PICOSTRAIN Span & Offset Compensation

The PS021 offers a very elegant method to correct for the span and offset compensation. This is based on the fact that the PS021 separates span and offset compensation and can measure directly the span compensation resistor.

The span compensation resistor can be modfied by software (through multiplication with a programmable register value).

Span Compensation Resistor Advantages

  • Correct descaled Load Cells
  • Fine tuning of Load Cells without touching it
  • Higher quality of the Load Cell
  • Cost reduction in Load Cell production

In todays solutions offset and span Compensation are not independent. Therefore the offset of the bridge have to be adjusted by resistors near to zero. PS021 can handle both values independent form the other. There is no need for the adjustment of the Bridge offset in Loadcell production.

PS02 Advantages

  • No need of an offset compensation resistor
  • Software correction of offset temperature drift
  • Higher quality of the Load Cell
  • Cost reduction in Load Cell production
Schematic Diagram
(click to enlarge)

    Schematic Diagram
    (click to enlarge)

      Datasheet PS09 Vol. 2:
      CPU - Single-chip solution for strain gauges
      Datasheet PS09 Vol. 1:
      General Data and Front-end Description
      Datasheet PS09 Eval System:
      Evaluation Kit for PS09
      Datasheet PS09:
      DLC Evaluation Kit for PS09
      Datasheet PS081:
      Enhanced Single-chip solution for weigh scales
      Datasheet PS081 Eval System:
      Evaluation kit for PS081
      Datasheet PS021:
      PICOSTRAIN front-end for strain gauge sensors
      Datasheet PS021 Eval System:
      Evaluation system for PS021
      Datasheet ALCS350-V2:
      Load cell simulator
      Datasheet PicoProg081:
      Production Programmer for PS081
      Application Note 030:
      PS09: Using PICOSTRAIN with piezoresistive sensors
      Application Note 025:
      PS081: EMI Countermeasures for a Digital Load Cell
      Application Note 023:
      PS081: Design Guideline for Building a Solar Body Scale
      Application Note 022:
      PS081: Design Guideline for Building a Solar Kitchen Scale
      Application Note 021:
      General: Compensation of gain error for uncompensated load cells
      Application Note 018:
      PS08: Metrological investigations of PS08, Determining Zero Drift and Gain Drift
      Application Note 012:
      General: Strain gage wiring with PICOSTRAIN
      Product Report 0909:
      PS081: ESD reliability report
      White Paper 004:
      PS081: How to build Digital Load Cells with PICOSTRAIN conveniently
      White Paper 003:
      PS08: Millikelvin Resolution with only a few Microampere
      White Paper 002:
      PS08 / PS081: How to lower gain and offset drift of a load cell
      White Paper 001:
      PS08: Construction guideline for solar driven scales
      Screencasts PS09:
      38:11 min. webcast introduction into the PS09 evaluation kit
      Screencasts PS09:
      19:01 min. webcast shows how to do a correct temperature compensation of a load cell using PS09 using the load cell's Rspan
      Screencasts PS09:
      17:54 min. webcast shows how to do a correct temperature compensation of a load cell using PS09 using the PS09 internal temperature sensor
      Screencasts PS081/PS09:
      20:28 / 40:30 min. webcast introducing a new concept for digital load cells
      Screencasts PS08/PS081:
      13:36 min. webcast shows how to do a correct scaling of the HB0 measurement result
      Screencasts PS08-EVA-KIT:
      23:23 min. movie introducing the PS08 assembler software
      Software PS09:
      PS09-EVA-KIT, PICOPROG v2.0
      Software PS081:
      PS081-EVA-KIT, PICOPROG v2.0
      Software PS081:
      PS081-DLC-KIT, PICOPROG v2.0
      Software PSA021/PS021:
      PSA021 / PS021-EVA-KIT, COM Port
      Software PicoProg V3.0 (V2.0):
      Driver Installer Stand-Alone
      Software National Instruments:
      NI VISA runtime engine for Windows 7
      Software National Instruments:
      NI VISA runtime engine for Windows Vista
      Software National Instruments:
      NI VISA runtime engine for Windows XP
      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.

      Thanks to the pulsed drive easily controls the current through the whole system and, even more important, reduces the current consumption to re-markably less than comparable ADC systems.

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

      Measurement Principle
      (click to enlarge)

      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).

      TypePart numberPackageRoHS compl.Shipping package
      PS09MNR 1783DiceYesWaffle pack
      PS09FNMNR 1840QFN40Yes
      PS09-EVA-KITMNR 1785PCBYes
      PS09-DLC-EVAMNR 1927PCBYes
      PS081MNR 1615DiceYesWaffle pack
      PS081FNMNR 1612QFN56Yes
      PS081-EVA-KITMNR 1525PCBYes
      PS021MNR 1002TQFP48YesTray
      PS021FNMNR 1001QFN48YesTray
      PSA21-STDMNR 984PCBYes
      PSA21-WSBMNR 985PCBYes
      PSA21mini-STDMNR 990PCBYes
      PSA21mini-WSBMNR 991PCBYes
      PicoProg Uni System V2.0MNR 1723PCBYes

      picostrain advantages


      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.


      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.


      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.


      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.


      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.