Texas TMS1232 Delta Sigma converters

Hello Adrian,

As John said, 50/60Hz easily gets in. Use a coupling transformer between your source (generator?) and the ADC board.

Make sure you have one solid ground plane. Use SMT bypass caps at all supply pins, right from pin to ground plane, no wires. Filter the heck out of your power supply to make sure no low frequency stuff gets in. Bench power supplies can exhibit huge noise levels.

Regards, Joerg

Past about 18 bits, the reference problem is serious. Most references have a heap of 1/f noise, which will be a problem around 10 Hz... bandgaps are bad. We're using an Analog Devices Xfet ref, ADR421, which is pretty good. The Intersil things are interesting too, basicly a voltage trapped on a capacitor forever.

One could build a 0.xx Hz active lowpass filter (a non-trivial problem) or maybe average a bunch of references, four maybe, but the math is discouraging here, improvement as to the square root of the number of refs.

John

Hi All,

Just got some samples of these very nice Delta-Sigma A/D converters, complete with built-in amplifiers for a new project.

A quick bench lash-up hooked to an AVR processor development system shows promising results when used direct with a gain of 64 ( I have a bridge output type sensor with a full scale 25 mV output ). I didnt expect too good a noise result on just a lash-up, and in fact only achieved about 16-17 bit samples above the noise. But I need at least

Anyone played with these ? Any good tips on layout and input filtering to remove as much noise as possible ?

Sorry, I meant ADS1232, too used to typing T for Texas parts.

They did some neat tricks in the internals of this, using a 10 Hz sampling frequency automatically rejects a lot of 50/60 Hz signal.

One of the objects in using this widget is that we can cut the external part count down from around 60 precision components on an existing design to less than 10 non precision ones, so I prefer not to add analog filtering unless we *really* have to.

My bench power supply is quiet, 5.00 volts with less than 0.1mv residual noise, and using fully isolated double transformer with separate inter-winding shields. Still get some hum pickup on the bench of course, but its about as low as I can get till I do a proper board layout.

The app isnt for weighing, and I can jigger the test transducer so its inputs are essentially true zero for test purposes, its just noise picked up between the transducer and the ADC I'm trying to eliminate. As well as the ref noise and stuff of course.

Thanks for the tips.

One can just make the measurement ratiometric, i.e. use the "reference" to supply the transducer excitation as well as the ADC ref input.

Hello Adrian,

Watch out for all those SMPS around you and other sources of noise. Laptops, printers, "modern" wall warts, fluorescent lighting and, yes, energy saver bulbs.

Once I picked up more than 6dB SNR simply by unplugging a (new) ink jet printer. Since I also needed it to print I had to build a filter. One that also took care of the enourmous differential mode noise, something that many commercial filters don't do very well.

Regards, Joerg

All good stuff.

A lash-up in this kind of situation is (as you said) not going to be the most utterly quiet thing ever. However, 50/60Hz will at least be rejected a great deal at 10sps, even if it shows up differentially, which it probably will.

Noise doesn't only get in through the reference and inputs; it can easily couple in over the digital lines, and if they're nice and long the input ones can act as beautiful antennas. Some people like to put EMI filters and resistors on all the digital lines for that reason, though that may be a bit excessive.

On a real layout this is less of a problem, since (presumably) the distances are shorter, wires aren't flying through the air and picking up fields, etc.

By the way, there is a reference board for this device; the user guide has the layout on page 18:

cheers -- mpa

Thanks for the info, and the link, I didnt spot it when I first looked into these devices.