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.
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…
The resulting voltage is by idea "differential" since it is V+ - V-. However, V+ and V- are sample at different times in each excitation voltage cycle. Since the 2 electrodes are switched in each cycle…
Thank you very much! That entirely answers my question here.
Ok, another silly question: What is the purpose of the 10x gain instrumentation amp? I know that sometimes op amps are used to impedance match, like when measuring pH. Here, it seems like you wouldn't need that. Is it due to the ADC expecting voltage in the ~1v range (vice the 100mA range of the probe)? A downside seems to be that it introduces errors in the amplification factor you offset for (The 0.1 factor in software), from the ref resistor and amp itself.
If you use an ADC that can handle 100mA range voltages, would it be better to leave out the amp? Thank you. The goal would be increased accuracy from not dealing with amplification factor error, and reduced circuit complexity. I guess I'm looking for a reason to keep it that outweighs these.
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.
Thank you very much for the explanation - that answers my question entirely, re why the amp, and how to offset for the effect it and its resistor have on the signal.