This is schematic of TDS (Total Dissolved Solids ) sensor module. Can someone help me in understanding the purpose of highlighted part of this circuit.

for more details about this sensor , visit https://www.dfrobot.com/wiki/index.php/Gravity:_Analog_TDS_Sensor_/_Meter_For_Arduino_SKU:_SEN0244

 

Welcome to AAC!
The highlighted part shows two precision rectifier circuits connected in parallel. The reason for such a connection escapes me.

 

can anyone explain in detail purpose of using 2 percision rectifier amplifier.

 

The input to the top LMV324 + input is grounded, and the lower LMV324 has the + input coming from the common op-amp ... so something is different.

 

Full wave precision rectifier instead of half wave?

 

Yup, that would be it.

 

That highlighted portion looks a whole lot like what is called a "window comparator", where the output state is different if the input voltage is between two limits, while the output is the other state when the voltage is either above or below those limits. Such a comparator scheme is often used for Accept/Reject testing.

 

That highlighted portion looks a whole lot like what is called a "window comparator", where the output state is different if the input voltage is between two limits, while the output is the other state when the voltage is either above or below those limits. Such a comparator scheme is often used for Accept/Reject testing.

Wouldn't the feedback loops be totally different in a comparator setup?

I always mix up my terminology, but I believe this is using negative feedback to provide a controlled voltage follower/rectifier/amplifier effect, as opposed to the positive feedback used in comparator circuits to provide hysteresis.

 

Wouldn't the feedback loops be totally different in a comparator setup?

I always mix up my terminology, but I believe this is using negative feedback to provide a controlled voltage follower/rectifier/amplifier effect, as opposed to the positive feedback used in comparator circuits to provide hysteresis.

OK, I did not look close enough. What the whole thing is for is producing a positive output for either polarity of input. Also called a precision rectifier, although that is not the purpose in this application. But it does provide a positive=going output no matter if the input is positive or negative. Probably the greater the signal in either direction, the greater the amount of disolved solids, or something like that.

 

Am i right that in water there can be both positive and negative charged particles, for positive particles , current flows from 2 to 1 (in J1) and in case of negative charged particles, the direction of current is reversed, so in this case precison rectifier is used.... please correct me

 

Yes. U2A is pumping out square waves with alternating polarity. It is the resulting alternating current flow through the test medium which is being sensed by U2C, which gives a proportional output voltage.

 

And the reasons for using square waves going both positive and negative are to avoid "polarizing the electrodes" and to provide results in either direction. Also it hepls to reduce some kinds of drift, I think.

 

I'll chuck in my bit as old time oceanographer.
Total Dissolved Solids is what you get when you filter out the non-soluble solids from a water sample and then evaporate all the water and weigh the remaining solids. There are many variations on this. For seawater (3.5% dissolved solids), there is a good correlation between conductivity and dissolved solids, so conductivity (reciprocal resistance) is the method of choice.
Greatest accuracy is obtained using an inductive salinometer.
For most other measurements, conductivity does.
Electrodes, commonly graphite or stainless steel can be used in 2, 3, or 4 electrode arrangements, depending on how accurate the measurement needs to be.
The main thing is that they all use an alternating current, not DC, to avoid electrolysis or plating.
National Semiconductor used to do a very useful IC which if memory still works, was an LM1890 or very similar. This had all the circuitry required and drove the AC at about 1-kHz.
The other main requirement is to provide a temperature measurement to compensate for the change in conductivity with temperature.
Conductivity is generally reported at standard temperatures, either 20 or 25-degrees C.
For what it's worth, all salinity measurements and instruments are calibrated by using Standard Seawater which used to come from a particular location in the Baltic Sea off Copenhagen