Reading a strain gauge bridge

TI MSC1210 for example, though they are 8051s as well.

Mike

Is that 1KHz per channel ? Most SD-ADC devices do not multiplex well, as only special ones have single-cycle settling times, and 1KHz rates are also high for strain guage, and outside the range of the single-cycle settling ADCs

The C8051F06x series have faster SAR ADCs and so would easily give the data rate, and MUX handling for all 3 bridges, but with higher DC noise floor than the F350.

-jg

Anton Erasmus wrote in news: snipped-for-privacy@4ax.com:

Anton,

Strain gauge bridges to measure strain, force, pressure, acceleration/ vibration are not the most accurate circuits. You're lucky to reach

1 promille of full scale. So a 12 bit A/D converter is usually sufficient. If this is the case for you, it widens your possibilities.

Cheers,

Sam

Assuming that you are going to limit yourself to a maximum of 16 bits (as any more would be a waste anyway) then the C8051F350/1/2/3 devices from Silicon Labs would seem to be a reasonable choice. From the strain gauge input you will need to make use of the in-built amplifier of the chip. The C8051F350/1/2/3 range are, apparently, available in LQFP-32 packages which are 9mm square including the pins. Doesn't leave much room for all the other bits around it if you really need to stay within the 20mm square you indicate.

As someone else has already stated, limiting to 12 bit ADC's might offer you a wider selection of devices.

You can get away with one constant voltage source for the bridge excitation side of things. 10V is usual for this purpose. You can expect a full scale of the strain gauge in the region of 25mV (2.5mV per volt). That would save you some board space. You will need the RS485 interface device as well. Quite a squeeze really. Can you negotiate more room or even consider putting the interface conditioning on a mezzanine?

--
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Paul E. Bennett ....................
Forth based HIDECS Consultancy .....
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You might try Analog Devices, the AD7798/9 has 3 channels and comes in

16pin TSSOP. Sadly, they only do 470 SPS, but maybe they've got others.

Hi,

12-bit of data would be OK, but each sensor can have quite high offsets. These must either be trimmed out with some sort of front-end, or I need more bits to be able to take out the offsets in the MCU. I have looked at using the PGA309 from TI. With this front-end, a 12bit ADC should be OK. The problem is that 3 of these are quite big. Anyone know of similar products from other manufacturers which is available in a smaller package ?

Regards Anton Erasmus

Currently I have only prelimanary requirements. The existing system uses a number of ADC cards in a PC to read all the strain gauges. Over 60 in total. The problem with the current setup, is that the harnesses to get all these signals out are huge and difficult to handle. Wires also have a tendancy to break off, so the system is fairly unreliable. We have been asked to maek it more robust, and I am investigating a solution whereby 3 sensors are read with a small MCU and then the data is sent over a serial link. This will cut down the number of wires dramatically. In the current configuration I only have about 20 x 20 mm^2. It might be possible to cut away some material and get a slightly bigger area. Using a rigid-flex PCB that folds into the space might also work. Finding something that will fit into the 20 x 20 mm^2 will of course be holy grail.

Regards Anton Erasmus

My current info is that it must be 1kHz per channel. It is also quite desirable, but not esential that the 3 channels are sampled simultanously.

The current system have quite long wires, so using an ADC with a higher noise floor close to the sensor might be OK.

Regards Anton Erasmus

That points to the C8051F06x, as that easily meets the Sampling rate, and even precison specs. It also has DACs for trimming. With double-sided component placement this should be do-able on 20x20mm It could do more than 3 channels, if that helps physically.

For the PGA side, I've noticed Digital Pots are improving all the time, and they may give a small PCB area, and very flexible config.

-jg

You mentioned that you may have to deal with offset values. The parts I indicated do have DAC's, as do some other chips, that can apply the offsets required.

As for the space available, what is the volume of the space like? I have done systems in the past where the only solution because of the environment was to stack several boards together. Not much floor-plan area but sufficient height for a stack of 10 boards.

Might the configuration of your equipment allow local boxes to give you more room to play with. In which case you may find there are other solutions that could be explored which will save some time and/or effort. This would only be if the harnesses from the strain gauges to the local boxes didn't land you the same wiring problems.

I know what your concerns are with large distances between sensor and ADC as I have a system that I am dealing with where the closest I can get the electronics to the strain gauge is 150 metres (due to radiation and magnetic field strengths in the measurement environment). We still only use

12-bit ADC's to discern up to 320kg of applied force with sufficient accuracy for our purposes.

--
********************************************************************
Paul E. Bennett ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************