CN0411 ec: Questions

Question

Hello. I'm interested in incorporating the CN0411 conductivity-measurement design into a circuit. ( www.analog.com/.../cn0411.html) I plan to use the analog components as listed, but with a different MCU and ADC. 
 1: In the flowchart that describes how to set the gain etc (Figure 12) What is V_DEF? Is it the 2.5V vref? The 0.4V default DAC output? 
 2: How long should the PWM 1 and 2 pulses be? (the ones that go in the middle of the V_EXC pulses. Is there a guide on configuring their on/off durations? The document provides the 2 standard frequences and a diagram (Figure 9). PWM0 is described as either 94Hz or 2.4kHz. (I assume that means cycle at these intervals), but I'm not clear on PWM 1 and 2. 
 3: Does this design supersede CN0349 and CN0359? The three documents don't mention each other. They were published in date that goes with their number, and all appear to accomplish the same task. 
 Thank you very much.

rogueC · Accepted Answer

Hi, 
 1. The flowchart in Figure 12 shows the auto-ranging of the measurement by setting the appropriate gain for the conductivity range of the solution. V_DEF is the user defined desired applied voltage across the conductivity electrodes. I realize the terms excitation and applied voltage are easily confused. In this board the excitation voltage V_EXC refers to the voltage output of the DAC. V_DEF refers to the desired voltage across f the electrodes of the conductivity probe. If you rearrange the equation , V_EXC cancels off and the ratio of V_DEF and the measured voltage across the electrode is unity which is one of the goals of the pre-measurement routine. The idea of the conductivity measurement is to apply a fixed voltage across the electrodes and measure the current. The voltage and current values are used to calculate conductance. In this case, the routine starts off by applying a very small voltage across the electrodes through a resistive divider and continuously changing the gain resistors until the measured voltage across the electrodes matches the desired range. Then the DAC output voltage V_EXC is adjusted until the measured voltage across the electrode matches the desired value V_DEF. Using the DAC output excitation voltage and the current gain resistance, the current across the electrode can be computed. 
 2. PWM1 and PWM2 are the track-and-hold times used to sample the positive and "negative" voltages in the conductivity cell. (The "negative" meaning the part of the cycle where the electrodes connections are inverted). There is no exact guide as to the duration of both but the general rule is for them to be at least 10% of the period of PWM0 or the excitation frequency. These cannot be configured and scale automatically based on the excitation frequency which is either 94Hz and 2.4kHz. 
 3. CN0349 and CN0359 offer a higher degree of accuracy in conductivity readings. The CN0411 was designed for single supply operation with a lower power consumption to measure total dissolved solids in portable/handheld instrument applications. CN0359 and CN0349 support 4-wire conductivity probes while the CN0411 supports only 2-wire probes. 
 If you can discuss some details of your intended application, I can direct you to which board is more suited for you to use as reference. 
 Regards, 
 Nikko

rogueC · Answer

Hi, 
 You are correct the gain of 10 is to amplify the input to at least 1V. To compensate for errors introduced by this, there are 20ohm and 200ohm calibration resistors in the board which are used by software calibration. 
 The AD7124-8 ADC used in the board can accept ADC inputs down to mV ranges. But with the sample-and-hold design referenced to ground, placing a gain > 1 in the ADCs internal PGA will clip 50mV from the signal. You can however work with the design with the inst. amplifier at a gain of 1 and the AD7124-8 at a gain of 1. This board was made so that the gain resistor of the amplifier can easily be removed. 
 Regards.

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CN0411 ec: Questions

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