Measurement of a strain gage is a big challenge of sensor technology. Due to the small excitation of the strain gage of only 0.2 % (2000 ppm) full scale, and the high resolution of 16 Bit plus, the resistors must be measured precisely in the ppb range. This is accomplished if you measure the ratio of the resistors.

The PICOSTRAIN strain gage measurement principle is breaking new grounds. The ratio of the resistors is ascribed to a time interval measure, not to a voltage difference like in a Wheatstone bridge. The sensor resistors together with a capacitor act as low-pass filters. The capacitor, charged to Vcc, is discharged through the sensor resistors. The discharge time to trigger level (selectable) is precisely measured by a Time-to-Digital Converter (TDC).

PICOSTRAIN® Circuits

Strain Gage Applications

The integrated circuits are realized without any analog component.

Acam uses it's TDC measurement technology to set new standards in circuit design.

The capacitor is charged to the supply voltage and then discharged through one of the SG resistors. The discharge time down to an arbitrary trigger level is measured with ultra-high precision using a Time-to-Digital Converter. The discharge time is in the range 100 µs. The circuits used have a typical single-shot resolution of less than 20 ps.

This measurement process is repeated in time-multiplex with both resistors of a half-bridge, using the same capacitor and the same comparator. Calculating the ratio of the results will turn out the absolute values and temperature dependencies of the capacitor and the comparator.

Additional patented circuits and algorithms inside the PICOSTRAIN products compensate for further error sources like switch-on resistance of the output drivers (R_dson) and propagation delays of the comparator. The result is highly precise, nearly free of gain errors and very stable with temperature. In total, each single measurement is made of 8 discharge/charge cycles to solve compensation tasks.

Due to the measurement principle, PICOSTRAIN does not need a full-bridge. A half-bridge is sufficient. The supply of the half-bridge is provided directly by the PICOSTRAIN circuits. There is no need for a separate supply of the strain gage and reference voltage is not required.

Thanks to the pulsed drive PICOSTRAIN easily controls and reduces the current in the whole system. PICOSTRAIN overall uses ultra low current. Current consumption is remarkably less than comparable ADC systems.

The PICOSTRAIN measurement principle shows a new approach to strain gage measurement. Contrary to the Wheatstone bridge, where the variation of resistance is transformed into a variation of voltage, PICOSTRAIN solutions transfer it into a high-precision time interval measurement. For this purpose the strain gage resistors are connected to a capacitor, forming a low-pass filter.

measurement basis

PICOSTRAIN® - Basis for the Measurement Principle

A metal strain gage will change its value with mechanical deformation, especially a variation in length. The strain e designates the relative variation in length of the strain gage:

Strain (e) = dL/L

Common strain gage has a maximum strain of typical - e(max) = 1000 µ (1000 x 10-6 or 0.1 %).

The ratio of the resistance variation to the length variation is designated K-factor or strain gain.

dR/R = K x dL/L

For a metal strain gage the K-factor is typically of value 2.
The maxim variation of the strain gage resistance is then given as:

dR(max)/R = e(max) x K = 2000 ppm

If the strain gage is connected in the manner of a Wheatstone bridge, this corresponds to a maximum signal output voltage of 2 mV/V. The resistance of a common metal strain gage is typically 350 Ohm or 1000 Ohm. The maximum variation in resistance and therefore the effective measurement range is within 0.7 Ohm to 2 Ohm. This small variation must be resolved according to the measurement task. The range of the resolution needed is very wide. It is between 10 ENOB (e.g. for pressure sensors) and 18 ENOB (e.g. calibrated scales). In the upper range the precision of the measurement has to be:

Resolution : 2000 ppm/218 = 0.008 ppm eff. or 26.9 ENOB referenced to the full resistance.

The typical measurement rates are in between 2 - 8 Hz (e.g. scales) and 4 - 10 kHz (e.g. fast pressure sensors).

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Acam USA Newsletter

PICOSTRAIN® - Sets Standard for Digital Strain Gage Measurement