programmable UVLO threshold

• Accurate windowed PGOOD

• Forced PWM and skip operating modes

• Over temperature, over voltage and short circuit protection

• 3mm x 3mm FC2QFN

• Operating Temperature Range: –40°C to +125°C
  The MAX38902A/B/C/D are low-noise, linear regulators that deliver up to 500mA output current with only 12μVRMS output noise over a frequency range of 10Hz to 100kHz. These regulators maintain ±1% output accuracy over a wide input voltage range and require only 100mV of input-to-output voltage headroom at full load. The no-load supply current is 365μA, independent of the dropout voltage. It also has fixed voltage output and resistor-adjustable output voltage, ranging from 0.6V to 5.3V. The MAX38902B also includes an active-low POK output.

• 1.7V to 5.5V input voltage range

• 0.6V to 5.3V output voltage range

• 12μVRMS output noise, 10Hz to 100kHz

• 365µA operating supply current

• 70dBPSRR@10kHz

• 500mA maximum output current

• ±1% DC accuracy over load, supply, and temperature

• 100mV (max) dropout @ 500mA load (3.6VIN)

• Shutdown current less than 0.1µA

• Very stable with 2µF (minimum) output capacitor

• Programmable soft start ramp rate

• Overcurrent and overheating protection

• Output to input backfeed current protection

• Active low POK output

• Pin-compatible with MAX8902 (TDFN)

• 1.22mm x 0.82mm, 2 x 3 ball, 0.4mm pitch WLP package, or 2mm x 2mm, 8-pin TDFN package

  
  

ADI's pH meter application solution can help customers conduct efficient pH testing and is an excellent choice for water quality monitoring solutions. ADI also provides flexible and rich product solutions to meet customers' individual needs. Its authorized agent Excelpoint can provide customers with technology Support helps customers shorten product development cycles and accelerate product application deployment.