Right now we are targeting something around 0 and 100 degree Celsius. A point of notice is that the read out won't be anywhere close to the heat source. So maybe temperature right now is not of great concern.

Definitely yes. I'm still in the design and prototyping phase so adjustments (small and big) are allowed

After some thoughts we have decided to leave the low power consumption to a next design stage. So right now power is not an issue.

Yes! the uC will take care of the synchronization between signals, clear signals, and external communication with the user vua USB.

The capacitance depends on the geometry of the device. There's some room for adjustment but in the same order of magnitude (hundreds of pico Farads to couple of nano Farads

Also about burst measurement we have decided that if needed will be implemented in the next stage of the project. I have decided to go for this design that could be implemented also with a CPLD to allow more flexibility.

Maybe a note on the overall design. This is not an industrial or consumer design. It's more for research purposes and therefore the requirements can be less stringent (good for me ).


Agree! But I need to see how our "sensor" behaves before changing something in the design.


Well, questions are the first step to improve...they are always welcome (especially when they are extremely on the point )

1.Since you are currently unable to specify an operating temperature range for the measuring circuit, we can realistically only aim for a design that operates stably at room temperature. The alternative of operating over a range of 0°C to 100°C would make design extremely difficult and complex.
2.By "adjustments during manufacture" I was referring to trimpots and cap trimmers that would be a permanent part of the measuring circuit design; essentially they would be required calibration adjustment for each unit. Of course in some cases calibration/adjustment can be made via software, but also entailing a calibration phase during manufacture of each unit.
3.Loosening the requirement for lowest power should ease the task of getting to an acceptable design.
4.By ranges I was asking whether a design that used multiple test ranges (e.g. via a switch or electronic control) to cover the full range of CUTs would be acceptable. For now, we will assume that only one range is allowed and the CUT value can range from 500pF to 3500pF, with a tentative goal of accuracy in the ±10pF range. (Tentative goal because I do not yet know what difficulty will arise in meeting a ±10pF goal.)
5.Question still open: How frequently must a measurement be made?
6.Regarding whether to use RC or constant-current approaches. In my opinion they will yield about the same accuracy but the RC approach is much simpler and more easily made stable vs temperature.
7.New Question: Is +5V the only power available? (That is, +5V and some higher voltage, such as +9V or +12V, is not an option?)

I encourage you to give serious consideration to simply using a commercial cap measuring instrument for your first measurements, until you know more about what you need. There are DMM's with suitable cap ranges and there are low-cost "Chinese" boards that measure capacitance. There are also online many schematics/projects for measuring capacitance. You could save yourself much frustration, time, cost, etc by first defining better goals for the project before attempting a real design.

 

2 - Yes that is definitely an option then. I haven't though about it but it's something that can be added to calibrate things on board
5 - right now I'm measuring 10kS/s (which is also the frequency of charging oscillator)
6 - Can you develop a little bit more on the RC approach?
7 - Yes 5V (from USB) and 3V3 (from the 5V). Would you use higher voltages to charge the capacitor faster?

I encourage you to give serious consideration to simply using a commercial cap measuring instrument for your first measurements, until you know more about what you need

We have a lock-in and an LCR meter in the lab that we have used to measure some "sensors". The main problem is that the manufacturing process is not very accurate and therefore they have a large variation. What I like about this design is that in principle I could change the discharging resistor and use always the same design even if the capacitance values change (of course as long as they are in the same order of magnitude).

You could save yourself much frustration, time, cost, etc by first defining better goals for the project before attempting a real design.

True But I have the feeling this design I have implemented works quite well in standard conditions (no crazy temperature variations or stuff like that). The issue I have is that delay at the rising edges. As @TeeKay6 suggested I will try to insert a delay on the Vin1 line and try to adjust it to match the two outputs.

 

2 - Yes that is definitely an option then. I haven't though about it but it's something that can be added to calibrate things on board
5 - right now I'm measuring 10kS/s (which is also the frequency of charging oscillator)
6 - Can you develop a little bit more on the RC approach?
7 - Yes 5V (from USB) and 3V3 (from the 5V). Would you use higher voltages to charge the capacitor faster?


We have a lock-in and an LCR meter in the lab that we have used to measure some "sensors". The main problem is that the manufacturing process is not very accurate and therefore they have a large variation. What I like about this design is that in principle I could change the discharging resistor and use always the same design even if the capacitance values change (of course as long as they are in the same order of magnitude).

True But I have the feeling this design I have implemented works quite well in standard conditions (no crazy temperature variations or stuff like that). The issue I have is that delay at the rising edges. As @TeeKay6 suggested I will try to insert a delay on the Vin1 line and try to adjust it to match the two outputs.

@Vinnie90
By frequency of measurement I am referring to how often must you perform the charge/discharge cycle, not how fast you are clocking the counter. Restated, after one measurement is complete, how long can you wait before starting the next measurement?

 

@Vinnie90
By frequency of measurement I am referring to how often must you perform the charge/discharge cycle, not how fast you are clocking the counter. Restated, after one measurement is complete, how long can you wait before starting the next measurement?

@Vinnie90
The RC approach is the approach you are currently implementing in your schematic. Contrast that with a circuit in which the discharge resistor is replaced with a charge-current (or discharge-current) source. The current source approach gives a more linear discharge, but that is not much help since the discharge of a C through an R is extremely repeatable and the nonlinearity of discharge does not in any way interfere with the measurement.

A higher voltage allows more choice in circuitry. The measurement becomes more accurate (and more easily implemented) for higher voltage compared to lower voltage (due to leakage current and offset voltage errors).

 

By frequency of measurement I am referring to how often must you perform the charge/discharge cycle, not how fast you are clocking the counter. Restated, after one measurement is complete, how long can you wait before starting the next measurement?

The charging voltage is a 10kHz square wave (with 20% duty cycle). So I make a measurement for every rising edge of the excitation voltage (10kS/s).

I'm trying to implement these changes in the circuit. I'll keep you posted on how it goes

 

The charging voltage is a 10kHz square wave (with 20% duty cycle). So I make a measurement for every rising edge of the excitation voltage (10kS/s).

I'm trying to implement these changes in the circuit. I'll keep you posted on how it goes

Ok, You will charge the RC circuit for 20us and then discharge it to 0.4V, right? And your R=10K and C=3.5nF max. At t=0 start charging via a 1N4148; at t=20us the cap will have about 4.4V charge (provided that your driving pulse can supply sufficient current). By about t=130us the C will discharge to 0.4V. Think about that. Without including any delay for the comparators, etc, the time allowed for the test ended 30us before C reached the test voltage. If you add in 30us for comparators, the test period is 60us short. There are things we can do to shorten the test slightly, but not enough to make it feasible.