PicoStrain Strain Gauge Sensors

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PicoStrain Measurement Principle

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 TDC (Time-to-Digital Converter). The discharge time is in the range 100 µs. The TDC unit used have a typical single-shot resolution of less than 20 ps.

This measuring 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 products compensate for further error sources like the switch-on resistance of the output drivers (Rdson) and the propagation delay of the comparator. The result is very 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 this compensation task.

Due to the measuring principle, does not need a full-bridge but a half-bridge is sufficient. The supply of the half-bridge is provided directly by the circuits. There is no need for a separate supply of the SG. Also the reference voltage is not required.

Measurement Principle

PicoStrain Background : Measurement Task

Metal strain gages (SG) change their value with mechanical deformation, especially a variation in length. The strain e designates the relative variation in length of the SG:

Strain (e) = dL/L

Common SG have 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/

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

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

If the SG is connected in the manner of a Wheatstone bridge, this corresponds to a maximum signal output voltage of 2 mV/V. The resistance of common metal strain gauges 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).

PicoStrain Advantages

Advantage 1

Extremely low current consumption for your complete bridge sensor system

The ultra-low power PICOSTRAIN measuring principle shows an extremely low overall current consumption of the total system. It is possible to reduce the power consumption by a factor 10 to 20 compared to conventional AD-Converter solutions.

Advantage 2

One solution suits to (nearly) all applications, from low cost up to highest precision

PICOSTRAIN allows to build systems with 1.500 … 150,000 peak to peak divisions (at 2 mV/V strain) and update rates up to 1 kHz. One and the same chip covers the requirements of many applications, just by variation of your software.

Advantage 3

Reduced overall system costs

PICOSTRAIN reduces or saves the costs of the power supply. Power plugs will be replaced by batteries, batteries will be reduced in size. In many applications a single coin cell battery may last for 10 years under normal operation. The number of external components is far below the usual.

Advantage 4

New products that could not be done up to now

There are new product options mainly due to the low current consumption and low operating voltage. First to mention are solar driven applications. Also 1.55 V silver-oxide batteries are possible although with some restrictions (e.g. re-programming). The measurement quality is impressive even at those low voltages and it might be difficult to find something comparable in the market.

Advantage 5

Improve your Quality and lower your production cost

PICOSTRAIN offers unique possibilities to simplify the production of your bridge sensor and to improve its quality. The bridge offset can be simply adjusted by setting a register, gain drift can be reduced through software correction of Rspan. This method is more precise than with standard solutions. At the same time costly mechanical trimming, wich is still common practice, becomes redundant

Ordering Numbers

Ordering Numbers