Capacimeter
Technical specification
Measurement specifications
| Parameter | Rating |
|---|---|
| Number of channels | Grounded connection : 24 Floating connection : 12 |
| Measurement range | 0 - 500 pF |
| Accuracy | ±0.5 pF |
| Resolution | 0.001 pF |
| Noise | 0.01 pF RMS |
| Type of capacitance connection | Grounded Floating |
| Sampling rate | The sampling rate is defined by a sampling interval expressed in milliseconds. Logger mode : 1 channel @ 2 ms (500 sps), 2 channels @ 3 ms (333sps), 4 channels @ 4 ms (250 sps), etc. down to 24 channels @ 10 ms (100 sps). BLE mode : the minimal sampling interval is fixed to 50 ms (20 sps). |
Data storage
| Parameter | Rating |
|---|---|
| Type of micro-SD card supported | micro-SD micro-SDHC micro-SDXC |
| Maximum micro-SD storage size | 2 TB |
| Supported file systems | FAT FAT32 exFAT |
| File format | CSV (comma separator and decimal point) |
Electrical characteristics
| Parameter | Rating |
|---|---|
| Battery storage duration | 6 months |
| Autonomy in logger mode | >1000 hours for 24 channels at 1 sample per second in grounded mode >500 hours for 24 channels at 10 samples per seconds in grounded mode |
| Full charge duration | < 2h theoretical |
| Power input | 5VCC over USB-C |
Case characteristics
| Parameter | Rating |
|---|---|
| Length | 78mm |
| Width | 52mm |
| Height | 25mm |
Working principle
Core Measurement Method
Capacitance measurement is based on the principle of "discharge time measurement" of an RC network. This process occurs in three distinct phases:
- 1. Pre-charge phase: The capacitor is charged through a series resistor to a level close to the supply voltage. This approach reduces mechanical stress on the capacitive sensor, (particularly important for MEMS applications).
- 2. Full-charge phase: The capacitor is then completely charged to VDD without any series resistor, ensuring a complete charge.
- 3. Discharge phase: The capacitor is discharged through a calibrated discharge resistor to 0V. The discharge time is directly proportional to the capacitance according to the relationship: τ = k × R × C where τ is the discharge time, R is the discharge resistance, and C is the measured capacitance.
Ratiometric Approach
To eliminate systematic errors, the system uses a ratiometric approach, comparing the measured capacitor to a reference capacitor: τN/τref = CN/Cref This method eliminates most influences of temperature, voltage, and other environmental factors, as they affect both measurements identically.
Digital Signal Processing
After measurement, an integrated 32-bit DSP: - Calculates capacitance ratios - Applies compensation algorithms - Performs linearization via polynomial calculations - Compensates for temperature effects - Produces results with resolution reaching 20 bits
Let's measure !
Get you capacimeter ready : the connection
The capacimeter have two ways to measure capacitances, whether they are all connected to a common ground or floating capacitances.
- Grounded connection Capacitances are said to be "grounded" if they are connected according to the schematics shown below. Set the connection parameter in the configuration file as "grounded".
In this setup, the pinout of the capacimeter's connector is as depicted in the figure below (front view). When using "grounded" connection configuration, 24 channels are available as only one pin of the capacitance needs to be connected to the capacimeter. Any of the connector's ground pins can be used as reference because they are all connected together within the device.
- Floating connection
Capacitances are said to be "floating" when they are not (or must not) be connected to ground. Set the connection parameter in the configuration file as "floating". In this configuration, two pins of the capacimeter are needed to measure one capacitance which must be connected according to the schematics shown below.
When the capacimeter is set to "floating" connection mode, the pinout of the connector adheres to the diagram depicted below. It offers up to 12 channels, and there is no need to connect the ground pins.
Capacimeter configuration (sensor setup)
This device is compatible with Contextual Instruments' mobile app. Please visit the Apple Store or Google Play to download it. In LIVE mode, you can specify the parameters of your acquisition in a sensor setup. The capacimeter sensor setup is composed of the following elements.
- connection: Specify "grounded" or "floating" to indicate whether the capacitance being measured is connected to ground or left unconnected. Refer to the following section, "Get you capacimeter ready : the connection" for further details.
-
sampling_period: The sampling period, measured in milliseconds, is adjustable from 2 (equivalent to 500 samples per second per channel) to 10000 (equal to 0.01 samples per second per channel). The minimal authorized value depends on the number of channels selected. Please refer to following table for minimal values. If ble is set to true, the minimal interval is 50 ms (20 sps) whatever the number of channels is selected.
Number of Channels Minimum sampling interval Equivalent sampling rate 1 2 ms 500 sps 2 3 ms 333 sps 3 - 4 4 ms 250 sps 5 - 8 5 ms 200 sps 9 - 12 6 ms 167 sps 13 - 16 (grounded only) 7 ms 143 sps 17 - 20 (grounded only) 8 ms 125 sps 21 - 24 (grounded only) 10 ms 100 sps -
channel_number: The number of channels to acquire. In grounded mode this can be up to 24 while in floating mode, the maximum number of channels is 12.
- ble: Configure as true or false to determine whether the data should be sent via the Bluetooth Low Energy wireless interface. If enabled, the software application may be used to collect data that will be sent on the dashboard for easy access.
- sd: Configure as true or false to determine whether the data should be written to a CSV data file on the micro-SD card or not.
Capacimeter states
| State | LED Status | Definition |
|---|---|---|
| Charging | off | A suitable power source is connected and the device is fully charged |
| Charging | solid yellow | A suitable power source is connected and the device is charging |
| Sleep mode | off | The device is in sleep mode (or the battery is depleted) |
| IDLE | blinking green | Pressing the button while the device is in sleep mode wakes it up. Then the device is in IDLE mode, ready to measure |
| IDLE | blinking red | The device is in IDLE mode and battery is inferior to 20% |
| Preparing | solid blue | The device is connected to the smartphone and ready for the measurement to start |
| Measurement | solid blue | The device is measuring |
| Error | blinking red | The device has a problem with the data storage |
Reseting the device
Unplug the Capacimeter from any external power source by simply disconnecting the USB cable if it is connected. Then, use a small pin, such as a paperclip, to press the reset button through the tiny hole in the case.