PS081 FAQ and Anwers

Q: What is the format of the HB0 result?

A: HB0 provides the compensated fullbridge result as a 24-bit value in two‘s complement. It gives the resistance ratio in ppm, multiplied by 100. According to that a load cell with 2mV/V sensivity gives 200000 as HB0 result at full scale. Additionally, the HB0 result can be scaled with the Mult_HB factors(config register 4 to 7). Setting them greater than 1 multiplies the result , e.g. Mult_HBx = 4 and would give you 800000 as HB0 value for the previous example.

Operating the PS081 in standalone mode, without the need of an external microcontroller, the HB0 result has to be read from RAM address 20. Using the PS081as converter front-end, in combination with an external microcontroller, the HB0 result is provided on RAM address 0.

Examples:

1.5 mV/V load cell, PICOSTRAIN wiring, Mult_HBx = 1:
1.5 mV/V = 1500 ppm, HB0 result at maximum strain: 150,000 (0x0249F0)

2 mV/V load cell, Wheatstone wiring, Mult_HBx = 1:

2 mV/V means 1.333 mV/V in Wheatstone = 1333 ppm (due to a reduction in strain),HB0 result at maximum strain: 133,333 (0x0208D5)

1 mV/V load cell, Picostrain wiring, Mult_HBx = 4:

1 mV/V = 1000 ppm HB0 result in PS081 at max. strain: 400,000 (0x061A80)

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Q: All noise values in the datasheet are related to 2 mV/V and PICOSTRAIN wiring. How do I calculate the noise for a sensor with lower sensivity or Wheatstone connection?

A: The base converter precision for a half bridge at averaing rate rate = 1 and a recommended discharge time of 90 to 150 µs in fast settling mode and 2 mV/V excitation is:
13.3 Bit eff. with internal comparator
13.8 Bit eff. with external bipolar comparator
In applications with Wheatstone bridges the converter's base precision is reduced by 0,6 Bit to

12.7 Bit eff. with internal comparator:
13.2 Bit eff. with external bipolar comparator

According to these values the converter's precision at higher avaraging rates can be calculated as follows:

The Bridge-factor is 2 for a full bridges and 4 when workink with quattro bridges. Further it has to be considered, that the achievable effective resolution linearily depends from your sensor's sensivity. Above values refer to a sensor with 2mV/V or 2000 ppm sensivity. Using a 1mV/V or 1000ppm sensor decrease the achievable effective resolution by a factor of two (equals 1 Bit) to

11.7 Bit eff. with internal comparator:
12.2 Bit eff. with external bipolar comparator

Example 1:

avrate = 12, Quattro bridge, internal comparator, 2mV/V

Resolution = 13.3 + ln(√(12*4))/ln(2) = 13.3 + 2.8 = 16.1 Bit eff. = 70,000 effective divisions = 10,000 peak-peak divisions in fast settle mode (without SINC-filter).

Example 2:

avrate = 450, Full bridge, external comparator, 2mV/V

Resolution = 13.9 + ln (√(450*2))/ln(2) = 13.8 + 4.9 = 18.7 Bit eff. = 425,000 effective divisions = 70,000 peak-peak divisions in fast settle mode.

Example 3:

avrate = 450, Full bridge, external comparator, 0.6mV/V

Resolution = 10.4 + ln (√(450*2))/ln(2) = 10.4 + 4.9 = 15.3 Bit eff. = 40,000 effective divisions = 6,000 peak-peak divisions in fast settle mode.

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Q: I read about different operation modes in PS081, continous mode, single conversion and stretched mode. What are the differences?

A: The PS081 has 3 basic operating modes as well as combinations of them. As they have an impact to the timing of the charge-/discharge cycles they are related to the sampling frequency and the active time of the 4 MHz oscillator. Therefore, the selection has infuence on the stability of the result and the current consumption. The following table provides an overview and gives some recomendations:

Applications	Mode		Parameters	Description
Highest resolution with no current limitation Standard mode for all applications with > 500 µA current capability	Continuous	Continuous mode	single_conversion = 0 stretch = 0	Continuously measuring, 4 MHz oscillator on all the time
High resolution but lower current	Continuous	Stretched continuous mode 2	single_conversion = 0 stretch = 2 or 3 cycle time = cytime*100µs	Continuously measuring. 4 MHz oscillator on only during the discharge time measurement.
Lowest current consumption Mechanically stable applications like pressure sensors	Single conversion	Single conversion mode	single_conversion = 1 stretch = 0	option with lowest current consumption, undersampling -> no suppression of mechanical vibrations
High resolution but lowest current, e.g. solar scales	Single conversion	Stretched single conversion mode 2	single_conversion = 1 stretch = 2 or 3 cycle time = cytime*100µs	option with very low current consumption and oversampling for suppression of mechanical vibrations.

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Q: How shall I setup the PS081 when I make first steps?

A: The recommended and easiest way is, to start with the PS081 evaluation system in combination with the evaulation software. Then you ha ev an ideal platform for test and evaluation, that contains preconfigured samples for different applications. THis ensures to start up in the shortest possible time. IN this softwaer the PS081 key parametere are indicated by a blue background, so they can be modified easily for test purposes. Once a suitable configuration is running, you can copy it to the PS081-Assembler and use it as a basis for your application specific assembler software.

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Q: What material shall the load capacitor be of?

A: The load capacitor, especially its material has significant influence to the measurement quality. So a good choice is simportant to get best results for your applications. Here are some recommandations the should be considered:

X7R Capitors: Use them only in low accuracy or low cost applications.

C0G/NP0 Types: Are recommended for highest accuracvy and tanperature stability, e. g. in calibratable scales.

CFCAP: Is quite an alterbative to the C0G types, but the temperature drift woll slightly increase.

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Q: What are the typical discharging times?

A: The optimal range would be between 70 µs and 120 µs. However, it can be smaller e. g. for faster measurements, in order to get higher data rates. The discharge time T is calculated T = 0.7 x R x C. where R specifies the strain gauge resitance and C the capacitance of your load capacitor.

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